Battery-Supercapacitor Hybrid System for High-Rate Pulsed Load Applications

Donghwa Shin, Younghyun Kim, Yanzhi Wang and Massoud Pedram Jaeam Seo, and Naehyuck Chang University of Southern California Seoul National University {yanzhiwa, pedram}@usc.edu {dhshin, yhkim, jaseo, naehyuck}@elpl.snu.ac.kr

Abstract—Modern batteries (e.g., Li-ion batteries) provide batteries. For portable applications where the size is a con- high discharge efficiency, but the rate capacity effect in these straint and cost is a factor, size of the supercapacitor should batteries drastically decreases the discharge efficiency as the load be minimized while achieving a reasonable energy efficiency. current increases. Electric double layer capacitors, or simply supercapacitors, have extremely low internal resistance, and a Another concern is the terminal voltage variation coming battery-supercapacitor hybrid may mitigate the rate capacity from the characteristics of a capacitor in the sense that the effect for high pulsed discharging current. However, a hybrid terminal voltage is linearly proportional to the state of charge architecture comprising a simple parallel connection does not of the supercapacitor. The terminal voltage increases or de- perform well when the supercapacitor capacity is small, which creases dynamically as the supercapacitor is charged or dis- is a typical situation because of the low energy density and high cost of supercapacitors. charged. The variation of the supercapacitor terminal voltage This paper presents a new battery-supercapacitor hybrid is much higher than that of ordinary batteries. As a result, the system that employs a constant-current charger. The constant- efficiency of power converters, which are connected to a su- current charger isolates the battery from supercapacitor to percapacitor varies significantly by the difference in their in- improve the end-to-end efficiency for energy from the battery to put and output voltage levels. the load while accounting for the rate capacity effect of Li-ion batteries and the conversion efficiencies of the converters. We must simultaneously consider the energy efficiency and energy density to optimize the battery-supercapacitor hybrid I.INTRODUCTION for portable applications. More precisely, we propose a new Rate capacity effect in batteries significantly degrades battery-supercapacitor hybrid energy storage system that em- their discharge efficiency under high load currents. Electronic ploys a constant-current charger isolating the battery from su- systems commonly exhibit large fluctuation in load current, percapacitor to maximize the deliverable energy density i.e., which defy the maximum discharge capacity of batteries. the end-to-end energy delivery per unit of volume of energy Typical electronic systems determine the battery size based storage elements, while accounting for the rate capacity ef- on their expected average power consumption, and thus a fect of Li-ion batteries and the conversion efficiencies of the large pulsed discharging current with peak greatly exceeding charger and the regulator. the average value can significantly shorten the battery service II.RELATED WORK life in a charge-discharge cycle. We need to reduce the peak current draw in battery pow- Supercapacitors are widely used for energy storage in ered electronics with the highly fluctuated load. Electric dou- various applications. Specifically, supercapacitors are gaining ble layer capacitors, more commonly known as supercapaci- more attention as energy storage elements for renewable tors, are widely exploited to mitigate such load current fluctu- energy sources which tend to have a high charge-discharge ations in the batteries. They have a superior cycle efficiency, cycle frequency, and demand high cycle efficiency and good which is defined as the ratio of the energy output to energy depth-of-discharge (DOD) properties [1]. There are several input, which reaches almost 100%, and so they are suitable related battery-supercapacitor hybrid architectures in the lit- for energy storage with frequent charge-discharge cycles. erature on hybrid electric vehicles (HEVs). A bidirectional Generally, the larger the supercapacitor is, the higher the converter-based approach is introduced for the regenerative energy efficiency will be. However, supercapacitors have a brake-equipped HEVs [2]. A DC bus-based architecture for significant disadvantage in terms of their volumetric energy the battery-supercapacitor hybrid system is described in [3]. density and cost per unit of stored energy compared to the However, it is difficult to directly apply these architectures to portable applications because they are designed for the HEV This work is supported by the Brain Korea 21 Project, IC De- which involves high-power operation. In contrast, one must sign Education Center (IDEC), and Mid-career Researcher Program address many other factors such as size, weight, cost, and through NRF grant funded by the MEST (No. 2010-0017680). The circuit complexity in portable battery-powered systems. ICT at Seoul National University provides research facilities. N. Chang is the corresponding author. A supercapacitor in parallel with a Li-ion battery forms a Copyright notice: 978-3-9810801-7-9/DATE11/ c 2011 EDAA. hybrid energy storage that supports a higher rate of discharg- v v 10 b o Regulator ib io is 88 1C 2C 6C 4C 66 5.3 min 14.6 min 33.2 min 76.3 min

Battery voltage (V) 4.3 kJ 8.2 kJ 10.0 kJ 10.7 kJ

Battery voltage (V) Fig. 2. Parallel connection battery-supercapacitor hybrid systems. 44 10 2020 30 40 50 60 70 Constant-current DischargeDischarge time (min) operation

= Preg ichg v ih (a) Discharging at a constant current of 1C, 2C, 4C, and vb s vo 6C. Charger Regulator ib is io 10 ηreg Pchg ηchg Pulsed current Constant currrent Cs Rload 88

