Advanced Electric Vehicle Fast-Charging Technologies

energies

Review Advanced Electric Vehicle Fast-Charging Technologies

Ryan Collin 1,* , Yu Miao 2 , Alex Yokochi 2 , Prasad Enjeti 3 and Annette von Jouanne 1 1 Department of Electrical and Computer Engineering, Baylor University, Waco, TX 76798, USA; [email protected] 2 Department of Mechanical Engineering, Baylor University, Waco, TX 76798, USA; [email protected] (Y.M.); [email protected] (A.Y.) 3 Department of Electrical and Computer Engineering, Texas A&M University, College Station, TX 77843, USA; [email protected] * Correspondence: [email protected]; Tel.: 360-470-1180

Received: 5 March 2019; Accepted: 9 May 2019; Published: 15 May 2019

Abstract: Negative impacts from the dominant use of petroleum-based transportation have propelled the globe towards electriﬁed transportation. With this thrust, many technological challenges are being encountered and addressed, one of which is the development and availability of fast-charging technologies. To compete with petroleum-based transportation, electric vehicle (EV) battery charging times need to decrease to the 5–10 min range. This paper provides a review of EV fast-charging technologies and the impacts on the battery systems, including heat management and associated limitations. In addition, the paper presents promising new approaches and opportunities for power electronic converter topologies and systems level research to advance the state-of-the-art in fast-charging.

Keywords: fast-charging; electric vehicle; high power charging; battery energy storage

1. Introduction In the past few decades there has been a steady transition from petroleum–based to electric-based transportation in all sectors, including aircraft, trains, ships, and electric vehicles (EVs). This shift is expected to rapidly advance, particularly with EVs, as the benefits, political incentives, and falling prices, including due to large-scale production, aid the market [1–3]. The U.S. Energy Information Administration states that the world has an adequate crude oil supply until about 2050 [4]. Clearly, alternatives to fossil fuels need to be developed for transportation, electricity generation, etc. For transportation, EVs have been a recognized solution and continue to become widely accepted as the technology develops and economic feasibility becomes a reality. EVs offer increased efficiency (energy savings) through better fuel economy, reduced emissions/pollution (especially when the electricity is generated from renewable resources, such as wind and solar), and EVs help the U.S. to have a greater diversity of fuel choices for transportation. Almost all U.S. electricity is produced from domestic sources, including natural gas, coal, nuclear, and renewable sources. While only 17% of the U.S. petroleum demand was imported in 2017 [5,6], this may change as reservoirs become depleted. Charging times have been an Achilles heel of electrified transportation technology, as it has been much faster and more convenient, historically, to refuel from petroleum-based sources. Charging of EVs has been categorized by the U.S. Department of Energy in three levels, as follows: Level 1 is standard charging that has a charge power less than 5 kW, Level 2 is fast-charging that occurs between 5 kW and 50 kW, and Level 3 is super-fast charging that is greater than 50 kW [7]. Level 3 charging consists of an off-board charger, meaning the charger is exterior to the vehicle. The high-power charging levels of Level 3 charging make it impractical to carry the required power electronics onboard, due to size constraints. In order to reduce the mass of on-board electronics, Level 3 charging usually conveys the

Energies 2019, 12, 1839; doi:10.3390/en12101839 www.mdpi.com/journal/energies Energies 2019, 12, 1839 2 of 26 power to the vehicle as DC, while Level 1 and Level 2 charging often times contain onboard electronic converters, allowing for AC energy transfer. Further detail on this topic is given in Section6. If one focuses attention on recharging an electric vehicle to enable the average daily drive of approximately 31.5 miles/day for a US driver at 10 kW charging [8], and given that typical EVs have efficiencies in the range of 240–300 Wh/mi with performance-based variations [9], this is still ~1 h of charging. Thus, if the vehicle is needed again, e.g., for an additional evening trip or emergency, it would be wise for alternative faster charging solutions to be available. For comparison, refueling stations in North America can dispense fuel at up to 10 gallons/minute (US) or 38 Liters/minute (Canada), which corresponds to an energy transfer rate of approximately 1.3 GJ/min, or approximately 21.5 MW [10,11]. Even if it is assumed that only approximately 25% of that energy is used as power to wheels (i.e., the energy in the fuel that is actually used), that would still correspond to an effective energy transfer rate of approximately 5.4 MW [12]. Clearly, the need for fast charging exists for consumer convenience as to not limit drivers to the daily average driving distance (31.5 miles/day), while also improving the energy transfer rate to closer values to that of gasoline and diesel refueling stations. Therefore, the scope of this review paper is super-fast charging (Level 3 charging, energy transfer rates >50 kW). The motivation behind developing super-fast charging technologies and standards will be given by comparing one of the first production EVs with the 2018 Chevy Bolt and by introducing consumer challenges that exist due to a lack of a standardized conductive charging plugs. A review of the chemical processes within the battery reveal the effects of high-power charging and the limitations for fast-charging (Section3). Branching off from that, modern charging methodologies are explored in light of the battery limitations and recent advancements are discussed that improve performance. Implementation of the battery charging technology is explored in Section 4.2 and ways of handling issues associated with the battery due to super-fast charging are introduced in Section5. Section6 explores the existing converters that make charging possible and, finally, potential research opportunities that will expand current technologies are proposed in Section7.

2. Further Motivations

2.1. EV Comparison: GM EV1 vs. 2018 Chevy Bolt The General Motors (GM) EV1 was one of the first mass produced EVs of the modern era. In 1996, the first-generation models were released with a 16.5 kWh lead-acid battery and a driving range of 60 miles. This car had a few different charging capabilities, as follows: A 6.6 kW charger that took 3 h to completely charge the battery, a 1.2 kW charger that took 15 h to fully charge, and, later, a 50 kW charger was developed that could charge the battery from 20% to 80% of full charge in 10–15 min. Fast-forwarding to today, the 2018 GM Chevy Bolt has a 60 kWh lithium-ion (Li-ion) battery and a driving range of 238 miles [13]. The Bolt has 3 charging options, as follows: Basic charging utilizes a 120 V wall outlet and gives ~48 miles per 12 h of charge time (~1 kW), fast-charging is a 240 V option that charges at 7.68 kW, and super-fast charging is a DC charging option that replenishes the battery at ~50 kW [14]. As a consequence of the increase in battery capacity and, thus, driving range, charging the Bolt’s 60 kWh battery at the super-fast charging rate of 50 kW takes 1 h and 15 min. Therefore, while EV driving ranges have become competitive with internal combustion engine vehicles (ICEVs), the recharge/refuel times still lack in comparison. While it should be acknowledged that, for maximal operational lifetime of the battery pack in the EV, charging should be done at slow rates, for the widespread adoption of EVs that deliver environmental benefits with convenience comparable to that of current internal combustion based engines, the charging must occur at higher power rates to decrease the charge time. A higher charge rate means a faster effective trip speed, but there are technical limitations at each step of the electric car charging process, as follows: (1) Batteries heat up while being charged, due to inner resistance and other factors, and the heat can cause damage and safety issues; (2) the wiring and connectors have a maximum amperage that can be handled and a 50 kW Energies 2019,, 12,, 1839x FOR PEER REVIEW 3 of 26 amperage that can be handled and a 50 kW DC fast charge station at 400V (voltage typically for DC fast charge station at 400V (voltage typically for electric cars) operates at 125 A, which requires electric cars) operates at 125 A, which requires large conductors; (3) the size of the AC/DC converter large conductors; (3) the size of the AC/DC converter will also be large when the power is greater will also be large when the power is greater than 50 kW; (4) the power demand must be limited to than 50 kW; (4) the power demand must be limited to 80% of the rated capacity of the conductors and 80% of the rated capacity of the conductors and other equipment for a continuous load, while other equipment for a continuous load, while charging an EV fits within this definition, so the ratings charging an EV fits within this definition, so the ratings must be 125% of the continuous power must be 125% of the continuous power requirement; (5) fees and electrical service limitations by utility requirement; (5) fees and electrical service limitations by utility companies is another obstacle [15]. companies is another obstacle [15]. 2.2. EV Charging Plugs 2.2. EV Charging Plugs As EV technologies improved and electrified transportation has become more widespread, a As EV technologies improved and electrified transportation has become more widespread, a need need for standardized charging ports has developed. Just as a driver of an internal combustion engine for standardized charging ports has developed. Just as a driver of an internal combustion engine vehicle (ICEV) can fill up at any gas station, the same need arose for EVs. Even today, a lack of vehicle (ICEV) can fill up at any gas station, the same need arose for EVs. Even today, a lack of charging port standardization is a major hurdle that is holding back EV adoption, as the various EV charging port standardization is a major hurdle that is holding back EV adoption, as the various EV manufacturers still use different charging port configurations. The Society of Automotive Engineers manufacturers still use different charging port configurations. The Society of Automotive Engineers [16] [16] (SAE) exists with the purpose of uniting and developing technologies by creating industry (SAE) exists with the purpose of uniting and developing technologies by creating industry accepted accepted standards. In 1996, SAE released its first EV conductive charging coupler standard, which standards. In 1996, SAE released its first EV conductive charging coupler standard, which has been has been revised numerous times up until the present day [17]. An SAE J1772 connector standard revised numerous times up until the present day [17]. An SAE J1772 connector standard with a DC with a DC fast-charging combined charging system (CCS 1) can be seen in Figure 1. Other EV and fast-charging combined charging system (CCS 1) can be seen in Figure1. Other EV and plug-in hybrid plug-in hybrid electric vehicle (PHEV) standards that SAE has created over the years include the electric vehicle (PHEV) standards that SAE has created over the years include the following: J2954 following: J2954 (a wireless power transfer standard), J3068 (a three-phase conductive charging (a wireless power transfer standard), J3068 (a three-phase conductive charging standard), and other standard), and other various standards that address communications and vehicle performance various standards that address communications and vehicle performance measurement standards. measurement standards.

Figure 1. SAE J1772 EV connector standard with DC fast-charging combined charging system Figure 1. SAE J1772 EV connector standard with DC fast-charging combined charging system (CCS (CCS 1) [18]. 1) [18]. Although the SAE has developed standards such as J1772, the standard still hasn’t been adopted by allAlthough manufacturers. the SAE As has EV developed technology standards continues such to develop, as J1772, the the hope standard is that still the hasn’t manufacturers been adopted will byconverge all manufacturers. on one plug-in As port.EV technology Four of the con maintinues charging to develop, connectors the hope emerging is that onthe the manufacturers market today will are convergethe North on American one plug CCS-in port. 1, the Four European of the CCS main 2, charging the CHAdeMO connectors (a standard emerging developed on the market in Japan today and areChina), the North and the American Tesla Super CCS Charger. 1, the European The CCS CCS 1, as 2, well the CHAdeMO as the CCS 2,(a havestandard a few developed diﬀerent chargingin Japan andlevels, Chin as havea), and been the mentioned Tesla Super when Charger. detailing The the CCS Chevy 1, as Bolt well charging as the speciﬁcations.CCS 2, have aCHAdeMO few different is a chargingDC fast-charging levels, as plug have that been can mentioned charge at up when to 62.5 detailing kW, with the current Chevy work Bolt on charging technologies specifications. that can CHAdeMOcharge at 200 is kW a DC [19 ]. fast Recently,-charging CHAdeMO plug that and can the charge China at Electricity up to 62.5 Council kW, with have current been making work on a technologiesstrong push towardsthat can charge a global at EV 200 charging kW [19]. standard, Recently, invitingCHAdeMO auto and manufacturers the China Electricity from all overCouncil the worldhave been to participate making a strong in the push design. towards The newa global standard EV charging will allow standard, for a charginginviting auto power manufacturers upwards of ~900from kWall over (1500 the V, world 600 A) to [20 participate]. The high-power in the design. handling The capabilitiesnew standard will will enable allow the for standard a charging to be power used upwardsfor several of years ~900 as kW battery (1500 and V, charging 600 A) [ technologies20]. The high continue-power handling to develop, capabilities allowing for will higher enable power the standardcharging. to Tesla’s be used latest for ultra-fast several “Superchargeryears as battery V3” and can charging have a power technologies output continue up to 250 to kW dev perelop car, allowand willing ultimatelyfor higher cutpower the amountcharging. of chargingTesla’s latest time ultra by an-fast average “Supercharg of 50%,er as shownV3” can in have Figure a 2power.[21]. Autooutput manufacturers up to 250 kW are per currently car andsplit will betweenultimately the cut CHAdeMO the amount and of the charging CCS standards, time by withan average Asian carof 50%,makers as usingshown CHAdeMo in Figure 2. while [21]. a Auto majority manufacturers of European are and currently American split manufacturers between the useCHAdeMO the CCS, and the CCS standards, with Asian car makers using CHAdeMo while a majority of European and American manufacturers use the CCS, and Tesla uses its own charging coupler, which has increased

Energies 2019, 12, 1839 4 of 26 Energies 2019, 12, x FOR PEER REVIEW 4 of 26 inTesla popularity uses its recently. own charging The lack coupler, of a global which standard has increased has caused in popularity much confusion recently. for The consumers lack of a globaland, alongstandard with has high caused prices, much battery confusion recharge for consumers times, and and, range along anxiety, with high this prices, lack battery of a standard recharge times, has preventedand range widespread anxiety, this acceptance lack of a standardof EVs. Therefore, has prevented further widespread advancements acceptance in charging of EVs. technologies Therefore, andfurther standards advancements will lead into chargingthe public’s technologies adoption andof EVs. standards will lead to the public’s adoption of EVs.

Figure 2. Tesla’s latest ultra-fast V3 supercharger can cut the charging time by an average of 50% [21]. Figure 2. Tesla’s latest ultra-fast V3 supercharger can cut the charging time by an average of 50% [21]. 3. EV Battery Considerations 3. EV Battery Considerations 3.1. Kinetics of EV Battery 3.1. KineticsCurrently, of EV Li-ionBattery batteries are the preferred energy storage option in electric vehicles. The Li+ + chargeCurrently, transfer Li process-ion batte occursries are when the apreferred solvated Lienergyion instorage the electrolyte option in migrateselectric vehicles. into an electrode,The Li+ chargwhiche transfer simultaneously process occurs accepts when an electron a solvated from Li+ the ion electrode, in the electrolyte or vice versa.migrates For into example, an electrode during, + whichcharging simultaneously for a LiCoO2 /acceptgraphites an battery, electron at the from graphite the electrode cathode, the or vice Li desolvates versa. For from example, the electrolyte during chargingand crosses for a intoLiCoO the2/graphite Solid Electrolyte battery, at Interphase the graphite from cathode which the it thenLi+ desolvates makes its from way the to theelectrolyte graphite + material in a half cell reaction that is generally written as xLi + xe + C LixC as illustrated in and crosses into the Solid Electrolyte Interphase from which it then −makes6 →its way6 to the graphite Figure3a [ 22]. The SEI which is a dense, nanometer thick passivation layer formed on electrode material in a half cell reaction that is generally written as xxLi e  C66  Lix C as illustrated in + Figuresurfaces 3a from[22]. decomposition The SEI which of is electrolytes, a dense, nanometer which allows thick Li passivationtransport but layer blocks formed electron on electrode transport and prevents further electrolyte decomposition. surfaces from decomposition of electrolytes, which allows Li+ transport but blocks electron transport + and preventsAt the counter-electrode, further electrolyte which decompositi currentlyon functions. as an anode, a Li ion de-intercalates from the + LiCoO2 material and eventually results in a solvated Li in the electrolyte,+ with the half cell reaction At the counter-electrode, which+ currently functions as an anode, a Li ion de-intercalates from generally written as LiCoO2 xLi + xe− + Li1 xCoO2 [22]. Naturally, by connecting the two half the LiCoO2 material and eventually results in a solvated Li+ in the electrolyte, with the half cell → −+ cells we have an electrolyte, through which the Li ions migrate under the effect of electric fields and reaction generally written as LiCoOLieLiCoO212xx x [22]. Naturally, by connecting the two concentration gradients. half cells we have an electrolyte, through which the Li+ ions migrate under the effect of electric fields During discharge, the Li+ charge transfer process reverses, as shown in Figure3b. The process and concentration gradients. Li C xLi+ + xe + C occurs at the carbon electrode, which now acts as an anode, and x 6 − 6 + During+ → discharge, the Li charge transfer process reverses, as shown in Figure 3b. The process xLi + xe−+ Li1 xCoO2 LiCoO2 at the cathode [22]. Li CLieCxx− occur→ s at the carbon electrode, which now acts as an anode, and x 66Each of these processes (charge transfer processes, migration through the electrode) has an intrinsic xxLieLi CoOLiCoO at the cathode [22]. reaction rate,122 withx the slowest reaction eventually dominating. This reaction rate sets up the maximal charge/discharge rate, which is reflected directly in the charge/discharge current. The rate of reaction is fundamentally driven by the excess potential applied to the electrochemical system in the form of voltage. In an electrochemical system, the applied chemical potential is reflected as voltage through the relation, shown in Equation (1) as follows:

∆G = nFE, (1) − where ∆G is the excess chemical potential, also known as the overpotential [23], F is faraday’s constant 1 of 96,485 C mol− , and n reﬂects the number of electrons exchanged in a particular chemical process. At the equilibrium potential (E = 0) the potential to drive the reaction (∆G) is also zero and no reaction

Energies 2019, 12, 1839 5 of 26 takes place. Clearly, some excess potential must be applied (or lost) in order to cause the reaction to occur, known as the overpotential [24]. Therefore, if the applied electrical energy to charge the battery is essentially viewed as pure enthalpy (∆H), but the stored energy is reﬂected in the resulting voltage, which according to Equation (1) is reﬂected in ∆G. Then, because as given in Equation (2):

∆G = ∆H T∆s, (2) − where ∆s is the change in entropy in the system and T is the thermodynamic temperature (and in general, T∆s = ∆Q, or the waste heat generated in a process), the eﬃciency of charging the battery can be written as shown in Equation (3), as follows:

∆G ∆H T∆s T∆s ∆Q e f f iciency% = 100% = − 100% = 1 100% = 1 100%, (3) ∆H × ∆H × − ∆H × − ∆H × with the loss of eﬃciency as that fraction of the enthalpy applied as electric energy that is dissipated as heat. Clearly then, the higher the applied overpotential, the lower the charging eﬃciency and the Energieslarger the2019 amount, 12, x FOR of PEER heat REVIEW generated in the battery during charging. 5 of 26

Figure 3. AA schematic schematic view view of of the the Li+ Li+ chargecharge transfer transfer process process during during ( (aa)) charge charge and and ( (b)) discharge discharge [2 [222]]..

EachThe rate of these at which processes the reaction (charge then transfer occurs processes, is generally migration described through as a function the electrode) of the applied has an intrinsicvoltage as reaction given inrate, Equation with the (4): slowest reaction eventually dominating. This reaction rate sets up the [ ηαF ] maximal charge/discharge rate, which is reflectedi = nFkCe directly− kBT , in the charge/discharge current. (4) The rate of reaction is fundamentally driven by the excess potential applied to the i electrochemicalan equation commonly system knownin the form as the of Tafel voltage. Equation In an [ 25electrochemical] and where is system, the resulting the applied electrochemical chemical k potentialcurrent, η isis reflected the electrochemical as voltage through overpotential, the relationis, the shown reaction in Equation rate of the(1) as electrochemical follows: reaction taking place, C is the concentration of the electroactive species participating in the reaction, α is the Charge Transfer Coeﬃcient (the eﬀectiveness of theGnFE applied, electric potential in lowering any activation(1) whereenergy ΔG barrier is the to the excess electrochemical chemical potential reaction),, alsokB is known Boltzmann’s as the constant,overpotential and n[2, 3F],, andF isT faraday’shave the constantsame meaning of 96,485 as above.C mol−1, and n reflects the number of electrons exchanged in a particular chemical process. At the equilibrium potential (E=0) the potential to drive the reaction (ΔG) is also zero and no reaction takes place. Clearly, some excess potential must be applied (or lost) in order to cause the reaction to occur, known as the overpotential [24]. Therefore, if the applied electrical energy to charge the battery is essentially viewed as pure enthalpy (ΔH), but the stored energy is reflected in the resulting voltage, which according to Equation (1) is reflected in ΔG. Then, because as given in Equation (2): G   H  T  s , (2) where Δs is the change in entropy in the system and T is the thermodynamic temperature (and in general, TΔs = ΔQ, or the waste heat generated in a process), the efficiency of charging the battery can be written as shown in Equation (3), as follows:

GH  T  sT sQ  efficiency%100%100%  1100%  1100%   , (3) HHHH  with the loss of efficiency as that fraction of the enthalpy applied as electric energy that is dissipated as heat. Clearly then, the higher the applied overpotential, the lower the charging efficiency and the larger the amount of heat generated in the battery during charging. The rate at which the reaction then occurs is generally described as a function of the applied voltage as given in Equation (4):

Energies 2019, 12, x FOR PEER REVIEW 6 of 26

 F [] i n F k C e kTB , (4) an equation commonly known as the Tafel Equation [25] and where i is the resulting electrochemical current, η is the electrochemical overpotential, k is the reaction rate of the electrochemical reaction taking place, C is the concentration of the electroactive species participating in the reaction, α is the Charge Transfer Coefficient (the effectiveness of the applied electric potential in lowering any Energiesactivation2019 ,energy12, 1839 barrier to the electrochemical reaction), kB is Boltzmann’s constant, and n, F, and6 of 26T have the same meaning as above. From Equation (4) we can see that as we attempt to drive the charging reaction faster, we increase From Equation (4) we can see that as we attempt to drive the charging reaction faster, we increase the overpotential, which, according to Equations (2) and (3), results in a lower charging efficiency the overpotential, which, according to Equations (2) and (3), results in a lower charging eﬃciency and and higher generation of heat. higher generation of heat.

3.2.3.2. Current Limit ItIt shouldshould bebe furtherfurther notednoted thatthat asas thethe chargingcharging processprocess takestakes place,place, aa layerlayer ofof electrodeelectrode materialmaterial + thatthat isis “charged”“charged” startsstarts toto grow,grow, andand throughthrough whichwhich thethe LiLi+ ionsions nownow needneed to didiffuse,ﬀuse, slowingslowing downdown thethe overalloverall chargingcharging rate.rate. For this reason, thethe rate at which a battery charges for a given overpotential decreasesdecreases andand oneone may be tempted to merely increase the applied overpotentialoverpotential toto restore fasterfaster rates of charging. of charging. ThereThere isis naturallynaturally aa limitlimit toto thethe overpotentialoverpotential thatthat cancan bebe applied.applied. ItIt isis wellwell knownknown thatthat Li-ionLi-ion batteries are damaged when a certain upper voltage limit is exceeded, due to the occurrence of batteries are damaged when a certain upper voltage limit is exceeded, due to the occurrence of undesired secondary electrochemical reactions. For example, if the upper voltage threshold is undesired secondary electrochemical reactions. For example, if the upper voltage threshold is exceeded exceeded in LiNi2O4 based batteries, a second phase is formed due to the migration of Ni ions to Li in LiNi2O4 based batteries, a second phase is formed due to the migration of Ni ions to Li vacancies, vacancies, permanently reducing the capacity of the battery [26]. Likewise, it is known that applying permanently reducing the capacity of the battery [26]. Likewise, it is known that applying too high a too high a current may lead to electroplating of lithium on the negative electrode, which in turn current may lead to electroplating of lithium on the negative electrode, which in turn modiﬁes the modifies the solid electrolyte interphase and permanently degrades the performance of the battery solid electrolyte interphase and permanently degrades the performance of the battery [27,28]. [27,28]. It is commonly known that the acceptable charging current is a function of the battery’s state of It is commonly known that the acceptable charging current is a function of the battery’s state of charge (SoC), as shown in Figure4. charge (SoC), as shown in Figure 4.

Figure 4. Acceptable battery current during charging [29]. Figure 4. Acceptable battery current during charging [29]. As can be seen in Figure4, the acceptable charging current is a monotonically decreasing function of theAs state can of be charge seen that in Fig canure be determined4, the acceptable by battery charging parameters. current Therefore, is a monotonically manufacturers decreasing specify maximumfunction of initial the charging state of rates charge (to about that 70% can SoC) be determined which the battery by battery can tolerate parameters without. damagingTherefore, themanufacturers battery. Special specif precautionsy maximum are neededinitial charging to maximize rate thes (to charging about 70% rate and SoC) to which ensure the that battery the battery can istolerate fully charged without while,damaging at the the same battery. time, Spe avoidingcial precautions overcharging are needed [30]. Initially,to maximi itz ise possiblethe charging that rate the maximumand to ensure acceptable that the battery battery current is fully is charged higher thanwhile the, at power the same conversion time, avoiding capability overcharging of the charging [30]. equipmentInitially, it and is possible the charging that the rate maximum of the battery acceptable is therefore battery charging current equipment is higher limited, than the but power after someconversion fractional capability SoC is of achieved, the charging the maximum equipment acceptable and the charging rate of charge rate of then the becomes battery is lower therefore than thatcharging of the equipment charging equipmentlimited, but and after the some system fractional becomes SoC limited is achieved, by the the battery maximum system acceptable itself. This rate is the recharging rate maximizing strategy employed in the constant current-constant voltage charging method described below.

of charge then becomes lower than that of the charging equipment and the system becomes limited by the battery system itself. This is the recharging rate maximizing strategy employed in the constant current-constant voltage charging method described below.

3.3. C-Rate Charge and discharge rates of a battery are governed by C-rates. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 1 Ah should provide 1A for one Energies 2019, 12, 1839 7 of 26 hour. The same battery discharging at 0.5C should provide 500 mA for two hours and at 2C it delivers 2A for 30 minutes. Losses at fast discharges reduce the discharge time and these losses also affect 2Acharge for 30times. min. Table Losses 1 illustrates at fast discharges typical times reduce at thevarious discharge C-rates time [31 and]. these losses also aﬀect charge times. Table1 illustrates typical times at various C-rates [31]. Table 1. C-rate and service times when charging and discharging batteries of 1Ah [31]. Table 1. C-rate and service times when charging and discharging batteries of 1Ah [31]. C-rate Time C-rate5C 12 Timemin 5C2C 30 12min min 2C1C 1 30 h min 1C0.5C 2 h 1 h 0.5C 2 h 0.2C 5 h 0.2C 5 h 0.1C0.1C 10 10 h h 0.05C0.05C 20 20 h h

To obtainobtain aa reasonablyreasonably goodgood capacitycapacity reading,reading, manufacturersmanufacturers commonly rate alkaline and lead acid batteries at at a a very very low low 0.05C, 0.05C, or or a a20 20 hour h discharge. discharge. Even Even at at this this slow slow dischargedischarge rate,rate, leadlead acid seldom attains a 100 percent capacity,capacity, asas thethe batteriesbatteries areare overrated.overrated. Manufacturers provide capacity ooffsetsﬀsets to to adjust adjust for for the the discrepancies discrepancies if discharged if discharged at a higher at a higher C rate than C rate speciﬁed. than specified. Figure5 illustrates Figure 5 theillustrates discharge the timesdischarge of a leadtimes acid of a battery lead acid at variousbattery loads,at various expressed loads, inexpressed C-rate [31 in]. C-rate [31].

Figure 5. Typical discharge curves of lead acid as a function of C-rateC-rate [[3311].]. 3.4. Current and Future EV Battery Types 3.4. Current and Future EV Battery Types There are many derivations of Li-ion batteries existing in today’s EVs, many of which are under There are many derivations of Li-ion batteries existing in today’s EVs, many of which are under development. Researchers are able to select the electrolyte, positive, and negative electrodes in an development. Researchers are able to select the electrolyte, positive, and negative electrodes in an attempt to optimize the performance, safety, and longevity of the battery. A comparison of today’s attempt to optimize the performance, safety, and longevity of the battery. A comparison of today’s Li-ion batteries used in EVs is shown in Figure6. Li-ion batteries used in EVs is shown in Figure 6. The categories under comparison in Figure6 are the main concerns in today’s batteries. In Figure6, the battery types with the largest colored area are the most ideal, with Lithium Nickel Manganese Cobalt Oxide (NMC), Lithium Iron Phosphate (LFP), and Lithium Manganese Oxide (LMO) possessing the best overall characteristics. As covered in [33], there are many materials that researchers continually attempt to implement to improve batteries. For example, (1) Li metal electrodes possess great electrochemical potential that may decrease the mass of the battery, but have inherent safety issues due to the formation of dendrites across the electrolyte; (2) alloy based electrodes may enable a higher specific capacity of the battery, but the mechanical strain due to the alloying process causes deterioration of the electrode; (3) silicon based electrodes appear to be approaching commercialization, despite the high internal strains and high electrical resistivity; (4) aqueous electrolytes offer safety and environmental benefits, yet the restricted electrochemical voltage window is of concern; and (5) ceramic electrolytes have shown promising results, due to higher conductivity at grain boundaries. All of these materials and others [33] are under research and development for implementation and once a proper solution exists to make them viable for EV batteries, commercialization will occur. Energies 2019, 12, 1839x FOR PEER REVIEW 8 of 26

Figure 6. A comparison of Li-ion batteries used in today’s EVs [32]. Figure 6. A comparison of Li-ion batteries used in today’s EVs [32]. 4. Battery Charging Methods The categories under comparison in Figure 6 are the main concerns in today’s batteries. In Figure There are several different strategies and physical implementations employed when recharging 6, the battery types with the largest colored area are the most ideal, with Lithium Nickel Manganese EV batteries. Battery charging strategy refers to the shapes and magnitudes of the currents/voltages to Cobalt Oxide (NMC), Lithium Iron Phosphate (LFP), and Lithium Manganese Oxide (LMO) be used during charging, while the physical implementations of battery charging infrastructure means possessing the best overall characteristics. As covered in [33], there are many materials that how the energy is transferred to the vehicle physically. researchers continually attempt to implement to improve batteries. For example, (1) Li metal electrodes4.1. Battery possess Charging great Strategy electrochemical potential that may decrease the mass of the battery, but have inherent safety issues due to the formation of dendrites across the electrolyte; (2) alloy based electrodes4.1.1. Constant may Voltageenable a higher specific capacity of the battery, but the mechanical strain due to the alloyingIn a process constant causes voltage deterioration charge, the charging of the electrode voltage; is (3) maintained silicon based at the electrodes maximum appear voltage to that be approachingshould be applied commercialization, to a certain type despite of battery the high while internal the charging strains current and high slowly electrical decreases resistivity as the; full(4) abatteryqueous charge electrolytes is approached. offer safety This andis an environmental effective method benefits, when using yet thelower restricted voltages, electrochemical as temperature voltageusually isn’twindow an issue, is of butconcern lengthy; and charge (5) ceramic times are electr of concernolytes have [34]. shown promising results, due to higher conductivity at grain boundaries. All of these materials and others [33] are under research and development4.1.2. Constant for Current implementation and once a proper solution exists to make them viable for EV batteries, commercialization will occur. As the name implies, this charging method applies a constant current as the battery voltage 4.builds Battery up Charging to its full Methods charge value. Even if the constant current applied is within the rated current, the constant current to the battery can easily cause overheating and damage, compromising the life of the batteryThere are [34 ].several different strategies and physical implementations employed when recharging EV batteries. Battery charging strategy refers to the shapes and magnitudes of the currents/voltages to4.1.3. be used Constant during Current-Constant charging, while Voltage the physical (CC-CV). implementations of battery charging infrastructure meansOriginally how the energy referred is to transferred as simply to “Voltage the vehicle Controlled physically. Charging” [35], constant current-constant voltage charging is a common approach to battery charging where the charger applies a constant 4.1. Battery Charging Strategy current until the battery reaches a predefined voltage potential, at which point voltage is held constant and the current continues to decrease until a full charge is reached [36]. This is illustrated in Figure7 4.1.1. Constant Voltage and is the traditional method for charging batteries, yet it is limited in fast-charging applications becauseIn a batteryconstant polarization voltage charge, becomes the charging an issue. voltage As may is bemaintained expected, at the the CC-CV maximum method voltage has beenthat shouldfurther be modified applied toto includea certain multiple type of battery constant while current the steps,charging thereby current further slowly improving decreases theas the rate full of batterycharging charge of the is batteries approached. [37]. This is an effective method when using lower voltages, as temperature usually isn’t an issue, but lengthy charge times are of concern [34].

4.1.2. Constant Current

Energies 2019, 12, x FOR PEER REVIEW 9 of 26

As the name implies, this charging method applies a constant current as the battery voltage builds up to its full charge value. Even if the constant current applied is within the rated current, the constant current to the battery can easily cause overheating and damage, compromising the life of the battery [34].

4.1.3. Constant Current-Constant Voltage (CC-CV). Originally referred to as simply “Voltage Controlled Charging” [35], constant current-constant voltage charging is a common approach to battery charging where the charger applies a constant current until the battery reaches a predefined voltage potential, at which point voltage is held constant and the current continues to decrease until a full charge is reached [36]. This is illustrated in Figure 7 and is the traditional method for charging batteries, yet it is limited in fast-charging applications because battery polarization becomes an issue. As may be expected, the CC-CV method has been further modified to include multiple constant current steps, thereby further improving the Energies 2019, 12, 1839 9 of 26 rate of charging of the batteries [37].

Figure 7. Constant current-constant voltage battery charging [38]. Figure 7. Constant current-constant voltage battery charging [38]. As would be expected, different batteries have different characteristics when it comes to charging. ThroughAs the would “A Better be expected, Route Planner” different (ABRP) batteries online have forum’s different data characteristics collection by generous when it users comes [39 to], significantcharging. Through real-world the data “A hasBetter been Route collected Planner” on which (ABRP models) online can forum’s be built data for collection use in route by planning.generous Theusers data [39 shown], significant below real in Figure-world8 data is based has been on Tesla collected batteries on which from 801models Tesla can Vehicles, be built listed for use below in route in Tableplanning.2[ 40 ].The The data practical shown implementation below in Figure of 8 is the based constant on Tesla current-constant batteries from voltage 801 Tesla charging Vehicles protocol, listed canbelowEnergies be seenin 2019 Table, in 12, the x 2 FOR [ data.40 ]PEER. The REVIEW practical implementation of the constant current-constant voltage charging10 of 26 protocol can be seen in the data.

Table 2. A list of Tesla batteries used for the real-world data collection [40].

Battery Tesla Estimated Usable Capacity 10 kWh  50 kWh Charge Time Code Model (kWh) (min) 3 Long BT37 72.8 23 Range BT70 S70 65.7 33 BT85 S85 73.4 27 BTX4 S/X90 79.8 23 BTX5 S/X75 71.6 27 BTX6 S/X100 95.7 20 BTX8 Rare S/X75 25 Figure 8. Typical battery charging curves showing implementation of the CC-CV protocol [40]. Figure 8. Typical battery charging curves showing implementation of the CC-CV protocol [40]. Table 2. A list of Tesla batteries used for the real-world data collection [40]. 4.1.4. Pulse Charging Estimated Usable 10 kWh 50 kWh Battery Code Tesla Model Pulse charging sends pulses of current toCapacity the battery (kWh) in a fashionCharge that optimizes Time→ (min) the charging time while consideringBT37 polarization, 3 Long Range battery heating, 72.8 SoC, and variable battery 23 impedance [34]. The rest time of eachBT70 pulse period allows S70 the ions to diffuse 65.7 through the electrode 33materials, increasing the efficiencyBT85 of the charging process S85 [41]. A common 73.4 battery charging impedance 27 model is shown in Figure 9. BTX4 S/X90 79.8 23 BTX5 S/X75 71.6 27 BTX6 S/X100 95.7 20 BTX8 Rare S/X75 25

4.1.4. Pulse Charging Pulse charging sends pulses of current to the battery in a fashion that optimizes the charging time while considering polarization, battery heating, SoC, and variable battery impedance [34]. The rest time of each pulse period allows the ions to diﬀuse through the electrode materials, increasing the eﬃciency of the charging process [41]. A common battery charging impedance model is shown in Figure9.

Figure 9. Electrical equivalent circuit charging impedance model for a battery [42].

The associated input impedance for the battery circuit model value can be seen in Equation (5). It should be noted that the circuit parameters are dependent on battery SoC and temperature [42].

푅1 푍퐶ℎ푎푟𝑔푒 = 푅표 + (5) 1 + 푗푤푅1퐶1 As a consequence of the capacitive nature of the battery when charging, the charging impedance ZCharge varies with frequency, as does the charging current. By selecting the appropriate pulse frequency, the charging impedance can be minimized, which will maximize the charging current, enabling faster charge times. As can also be seen in Figure 9, the impedance values of the battery vary depending on temperature and SoC. Polarization in Li-ion batteries can be accounted for by controlling the duty ratio of the pulse. By employing a dynamic control algorithm [42], the optimal frequency and duty cycle can be constantly calculated and applied, minimizing the battery charge time while accounting for battery health. The results from [42] show charging times cut in half and a system that is 52% more efficient than constant current charging. When the duty ratio and frequency are properly applied, pulse charging shows significant improvements over constant current-constant voltage charging, with lifetime improvements of up to 100 cycles, which in vehicles charged once per week corresponds to approximately two calendar years of extended operation [43]. However, improper frequency and duty cycle selection can severely degrade the lifetime of the battery, as was

Energies 2019, 12, x FOR PEER REVIEW 10 of 26

Figure 8. Typical battery charging curves showing implementation of the CC-CV protocol [40].

4.1.4. Pulse Charging Pulse charging sends pulses of current to the battery in a fashion that optimizes the charging time while considering polarization, battery heating, SoC, and variable battery impedance [34]. The rest time of each pulse period allows the ions to diffuse through the electrode materials, increasing Energiesthe efficiency2019, 12, 1839of the charging process [41]. A common battery charging impedance model is 10shown of 26 in Figure 9.

Figure 9. Electrical equivalent circuit charging impedance model for a battery [42]. Figure 9. Electrical equivalent circuit charging impedance model for a battery [42]. The associated input impedance for the battery circuit model value can be seen in Equation (5). The associated input impedance for the battery circuit model value can be seen in Equation (5). It should be noted that the circuit parameters are dependent on battery SoC and temperature [42]. It should be noted that the circuit parameters are dependent on battery SoC and temperature [42]. R 1푅1 ZCharge = Ro + (5) 푍퐶ℎ푎푟𝑔푒 = 푅표 1++ jwR C (5) 1 + 푗푤1푅1퐶1 AsAs aa consequenceconsequence ofof thethe capacitivecapacitive naturenature ofof thethe batterybattery whenwhen charging,charging, thethe chargingcharging impedanceimpedance ZZChargeCharge variesvaries with with frequency, frequency, as does as does the charging the charging current. current. By selecting By selecting the appropriate the appropriate pulse frequency, pulse thefrequency, charging the impedance charging can impedance be minimized, can be which minimized, will maximize which will the chargingmaximize current, the charging enabling current faster, chargeenabling times. faster As charge can also times. be seen As can in Figurealso be9 ,seen the impedancein Figure 9, values the impeda of thence battery values vary of the depending battery vary on temperaturedepending andon temperature SoC. Polarization and in SoC. Li-ion Polarizati batterieson can in be Li accounted-ion batteries for by can controlling be accounted the duty for ratio by ofcontrolling the pulse. the By duty employing ratio of a the dynamic pulse. controlBy employing algorithm a dynamic [42], the control optimal algorithm frequency [4 and2], the duty optimal cycle canfrequency be constantly and duty calculated cycle can and be applied,constantly minimizing calculated the and battery applied, charge minimizing time while the accountingbattery charge for batterytime while health. accounting The results for battery from [42 health.] show The charging results times from cut[42] in show half charging and a system times that cut isin 52%half moreand a esystemfficient that than isconstant 52% more current efficient charging. than constant When current the duty charging. ratio and When frequency the duty are ratio properly and frequency applied, pulseare properly charging applied, shows significantpulse charging improvements shows significant over constant improvements current-constant over constant voltage current charging,-constant with lifetimevoltage improvementscharging, with lifetime of up to improvements 100 cycles, which of up in to vehicles 100 cycles charged, which once in vehicles per week charged corresponds once per to approximatelyweek corresponds two calendar to approximately years of extended two calendar operation years [43]. of However, extended improper operation frequency [43]. However, and duty cycleimproper selection frequency can severely and duty degrade cycle the selection lifetime can of theseverely battery, degrade as was the the lifetime case in [ 43of ]the for battery, a duty ratio as w ofas 0.2, due to the large current pulses. Pulse charger technologies must be able to find the correct charging parameters to maximize battery life, or else they will not yield improvements to the traditional constant current-constant voltage charging tactic.

4.1.5. Negative Pulse Charging Negative Pulse Charging methods, originally developed to enhance the efficiency of charging converters for lead acid batteries but now extended to lithium ion batteries, imposes small discharges to the battery during the pulse charging rest period [34]. The negative impulse decreases stresses in the cell and helps minimize temperature rise of the cell [44]. Since the negative pulse pulls a small amount of energy from the battery, circuit configurations that recapture that energy have been devised [45]. By occasionally depolarizing the cell, high currents can continually be pumped into the battery, enabling a higher charge rate and lower charge time. This method helps the chemical reactions within the battery and can significantly improve the life of the battery [46,47].

4.2. Physical Implementations of Battery Charging Infrastructure Conductive charging using a plug-in cable has been mentioned so far and is by far the most developed form of EV charging, but there are other methods being developed that provide alternative ways of transferring energy to the EV Battery. The main areas that are being developed are highlighted in Sections 4.2.1–4.2.3, with a comparison between each method given at the end of the section. Static Energies 2019, 12, x FOR PEER REVIEW 11 of 26

the case in [43] for a duty ratio of 0.2, due to the large current pulses. Pulse charger technologies must be able to find the correct charging parameters to maximize battery life, or else they will not yield improvements to the traditional constant current-constant voltage charging tactic.

4.1.5. Negative Pulse Charging Negative Pulse Charging methods, originally developed to enhance the efficiency of charging converters for lead acid batteries but now extended to lithium ion batteries, imposes small discharges to the battery during the pulse charging rest period [34]. The negative impulse decreases stresses in the cell and helps minimize temperature rise of the cell [44]. Since the negative pulse pulls a small amount of energy from the battery, circuit configurations that recapture that energy have been devised [45]. By occasionally depolarizing the cell, high currents can continually be pumped into the battery, enabling a higher charge rate and lower charge time. This method helps the chemical reactions within the battery and can significantly improve the life of the battery [46,47].

4.2. Physical Implementations of Battery Charging Infrastructure Conductive charging using a plug-in cable has been mentioned so far and is by far the most Energiesdeveloped2019, form12, 1839 of EV charging, but there are other methods being developed that provide alternative11 of 26 ways of transferring energy to the EV Battery. The main areas that are being developed are highlighted in Section 4.2.1–4.2.3, with a comparison between each method given at the end of the charging is deﬁned as charging the EV while in a stationary position, while dynamic charging supplies section. Static charging is defined as charging the EV while in a stationary position, while dynamic energy to the battery while the EV is in motion. charging supplies energy to the battery while the EV is in motion. 4.2.1. Inductive Charging (Static and Dynamic) 4.2.1. Inductive Charging (Static and Dynamic) Inductive charging is a wireless solution that is being researched for EV charging, both for static (stationary)Inductive charging charging and is dynamic a wireless charging, solution while that is the being vehicle researched is driving. for AsEV cancharging be seen, both in Figure for static 10, by(stationary) driving ancharging EV over and a dynamic large charging charging coil,, while a coil the within vehicle the is EVdriving can. couple As can tobe theseen charging in Figure coil, 10, allowingby driving power an EV to beover transferred a large charging through thecoil, magnetic a coil within coupling the ofEV the can two couple coils. to This the method charging has coil, the advantageallowing power of simplicity to be transferred and convenience, through e.g.,the atmagnetic charging coupling stations of and the dynamically two coils. This during method driving, has safetythe advantage for the driver of simplicity (as they won’tand convenience have to electrically, e.g., at plug charging the vehicle stations into and the grid), dynamically and eliminates during thedriving need, safety for the for correct the driver charging (as plug.they won’t have to electrically plug the vehicle into the grid), and eliminates the need for the correct charging plug.

Figure 10. Wireless power transfer is accomplished through the magnetic coupling of two coilscoils [[4488].].

Many inductive power transfer systems are being researched and developed, as there are many issues to to address. address. In In [49 [4] a9] 50 a kW 50 inductivekW inductive fast-charging fast-charging system system was developed was developed at ~25 kHz at frequency. ~25 kHz Byfrequency. achieving By resonance achieving in coils resonance via power in coils electronics via power switching, electronics the losses switchin wereg, minimized the losses via were zero currentminimized switching via zero and current a control switching system and was a control implemented, system was allowing implemented the system, allowing to accommodate the system vehiclesto accommodate with diff erentvehicles parameters. with different A 100 parameters. kW wireless A power 100 kW transfer wireless (WPT) power system transfer for EVs(WPT) operating system atfor ~22 EVs kHz operating was constructed at ~22 kHz inwas [50 constructed], where a 5”in distance[50], where separating a 5” distance the coupled separating coils the was coupled used. Thecoils experimental was used. The results experimental yielded anresults efficiency yielded of 96%.an efficiency of 96%. The development of inductive charging system systemss for electric vehicles is still at the beginning of the market cycle. cycle. There There are are still still se severalveral challenges challenges,, as as follows follows:: (1) (1) The The power power frequency frequency spectrum spectrum for forresonant resonant inductive inductive energy energy transfer transfer may mayresult result in interference in interference risks with risks other with vehicle other vehicle systems; systems; (2) the (2)geometrical the geometrical dimensions dimensions of the of inductive the inductive charging charging system system are are another another problem problem,, as as it it ne needseds to to be integrated into an existing vehicle platform package; and (3) with high energy transfer power levels, the inductive charging system needs additional thermal management, which comes with extra cost and need for space [51]. Although driver safety has the potential of being higher with inductive charging compared to conductive charging, as previously mentioned, magnetic field emissions are of concern for the operators of EVs that use inductive charging technology, as excessive exposure can cause long term effects. Thankfully, the stray magnetic field drops off rapidly with increased distance from the charging coil and several designs have been developed to shield the driver and passenger from the potential risks [52,53]. Inductive charging has rapidly increased in popularity and research in the past few years. The inductive WPT technology has lagged in acceptance and adoption compared to conductive charging, due to the traditionally lower power transfer levels and associated radiation issues, although emissions are being addressed and, as indicated, power transfer levels are becoming competitive.

integrated into an existing vehicle platform package; and (3) with high energy transfer power levels, the inductive charging system needs additional thermal management, which comes with extra cost and need for space [51]. Although driver safety has the potential of being higher with inductive charging compared to conductive charging, as previously mentioned, magnetic field emissions are of concern for the operators of EVs that use inductive charging technology, as excessive exposure can cause long term effects. Thankfully, the stray magnetic field drops off rapidly with increased distance from the charging coil and several designs have been developed to shield the driver and passenger from the potential risks [52,53]. Inductive charging has rapidly increased in popularity and research in the past few years. The inductive WPT technology has lagged in acceptance and adoption compared to conductive charging, due to the traditionally lower power transfer levels and associated radiation issues, although emissions are being addressed and, as indicated, power transfer levels are becoming competitive.

4.2.2. Capacitive Charging (Static and Dynamic) Capacitive charging is another solution that is being developed for EV charging. By operating at high frequencies, capacitive coupling can be achieved between a roadway plate and a large plate Energiesonboard2019 the, 12 , 1839EV. This capacitive coupling can enable high frequency power to be transferred12 of 26 wirelessly. As shown in Figure 11 [54], a 13.56 MHz 12 cm air-gap prototype capacitive WPT coupling power electronics solution was developed, which can achieve power transfers with efficiencies electronics solution was developed, which can achieve power transfers with efficiencies greater than greater than 90%. The present system transfers 884 W at an efficiency of 91.3%, achieving a power 90%. The present system transfers 884 W at an efficiency of 91.3%, achieving a power transfer density transfer density of 29.5 kW/m2. Targets for a 12 cm air-gap include, 50 kW/m2 with >90% efficiency. of 29.5 kW/m2. Targets for a 12 cm air-gap include, 50 kW/m2 with >90% efficiency. This specific This specific design can be implemented in the roadway, transferring power to the vehicle during design can be implemented in the roadway, transferring power to the vehicle during motion (dynamic) motion (dynamic) which allows for a simpler EV design that won’t require excessive amounts of which allows for a simpler EV design that won’t require excessive amounts of onboard energy storage. onboard energy storage.

Figure 11. A 13.56 MHz capacitive coupling system for wireless power transfer [54]. Figure 11. A 13.56 MHz capacitive coupling system for wireless power transfer [54]. Compared to inductive charging, the recent capacitive charging technology still has a long way Compared to inductive charging, the recent capacitive charging technology still has a long way to mature and there are several limitations to overcome, including low power density, low efficiency, to mature and there are several limitations to overcome, including low power density, low efficiency, and strong field emissions. First, in long distance applications, the power density of a capacitive and strong field emissions. First, in long distance applications, the power density of a capacitive charging system is only ~2 kW/m2, which is smaller than the nearly 40 kW/m2 of the inductive charging charging system is only ~2 kW/m2, which is smaller than the nearly 40 kW/m2 of the inductive system [55]. This is because the coupling capacitance is usually in the pF range when the transfer charging system [55]. This is because the coupling capacitance is usually in the pF range when the distance is in the hundreds of mm range. The effective way to increase power density is to increase the transfer distance is in the hundreds of mm range. The effective way to increase power density is to plate voltage and the switching frequency [56]. Second, the capacitive charging system can realize a increase the plate voltage and the switching frequency [56]. Second, the capacitive charging system DC-DC efficiency of 91.6% for electric vehicle charging process [57], while a DC-DC efficiency of 97% can realize a DC-DC efficiency of 91.6% for electric vehicle charging process [57], while a DC-DC can be achieved in an inductive charging system [58]. Since the switching frequency is usually higher efficiency of 97% can be achieved in an inductive charging system [58]. Since the switching frequency than 1 MHz, the skin effect of the wire becomes obvious and extra losses can be induced. The power is usually higher than 1 MHz, the skin effect of the wire becomes obvious and extra losses can be losses can be more significant if the switching frequency increases even further. Third, the electric field induced. The power losses can be more significant if the switching frequency increases even further. emission in a capacitive charging system is difficult to shield, because the electric fields can easily pass Third, the electric field emission in a capacitive charging system is difficult to shield, because the through metal material. Since a capacitive charging system usually requires high voltages on the plates electric fields can easily pass through metal material. Since a capacitive charging system usually to transfer power, either the four-plate parallel or four-plate stacked capacitive coupler cannot reduce requires high voltages on the plates to transfer power, either the four-plate parallel or four-plate the electric field emission. Although the six-plate coupler can reduce these emissions, its application stacked capacitive coupler cannot reduce the electric field emission. Although the six-plate coupler area is limited by the complicated structure [57]. Capacitive charging requires a significant amount of can reduce these emissions, its application area is limited by the complicated structure [57]. development before it will be adopted. Capacitive charging requires a significant amount of development before it will be adopted. 4.2.3. Dynamic Conductive Charging 4.2.3. Dynamic Conductive Charging Dynamic conductive charging refers to supplying electric energy to electric vehicles through a conductor while moving. It is also known as Electric Road Systems (ERS). The use of dynamic conductive charging system offers a possibility to significantly reduce the need for batteries [59]. According to the cost comparison between ERS and alternative charging infrastructures, it has been proved that ERS is a powerful tool to reduce the total societal cost for electrification of road transport [60]. The ERS supply can be made from three possible directions, from the top, side, and the road surface. All these have benefits and drawbacks. Supply from the top corresponds to overhead wires. It cannot be reached by smaller vehicles, but does not interfere with the road structure itself. However, they pose a safety hazard on the occasion of a fallen power conductor. Siemens opened the world’s first eHighway in Sweden [61] and Scania contributes electrically-powered trucks [62]. They have already been in operation on a public road, as shown in Figure 12a. Supply from the side is primarily used in subway train supplies and cannot be used by more than the outmost lane in a multi-lane road. It also presents safety issues for the space between the vehicle and the road side [40]. Honda has developed a system for conductive supply from the side, currently designed for 180 kW at 156 km/h but is aiming for 450 kW of power transfer at a 200 km/h vehicle speed, see Figure 12b [63]. Most of the development of ERS focus on supply from the road surface. It can be used by almost all vehicles but interferes with the road construction. It is subject to the environment in the form of water, snow, ice, dirt, and debris and poses a touch safety hazard. In Sweden, the following three Energies 2019, 12, x FOR PEER REVIEW 13 of 26

Dynamic conductive charging refers to supplying electric energy to electric vehicles through a conductor while moving. It is also known as Electric Road Systems (ERS). The use of dynamic conductive charging system offers a possibility to significantly reduce the need for batteries [59]. According to the cost comparison between ERS and alternative charging infrastructures, it has been proved that ERS is a powerful tool to reduce the total societal cost for electrification of road transport [60]. The ERS supply can be made from three possible directions, from the top, side, and the road surface. All these have benefits and drawbacks. Supply from the top corresponds to overhead wires. It cannot be reached by smaller vehicles, but does not interfere with the road structure itself. However, they pose a safety hazard on the occasion of a fallen power conductor. Siemens opened the world’s first eHighway in Sweden [61] and Scania contributes electrically-powered trucks [62]. They have already been in operation on a public road, as shown in Figure 12a. Supply from the side is primarily used in subway train supplies and cannot be used by more than the outmost lane in a multi- lane road. It also presents safety issues for the space between the vehicle and the road side [40]. Honda has developed a system for conductive supply from the side, currently designed for 180 kW at 156 km/h but is aiming for 450 kW of power transfer at a 200 km/h vehicle speed, see Figure 12b [63]. Energies 2019, 12, 1839 13 of 26 Most of the development of ERS focus on supply from the road surface. It can be used by almost all vehicles but interferes with the road construction. It is subject to the environment in the form of water,diﬀerent snow, ground-based ice, dirt, and conductive debris and ERS pose solutionss a touch are beingsafety developed hazard. In and Sweden, demonstrated: the following Alstom three APS, differentElways, ground and Elonroad,-based conductive shown below ERS in Figuresolutions 12 c–eare [being64–66 ].developed The Italian and company demonstrated Ansaldo: Alstom also has APS,developed Elways their, and ERSElonroad product, shown TramWave, below in see Fig Figureure 12 12c–fe [ 67[6].4–66]. The Italian company Ansaldo also has developed their ERS product TramWave, see Figure 12f [67].

Figure 12. (a) A Scania truck using Siemens eHighway system; (b) Honda’s conductive ERS from the side; (c) The Alstom APS system for trucks; (d) the Elways ERS seen from the surface; (e) the Elonroad

system; (f) the Ansaldo TramWave system.

While ERS shows great potential by reducing onboard energy storage requirements of EVs, the main concern with this technology is the infrastructure that would have to be implemented to give drivers the ﬂexibility to drive wherever they choose, as they will be limited to the roads and highways which can supply them power. A potential solution is to ﬁnd a compromise between embedded roadway conductive charging pads and onboard energy storage, which would allow drivers to leave the roadways that contain ERS for a short period of time.

5. Battery Thermal Management As mentioned above, a significant amount of heat can be generated while a battery is charged (and, likewise, when it is discharged) and thermal management is critical for Li-ion batteries. Without sufficient cooling, the cells can become locally damaged by formation of hot spots and can significantly reduce battery life (e.g., due to fragmentation of electrode materials due to thermal expansion mismatch) and, if thermal runaway occurs [68], permanently destroy the cells. Energies 2019, 12, 1839 14 of 26

There are two main problems caused by temperature. The ﬁrst is that the high temperature during charge and discharge leads to the possibility that temperatures will exceed permissible levels and decrease the battery performance. As discussed above, throughout all battery operation, heat is produced from the ohmic resistance of the battery and the chemical processes within the battery and fast-charging produces even greater amounts of heat. For this reason, several battery thermal management systems (BTMS) have been developed [69–71], some of which incorporate SoC within the management algorithm. Ideally, Li-ion batteries operate between 15 ◦C and 40 ◦C for optimal battery performance [72–74]. Operation outside of these temperature bounds is detrimental for the health and longevity of the battery, although temperatures lower than 15 ◦C are acceptable as the discharge of the battery produces its own heat [75]. During extreme heating, the electrolyte between the electrodes can melt, leading to a reduction in power delivery and, in severe cases, internal short circuits that destroy the battery and even cause explosions. Figure 13 shows that the internal resistance of the battery is a function of the battery temperature [72]. Battery operation outside of the 15 ◦C–40 ◦C range leads to reduced eﬃciencies, thus battery cooling and warming systems are required for peak performance, with battery cooling being the main concern. Another problem is that the uneven temperature distribution in the battery pack will lead to a localized deterioration. Therefore, temperature uniformity, within a cell and from cell to cell, is important to achieve the maximum cycle life of the cell, module, and pack [76]. Tesla has recently revealed a new technology to address such issues, known as “on-route battery warmup”, which heats the battery to the ideal temperature for charging when the driver begins to route the vehicle towards the Supercharger charging station, which can aid in the reduction of charge timesEnergies by 2019 25, percent12, x FOR [ PEER77]. REVIEW 15 of 26

Figure 13. Battery internal resistance is a function of battery temperature. The internal resistance is Figure 13. Battery internal resistance is a function of battery temperature. The internal resistance is minimized between 15 C and 40 C, leading to smaller internal heating losses [72]. minimized between 15◦ °C and 40°C◦ , leading to smaller internal heating losses [72]. To optimize the performance of a battery and pack/module, the thermal energy management The thermal management system may be passive (i.e., only the ambient environment is used) or system should have the following features: (1) Optimum operating temperature range for every active (i.e., a built-in source provides heating and/or cooling) and can be divided into three categories cell and all battery modules, rejecting heat in hot climates/adding heat in cold climates; (2) small based on the medium. temperature variations within a cell and module; (3) small temperature variations among various modules; (4) compact and lightweight, easily packaged, reliable, low-cost and easy for service; and 5.1. Air Cooling (5) a provision for ventilation if the battery generates potentially hazardous gases [78]. It is well known that, underAir cooling normal allows power for/current simple loadsdesigns and with ambient the advantages operating of conditions, direct contact the with temperature the battery within cells, therelatively battery lower can be cost, easily easier controlled maintenance, to remain less in mass the reasonable, and no potential range. However,for leaks. Although stressful conditions,simple and suchrequiring as high-power minimal drawmaintenance, at high cell air/ambient flow on temperatures,the cells isn’t asthe well most as defectseffective in method individual for cells,removing may greatlyheat as increase the system local cannot heat generation. be easily sealed A BTMS from must the environment. prevent suchpropagation In addition, withoutit is difficult over-designing to maintain thethe cooling uniformity system of temperature and complicating within the a single control cell of theor between battery performancethe cells in the [79 battery]. pack because of the smallThe thermal heat capaci managementty of air. Another system may drawback be passive of air (i.e., cooling only is the its ambientlimitation environment by the blower’s is used) power or activeand size (i.e., [80 a built-in]. Figure source 14a shows provides the m heatingechanism and of/or air cooling)-cooling and system can be [76 divided]. into three categories based on the medium.

Figure 14. (a) Mechanism of air-cooling battery thermal management system [76] and (b) the air- cooling system onboard the Kia Soul EV [81].

An air-cooling system is currently used in the Nissan E-NV200 and Kia Soul EV. Cooling in these two EVs can be based on a simple system that uses fans and ducts to redirect cooler air from the air conditioning system through the battery pack when necessary. The E-VN200 system proves mostly adequate to provide an extra cooling boost, in addition to the passive heat transfer, and results in preventing excessive heat built up in the battery pack. The Kia Soul EV has a very similar cooling system, as illustrated in Figure 14b [81].

5.2. Liquid Cooling Liquid cooling is an alternative method that is used. With a larger heat capacity, liquid can offer more effective heat transfer in a smaller volume even if the coolant is not in direct contact with cells. Liquid systems do require more maintenance and components, which adds to the complexity and cost of the system [80]. Figure 15a shows the mechanism of the liquid cooling system [76].

Energies 2019, 12, x FOR PEER REVIEW 15 of 26

Figure 13. Battery internal resistance is a function of battery temperature. The internal resistance is minimized between 15°C and 40°C, leading to smaller internal heating losses [72].

The thermal management system may be passive (i.e., only the ambient environment is used) or Energiesactive2019 (i.e.,, 12 a, built 1839 -in source provides heating and/or cooling) and can be divided into three categories15 of 26 based on the medium. 5.1. Air Cooling 5.1. Air Cooling Air cooling allows for simple designs with the advantages of direct contact with the battery cells, Air cooling allows for simple designs with the advantages of direct contact with the battery cells, relatively lower cost, easier maintenance, less mass, and no potential for leaks. Although simple and relatively lower cost, easier maintenance, less mass, and no potential for leaks. Although simple and requiring minimal maintenance, air flow on the cells isn’t the most effective method for removing heat requiring minimal maintenance, air flow on the cells isn’t the most effective method for removing as the system cannot be easily sealed from the environment. In addition, it is difficult to maintain the heat as the system cannot be easily sealed from the environment. In addition, it is difficult to maintain uniformity of temperature within a single cell or between the cells in the battery pack because of the the uniformity of temperature within a single cell or between the cells in the battery pack because of small heat capacity of air. Another drawback of air cooling is its limitation by the blower’s power and the small heat capacity of air. Another drawback of air cooling is its limitation by the blower’s power size [80]. Figure 14a shows the mechanism of air-cooling system [76]. and size [80]. Figure 14a shows the mechanism of air-cooling system [76].

Figure 14. (a) Mechanism of air-cooling battery thermal management system [76] and (b) the air-cooling Figure 14. (a) Mechanism of air-cooling battery thermal management system [76] and (b) the air- system onboard the Kia Soul EV [81]. cooling system onboard the Kia Soul EV [81]. An air-cooling system is currently used in the Nissan E-NV200 and Kia Soul EV. Cooling in these two EVsAn canair-cooling be based system on a simple is currently system used that in uses the Nissan fans and E-NV200 ducts to and redirect Kia Soul cooler EV. airCooling from in the these air conditioningtwo EVs can systembe based through on a simple the battery system pack that when uses fans necessary. and ducts The to E-VN200 redirect system cooler provesair from mostly the air adequateconditioni tong provide system anthrough extra coolingthe battery boost, pack in when addition necessary. to the passive The E-VN200 heat transfer, system andproves results mostly in preventingadequate to excessive provide heatan extra built cooling up in the boost battery, in addition pack. The to Kiathe passive Soul EV heat has atransfer, very similar and results cooling in system,preventing as illustrated excessive in heat Figure built 14 upb [81 in]. the battery pack. The Kia Soul EV has a very similar cooling system, as illustrated in Figure 14b [81]. 5.2. Liquid Cooling 5.2. Liquid Cooling Liquid cooling is an alternative method that is used. With a larger heat capacity, liquid can offer more eLiquidffective cooling heat transfer is an alternative in a smaller method volume that even is used. if the With coolant a larger is not heat in directcapacity, contact liquid with can cells. offer Liquidmore effective systems doheat require transfer more in a maintenance smaller volume and components,even if the coolant which is adds not toin thedirect complexity contact with and cells. cost ofLiquid the system systems [80 ].do Figure require 15 amore shows maintenance the mechanism and ofcomponents the liquid, cooling which systemadds to [76the]. complexity and costTwo of the types system of cooling [80]. Fig fluidsure 1 can5a shows be used the in mechanism the liquid cooling of the liquid system, cooling as follows: system Water [76] and. glycol solution, in indirect liquid cooling, and dielectric mineral oil, in direct liquid cooling [82]. The former one offers ease of handling, much lower viscosity, and a higher heat capacity, which leads to ease in increasing the coolant flow rate by the pump power and effective achievement of cell temperature uniformity. However, due to the added thermal resistance between the coolant and cell surfaces in the indirect cooling system, such as the jacket wall and air gap, the effective heat transfer coefficient is significantly reduced. The mineral oil direct contact liquid cooling system provides a much higher heat transfer coefficient at the expense of high-pressure loss in the coolant channel, thus, it is preferred in highly transient heat generating battery systems. Liquid cooling systems using circulating fluid and a radiator are widely used in EVs from Tesla, BMW, GM, Hyundai, and Renault. Figure 15b illustrates the liquid battery cooling system used in Tesla’s Model S. Tesla snakes a flattened cooling tube through their cylindrical cells, resulting in a very simple cooling scheme with very few points for leakage. The GM Volt PHEV uses thin prismatic shaped cooling plates in between the cells, with the liquid coolant circulating through the plate, as shown in Figure 15c. The scheme is very effective from a cooling point of view, but it is complicated [83]. Energies 2019, 12, x FOR PEER REVIEW 16 of 26

Two types of cooling fluids can be used in the liquid cooling system, as follows: Water and glycol solution, in indirect liquid cooling, and dielectric mineral oil, in direct liquid cooling [82]. The former one offers ease of handling, much lower viscosity, and a higher heat capacity, which leads to ease in increasing the coolant flow rate by the pump power and effective achievement of cell temperature uniformity. However, due to the added thermal resistance between the coolant and cell surfaces in the indirect cooling system, such as the jacket wall and air gap, the effective heat transfer coefficient is significantly reduced. The mineral oil direct contact liquid cooling system provides a much higher Energies 2019, 12, 1839 16 of 26 heat transfer coefficient at the expense of high-pressure loss in the coolant channel, thus, it is preferred in highly transient heat generating battery systems.

Figure 15. (a) Mechanism of liquid cooling battery thermal management system [76]; (b) the scheme of Figurethe liquid 15. (a cooling) Mechanism system of onboard liquid cooling Tesla’s Modelbattery S thermal [83]; (c) management the liquid coolant system circulating [76]; (b) platethe scheme used in ofthe the GM liquid Volt cooling PHEV system [83]. onboard Tesla’s Model S [83]; (c) the liquid coolant circulating plate used in the GM Volt PHEV [83]. 5.3. Phase Change Materials Liquid cooling systems using circulating fluid and a radiator are widely used in EVs from Tesla, Phase change composite (PCC) is a patented wax-graphite material that has been successfully BMW, GM, Hyundai, and Renault. Figure 15b illustrates the liquid battery cooling system used in used as a stand-alone thermal management system for a variety of Li-ion batteries, as shown below Tesla’s Model S. Tesla snakes a flattened cooling tube through their cylindrical cells, resulting in a in Figure 16a [76] and 16b [84]. As a solid, the material can absorb heat produced by the cell and very simple cooling scheme with very few points for leakage. The GM Volt PHEV uses thin prismatic undergoes a phase change into a liquid state, effectively removing heat from the cell and limiting the shaped cooling plates in between the cells, with the liquid coolant circulating through the plate, as temperature rise of the battery, as illustrated in Figure 16c [84]. Upon cessation of use, the PCC material shown in Figure 15c. The scheme is very effective from a cooling point of view, but it is complicated can reject the stored heat to the environment, return through phase change to ambient temperature, [83]. and be ready to use again [85]. The heat stored in the PCC material can also release the heat back to the battery cell when the temperature decreases during the cold winter, keeping the cells warmer than the 5.3. Phase Change Materials case without the PCC in place [76]. This technology is able to handle the cells on an individual level andPhase can prevent change a composite domino eff (PCC)ect from is happeninga patented ifwax one-graphite cell were material to overheat that [has80]. been successfully used asPCCs a stand stand-alone out thermal to be a practical, management passive, system inexpensive for a variety thermal of Li- managemention batteries, solution.as shown Itsbelow light inweight, Figure absence16a [76] of and moving 16b [8 parts,4]. As and a solid, free-maintenance the material make can absorb it an outstanding heat produced thermal by the management cell and undergoessolution. a Moreover, phase change the cellsinto withina liquid the state, PCC effectively based battery removing pack heat are from fully the in contactcell and and limiting enclosed, the temperaturewhich maximizes rise of the the heat battery transfer, as illustrated out of the cells, in Fig whichure 16 isc impossible[84]. Upon with cessation indirect of use, liquid the cooling. PCC materialThe higher can thermal reject the conductivity stored heat of to graphite the environment, in the PCC return can guarantee through phaseuniformity change of temperatureto ambient temperature,within the pack and andbe ready its higher to use heat again capacity [85]. The allows heat the stored absorption in the PCC of the material heat from can thealso cells release without the significantly increasing the temperature of the PCC coolant. The maximum temperature limit can be flexible by choosing the suitable wax with the desired phase change temperature [80]. EnergiesEnergies 2019 2019, 12, 12, x, FORx FOR PEER PEER REVIEW REVIEW 1717 of of26 26 heatheat back back to to the the battery battery cell cell when when the the temperature temperature decreases decreases during during the the cold cold winter, winter, keeping keeping the the cells cells warmerwarmer than than the the case case without without the the PCC PCC in in place place [7 [67]6. ]This. This technology technology is isable able to to handle handle the the cells cells on on an an individualindividual level level and and can can prevent prevent a dominoa domino effect effect from from happening happening if ifone one cell cell were were to to overheat overheat [80 [80]. ] . PCCsPCCs stand stand out out to to be be a apractical, practical, passive, passive, inexpensive inexpensive thermal thermal management management solution. solution. Its Its light light weight,weight, absence absence of of moving moving parts parts, and, and free free-maintenance-maintenance make make it itan an outstanding outstanding thermal thermal management management solution.solution. Moreover, Moreover, the the cells cells within within the the PCC PCC based based battery battery pack pack are are fully fully in in contact contact and and enclosed, enclosed, whichwhich maximizes maximizes the the heat heat transfer transfer out out of of the the cells, cells, which which is isimpossi impossibleble with with indirect indirect liquid liquid cooling. cooling. TheThe higher higher thermal thermal conductivity conductivity of of graphite graphite in in the the PCC PCC can can guarantee guarantee uniformity uniformity of of temperature temperature withinwithin the the pack pack and and its its higher higher heat heat capacity capacity allows allows the the absorption absorption of of the the heat heat from from the the cells cells without without significantlysignificantly increasing increasing the the temperature temperature of of the the PCC PCC coolant. coolant. The The maximum maximum temperature temperature limit limit can can be be Energies 2019, 12, 1839 17 of 26 flexibleflexible by by choosing choosing the the suitable suitable wax wax with with the the desired desired phase phase change change temperature temperature [80 [80]. ].

FigureFigureFigure 16. 16.16. (a( )(a a)Mechanism) Mechanism Mechanism and and and [7 [ 766[7]]( 6(]bb ())b Photo Photo) Photo of of Phase Phase changechange change compositecomposite composite (PCC)(PCC) (PCC) cooling cooling cooling system system system [84 [8 ];4[8] (;4c ]); (cprinciples)( cprinciples) principles of of how ofhow how PCC PCC PCC cooling cooling cooling system system system works works works [84 [8]. 4[8].4 ]. 6. Fast-Charging Power Electronics 6. 6.Fast Fast-Charging-Charging Power Power Electronics Electronics In general, fast-charging power electronics consists of 3 stages [86], as follows: An input filter for InIn general, general, fast fast-charging-charging power power electronics electronics consist consists ofs of 3 3stages stages [8 [86]6, ]as, as follows follows: A: nA ninput input filter filter the reduction of input harmonics, which also contributes to power factor optimization, an AC-DC forfor the the reduction reduction of of input input harmonics harmonics, which, which also also contributes contributes to to power power factor factor optimization, optimization, an an AC AC-DC-DC rectifier, and a DC-DC converter that transfers power to the battery, as depicted in Figure 17 for DC rectifierrectifier, and, and a DCa DC-DC-DC converter converter that that transfers transfers power power to to the the battery, battery, as as depicted depicted in in Fig Figureure 17 1 for7 for DC DC fast-charging of a plug-in hybrid electric vehicle (PHEV). For AC charging, the AC-DC rectifier and fastfast-charging-charging of of a pluga plug-in-in hybrid hybrid electric electric vehicle vehicle (PHEV). (PHEV). For For AC AC charging, charging, the the AC AC-DC-DC rectifier rectifier and and DC-DC converter are part of the onboard charger, which also illustrates an advantage of DC charging. DCDC-DC-DC converter converter are are part part of of the the onboard onboard charger, charger, which which also also illustrates illustrates an an advantage advantage of of DC DC charging. charging. The size of the onboard charging device is constrained by the space inside the vehicle. As the onboard TheThe size size of of the the onboard onboard charging charging device device is isconstrained constrained by by the the space space inside inside the the vehicle. vehicle. As As the the onboard onboard converter is small, the amount of power that it is able to deliver to the battery is typically low (3–6 kW). converterconverter is issmall, small, the the amount amount of of power power that that it itis isable able to to deliver deliver to to the the battery battery is istypically typically low low (3 (3–6– 6 In contrast, the DC charger is external to the vehicle and therefore not constrained in size or cost. In kW).kW). In In contrast, contrast, the the DC DC charger charger is isexternal external to to the the vehicle vehicle and and therefore therefore not not constrained constrained in in size size or or addition, DC fast chargers can interface with 3-phase power and enable adjustment of the charge level cost.cost. In In addition, addition, DC DC fast fast chargers chargers can can interface interface with with 3 -3phase-phase power power and and enable enable adjust adjustmentment of of the the to suit the battery state [87]. chargecharge level level to to suit suit the the battery battery state state [8 [87]7. ] .

FigureFigureFigure 17. 17. 17. DDC CD C fast fast-charging fast-charging-charging power power power electronics electronics electronics modules, modules modules from, from, from the the utility the utility utility grid grid to grid a plug-in to toa aplug plug hybrid-inin hybrid electric hybrid electricvehicleelectric vehicle (PHEV)vehicle (PHEV) battery(PHEV) battery [ 86battery]. [8 6[8].6 ].

TheTheThe EV EVEV charging chargingcharging station station station appears appears appears as as as a a DCa DC DC load load to to the the electric electric distributiondistribution distribution systemsystem system and and and the the the nature nature nature of oftheof the the AC-DC AC AC-DC-DC conversion conversion conversion process process process by by rectification by rectification rectification can can present can present present unwanted unwanted unwanted current current current harmonics harmonics harmonics on the on on grid’s the the distribution system, leading to negative effects on grid power quality. The input filter’s purpose is to filter unwanted harmonics and, in doing so, maintain a power factor for the distribution system that is near unity. Equation (6) gives the total harmonic distortion (THD) as a function of the harmonic input currents (Ih) over the fundamental component (Is1) of the total input current (Is). Equation (7) gives the displacement power factor (DPF), and Equation (8) gives the true power factor (PF) as a function of DPF and THD, emphasizing that true PF is a function of both the DPF and the THD.

s P ∞ I2 h h=2 (6) %THD = 100x Is1 Energies 2019, 12, x FOR PEER REVIEW 18 of 26 grid’s distribution system, leading to negative effects on grid power quality. The input filter’s purpose is to filter unwanted harmonics and, in doing so, maintain a power factor for the distribution system that is near unity. Equation (6) gives the total harmonic distortion (THD) as a function of the harmonic input currents (Ih) over the fundamental component (Is1) of the total input current (Is). Equation (7) gives the displacement power factor (DPF), and Equation (8) gives the true power factor (PF) as a function of DPF and THD, emphasizing that true PF is a function of both the DPF and the THD.

 2  Ih %THDx 100  h2 Energies 2019, 12, 1839 18 of(6 26) Is1

where Ih areare the harmonic input currents and Is1 isis the the fundamental fundamental component component of of the the total total input input currentcurrent IIss.. DPF = cos ϕ1 (7)

D P F c o s 1 (7) where φ1 is the phase shift between the fundamental input current and the input voltage. where ϕ1 is the phase shift between the fundamental input current and the input voltage. Is1 DPF PF = I (DPF) = DPF (8) PFDPF () Is s1 √1 + THD2 2 (8) Is 1THD Examples of chargers with power factor correction filters are detailed in [88,89]. The power factor correctionExamples of the of charging chargers systemwith power can be factor handled correction at the input filters filter are detailed and during in [8 the8] and AC-DC [89]. rectification, The power factore.g., using correction a synchronous of the charging/active rectifier. system can be handled at the input filter and during the AC-DC rectificationThe AC-DC, e.g., conversionusing a synchronous/active stage takes the grid rectifier voltage. and rectifies it to a DC bus. Researchers have beenT movinghe AC-DC towards conversion a generic stage DC takes bus the for grid EV voltage charging and stations, rectifies in it whichto a DC each bus. charger Researchers will drawhave beenpower moving from the towards same DCa generic bus. Alternatively, DC bus for EV a common charging AC stations, bus can in be which drawn each from charger for each will charger, draw powerthough from each the charger same willDC needbus. Alternatively, its own AC-DC a common rectifier stage.AC bus Additionally, can be drawn integration from for each of renewable charger, thoughsources each and localcharger energy will need storage its becomesown AC- simplerDC rectifier with stage. a shared Additionally DC bus, another, integration reason of arenewable common sourcesDC bus and is desired. local energy DC bus storage voltages becomes are usually simpler ~750 with V [ 86a shared]. DC bus, another reason a common DC busThe is DC-DC desired. converter DC bus volta stageges is are where usually the voltage ~750 V of[86 the]. DC bus is converted into the necessary chargingThe DC voltage-DC converter and current stage parameters, is where the as definedvoltage byof the DC EV battery.bus is converted This DC-DC into converterthe necessary will chargingalso implement voltage aand dynamic current control parameters method, as fordefined fast-charging by the EV applications, battery. This to DC minimize-DC converter the eff willects alsoof temperature implement increasea dynamic and control battery meth polarization.od for fast- Thischarging is also applications, where the pulseto minimize generator the wouldeffects beof temperatureimplemented increase for pulse and and negativebattery polarization. pulse fast-charging. This is This also converter where the needs pulse to be generator robust and would flexible be implementedin design, with fo ther pulse ability and to apply negative various pulse power fast-charging. levels to the This battery. converter Isolation needs is toalso be an robust important and flexidesignble aspectin design, of the with DC-DC the ability stage, asto theapply vehicle various owner power will levels need to attachthe battery. the charging Isolation port is also to their an importantvehicle without design being aspect exposed of the toDC the-DC DC stage, bus voltage. as the vehicle Many designsowner will contain need an to isolation attach the transformer charging portto improve to their drivervehicle safety without at the being charging exposed station to the [86 DC], as bus can voltage be seen. Many in Figure designs 18. contain an isolation transformer to improve driver safety at the charging station [86], as can be seen in Figure 18.

Figure 18. A full-bridge DC-DC converter topology with isolation [86]. Figure 18. A full-bridge DC-DC converter topology with isolation [86]. Bidirectional power electronic systems are also being developed for vehicle to grid (V2G) applicationsBidirectional [90]. power By replacing electronic the secondary systems are of thealso circuit’s being developed diodes with for switches, vehicle ato bidirectional grid (V2G) applicationsconverter can [90 be] realized. By replacing [91]. With the numeroussecondary switches, of the circuit’s switching diodes losses with become switches, an area a ofbidirectional concern for converterconverter ecanfficiency. be realized By employing [91]. With zero numerous voltage switches, switching switching (ZVS) and losses zero current become switching an area of (ZCS) concern [92] forstrategies, converter high efficiency. electrical By effi employingciencies can zero be realized.voltage switching By switching (ZVS) during and zero a zero current voltage switching or zero current (ZCS) state, switching losses are nearly eliminated. An advanced EV charger topology proposed in [93], and shown below in Figure 19, employs a direct AC to DC rectifier using a high frequency solid state transformer. This approach has high power density and efficiency, avoids the use of a DC link capacitor, provides robust control, and high-power quality. Energies 2019, 12, x FOR PEER REVIEW 19 of 26

[92] strategies, high electrical efficiencies can be realized. By switching during a zero voltage or zero current state, switching losses are nearly eliminated. An advanced EV charger topology proposed in [93], and shown below in Figure 19, employs a direct AC to DC rectifier using a high frequency solid state transformer. This approach has high Energiespower2019 density, 12, 1839 and efficiency, avoids the use of a DC link capacitor, provides robust control,19 ofand 26 high-power quality.

Figure 19. An EV direct switch-mode AC to DC rectifier with high frequency isolation [93]. Figure 19. An EV direct switch-mode AC to DC rectifier with high frequency isolation [93]. 7. New Areas of Research 7. New Areas of Research There are various areas in need of investigation and further development for EV fast-charging technologies.There are various areas in need of investigation and further development for EV fast-charging technologies. 7.1. Power Electronic Converters 7.1. PowerAlthough Electronic much Converters research has gone into effective unidirectional and bidirectional charging converters,Although advancements much research of existing has gone topologies into effective and work unidirectional to create new and topologies bidirectional are still charging needed. Thisconverters, will help advancement drive the communitys of existing towardstopologies a standardand work fast-charging to create new convertertopologies that are canstill becomeneeded. universal.This will help The drive ideal the power community electronic towards converter a standard will safely fast-charging charge an converter EV battery that in can 5–10 become min, haveuniversal. isolation The for ideal driver power safety, electronic allow forconverter bidirectional will safely power charge flow an (V2G), EV battery implement in 5– soft10 minutes, switching have to minimizeisolation for switching driver losses,safety, and allow achieve for bidirectional resonance for power maximum flow (V2G), efficiency. implement soft switching to minimizeAs mentioned switching in losses, Section and 4.1.4 achieve., pulse resonance charging for EV maximum batteries efficiency. is a promising new fast-charging method.As mentioned To implement in Section high-power 4.1.4., pulse pulse charging, charging advanced EV batteries converters is a promising need to benew developed fast-charging and optimized.method. To Controlimplement systems high- willpower also pulse need charging, to be developed advanced for converters pulse charging, need to as be they developed can actively and monitoroptimized battery. Control health systems and optimizewill also need the charging to be developed process. for Large pulse current charging, pulses as they will alsocan actively have a significantmonitor battery impact health on the and DC bus optimize and electric the charging grid operation, process. thus, Large developing current pulses filters and will power also have factor a correctionsignificant topologiesimpact on the will DC be bus of vitaland electric importance. grid operation, There is alsothus, thedeveloping issue of filters various and batteries power factor from dicorrectionfferent automotive topologies manufactures. will be of vital A fast-chargingimportance. systemThere is will also need the toissue be smart of various enough batte to recognizeries from thedifferent different automotive makes and manufactures. models of vehicles, A fast-charging adjusting system the charging will need parameters to be smart accordingly. enough to recognize the differentAs wide makes bandgap and (WBG) models devices of vehicles, are continually adjusting madethe chargi available,ng parameters they will needaccordingly. to be integrated into fast-chargersAs wide bandgap for compact (WBG) devices footprints are andcontinually high-power made density available, converters. they will need While to these be integrated devices willinto helpfast-chargers improve for electrical compact effi footprintsciencies, theyand high must-power be implemented density converters. through While a holistic these research devices andwill developmenthelp improve process electrical so that efficiencies, issues that they arise must may be be addressed. implemented Fast through inductive a and holistic dynamic research charging and technologiesdevelopment have process been so advancingthat issues inthat recent arise yearsmay be due addressed. to safety Fast and inductive compatibility and dynamic advantages. charging Still, fasttechnologies inductive ha andve dynamicbeen advancing charging in power recent electronicyears due converters to safety and need compatibility to be advanced advantages. further in Still, the research,fast inductive to catch and updynamic with conductive charging power fast-charging. electronic Asconverters with plug-in need fast-chargers,to be advanced inductive further in and the dynamicresearch,charging to catch converters up with conductive will need to fast be- echarging.fficient, flexible, As with and plug robust-in fas int design-chargers, for various inductive makes and anddynamic models charging of vehicles. converters will need to be efficient, flexible, and robust in design for various makes and models of vehicles. 7.2. Battery System Modeling 7.2. BatteryBattery System technologies Modeling are currently a hot area of research and their continued improvements will significantlyBattery technologies contribute are tocurrently increased a hot EV area adoption. of research New and battery their conti technologiesnued improvements are continually will introducedsignificantly to the contribute market and to e increasedffective electrical EV adoption. charging Newmodels battery are vital technologies for fast-charging. are continually Developed modelsintroduced will allow to the further market characterization and effective of electrical battery charging charging parameters, models enablingare vital the for quickest fast-charging. charge whileDeveloped balancing models battery will health.allow further It is commonly characteri knownzation thatof battery a complete charging drain parameters, and complete enabling charge the is detrimentalquickest charge to battery while health. balancing Work battery needs health. to be performed It is commonly to find theknown optimal that combinationa complete drain of battery and dissipationcomplete charge and charging; is detrimental especially to asbattery newer health. Li-ion Work battery needs technologies to be performed are released. to find While the this optimal paper focusedcombination on battery of battery charging, dissipation research and on charging; advanced especially battery swapping as newer technologies Li-ion battery is presented technologies in [ 94are]. 7.3. Impact on the Grid and Local Energy Storage A particularly important issue is the impact on the grid that EV charging can have [95], which is magnified by the introduction of fast charging technologies [96–98]. Approaches to the intelligent scheduling and aggregation of the new demand through the use of agents have been examined [99–102], Energies 2019, 12, 1839 20 of 26 as have novel ways to engineer the electrical distribution grid and its use, such as through locational marginal pricing [103]. With the introduction of improved fast-charging technologies EVs will be charging at rates of >100 kW. For a 10-car charging station, the local distribution system can easily change from no load to 1 MW in a matter of minutes, giving system operators little time to react. Work is being done to address this issue through local energy storage [104], though there is still much work to be done. The questions of the correct balance that allows minimizing the peak load to average load ratio and how much storage is necessary to supplement the highly variant EV charging station load need to be answered, in addition to how localized renewables play a part in the supplementation [105]. It should be further noted that this work has focused on the grid-to-vehicle aspects of fast charging, though through appropriate design, EVs can also behave as a distributed energy and power resource. In this way, EVs can act as local energy storage for areas that have higher power demand during certain times of the day or year. This concept, where EVs can supply energy to the grid is known as vehicle-to-grid (V2G) [106]. It is expected that the ancillary services, such as voltage and frequency control and spinning reserve provided by the V2G concept, can improve the efficiency, stability, and reliability of the electrical grid by offering reactive power support, active power regulation, and supporting the continued introduction of variable renewable energy sources. EV energy storage support for the grid can curve the effects of variable renewable resources, such as wind and solar. During low generation times where there is cloud cover or a lack of wind, EVs that are not in use can be used to support the grid. While V2G can accelerate battery degradation if not properly implemented, through proper application it may hold economic value for vehicle owners and grid operators [107]. An additional advantage is the concept of vehicle-to-home (V2H), in which the EV battery can become a source of energy to the driver’s home during times of power outages, for example during a natural disaster. This capability would enable the creation of microgrids for electricity consumers, which enables them to have less dependency on the electric grid for short periods of time. Finally, vehicle-to-vehicle technologies (V2V) have the capability to transfer energy between vehicles. All of these vehicle-to-x (V2X) technologies have the goal of increasing energy sustainability, grid reliability, grid efficiency, and reducing emissions by communicating with each through a common controller, usually called an aggregator [108].

7.4. Systems Level Research As can be seen from this paper, many technologies have already been developed to address fast-charging issues. A systems level approach combining new fast-charging technologies is required. For example, pulse charging could be used in tandem with a cooling system to analyze the additional charging improvements. Existing converter designs need to be equipped with control strategies that evaluate SoC, battery temperature, and health. Integrating the pieces together through a systems level approach will help fast-charging become a reality. For this reason, it is important that charging improvements are designed in a modular way, such that a systems level approach can work with multiple modules.

8. Conclusions This paper presented a review of fast-charging technologies, explained the issues associated with fast-charging, including impacts on the battery systems regarding heat management and limitations, presented solutions, and proposed new areas of research for future solutions. Petroleum based transportation will not sustain the globe in the decades ahead, thus, advanced technologies are needed that will move the world towards sustainable electrified transportation. Slow EV battery charging times is one of the significant concerns with EVs. Widespread consumer acceptance requires that recharge times are comparable with the time it takes to fill up an ICEV’s gasoline tank. This paper emphasized that fast and safe charging systems that do not damage the batteries, and do not negatively affect the electric grid must be developed. With continued charging research, technologies will be Energies 2019, 12, 1839 21 of 26 refined and standards will be developed, which will encourage the increased adoption of EVs, leading to a more sustainable energy future.

Author Contributions: Conceptualization, R.C., A.v.J.; methodology, R.C., A.v.J., and A.Y.; validation, R.C., A.v.J., P.E., and Y.M.; writing—original draft preparation, R.C.; writing—review and editing, R.C., A.v.J., and Y.M.. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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