66 Fig. 3. Battery-supercapacitor hybrid system using a constant-current charger. 269 sec 315 sec 4.3 kJ

Battery voltage (V) 3.5 kJ Battery voltage (V) 44 as a low pass filter that prunes out rapid voltage changes. 1 2 3 44 55 The battery-supercapacitor hybrid is thus effective in reducing DischarDischargege time (min) voltage variation. The supercapacitor shaves the short duration, (b) Discharging at a 6C constant current and 12C pulsed high amplitude load spikes and makes a wider duration but current of a 20 s period and a 50% duty cycle. lower amplitude which would result in better energy efficiency Fig. 1. Discharging a 350 mAh 2-cell Li-ion battery with (a) different constant current and (b) pulsed and constant currents. due to lower rate capacity effect in the batteries. The filtering effect of the supercapacitor is largely depen- ing current thanks to the high power density of the superca- dent on its capacitance un the parallel connection architecture. pacitor [4], and thus reduces the impact of the rate capacity A larger capacitance results in better filtering effect. As a effect. Under pulsed load conditions, the supercapacitor acts result, the parallel connection has a limited ability to reduce as a filter that relieves peak stresses on the battery. This type the rate capacity effect in the Li-ion battery when the capac- of parallel battery-supercapacitor connection storage has been itance value of the supercapacitor is not sufficiently large. characterized and evaluated with pulsed load current and com- Unfortunately, due to the volumetric energy density and cost pared to the battery-alone systems in [5]. A simplified model, constraints in its practical deployment, the supercapacitor which helps theoretical analysis in terms of performance en- capacitance is generally rather small. hancement is provided in [6]. Duty ratio, capacitor configura- C. Constant-Current Architecture tion and pulse frequency play important roles in performance optimization of such a hybrid storage [7]. We introduce a new hybrid architecture using a constant- current charger to overcome the disadvantage of the con- III.BATTERY-SUPERCAPACITORHYBRIDSYSTEM ventional parallel connection hybrid architecture as shown A. Rate Capacity Effect in Fig. 3. The constant current charger separates the battery and the supercapacitor. It maintains a desired amount of Fig. 1(a) shows the voltage drop and total amount of de- the charging current regardless of the state of charge of the livered energy from the battery with a constant discharging supercapacitor whereas, in the conventional parallel connec- current of 1C, 2C, 4C, and 6C, when using 2-cell series Li- tion architecture, the charging current is not controllable and ion GP1051L35 cells [8]. The discharge efficiency (defined as varies greatly as a function of the state of charge of the su- the ratio of energy delivered from the battery to the load to percapacitor. Consequently, the proposed hybrid architecture the nominal energy storage in that battery) at 6C load current can reduces variation in the battery discharging current even is merely 40.2% of the 1C discharge efficiency. In practice, with a small supercapacitor. intermittent large amount of discharging current is often ap- There are several problems that must be addressed in order plied to batteries due to significant load current fluctuation of to develop a constant-current architecture. In the next sections, a typical battery-powered electronics circuit or systems. Fur- we will find a practical way to use a constant-current charger thermore, as presented in Fig. 1(b), drawing a pulsed current for the battery-supercapacitor hybrid and then optimize the of 12C with a 50% duty cycle, which is 6C on average, results operating conditions of the proposed system. in only 81.3% delivered energy and a shorter service life compared to drawing a constant current of 6C. In this paper, we IV. CONSTANT-CURRENT ARCHITECTURE DESIGN target the high-rate pulsed load applications which seriously The amount of delivered power from the battery to the load reduce the battery service life due to the rate capacity effect. will depend on the terminal voltage of the supercapacitor with fixed supercapacitor charging current. Therefore, we need to B. Parallel Connection Architecture charge the supercapacitor up to certain voltage at the initial A battery-supercapacitor hybrid shown in Fig. 2 is a simple state to deliver enough power to the supercapacitor. The super- way of reducing the effect of load fluctuation on the supplied capacitor will be precharged up to a certain voltage level be- voltage level. The supercapacitor connected in parallel acts fore supplying power to the load as illustrated in interval a of Racc Ls R1 io ⓐ ⓑ ⓒ ipeak vs Cacc C1 iavg t vs ton toff Fig. 5. Simplified supercapacitor equivalent circuit model. vss vSOC Rs Rts Rtl

t R C vOC C C V tprechg ti ti+1 tcuto f f tend sd b ts tl b ib ib Fig. 4. Precharge and steady-state operation of supercapacitor.

Fig. 4. We use part of the pre-charged energy in the superca- Fig. 6. Li-ion battery equivalent circuit model. pacitor after the battery cut-off time, tcuto f f (which denotes the the battery to the load in the hybrid system is given by time after which the battery’s remaining capacity fall below Z tend Edeliver 1 20%). However during intervals a and c in Fig. 4, the vari- ηsystem = = iovodt, (3) ation in input and output voltage difference affect converter Estored Estored 0 efficiency. We consider this amount of energy to calculate the where Estored, and Edeliver denote the stored energy in the bat- end-to-end energy efficiency. The amount of reusable energy tery, delivered energy to the load, respectively. which is available for the load from the pre-charged energy in The energy density of the system may be calculated as the supercapacitor is given by Eb + Es ρhybrid = , (4) Z tend Hb + Hs Ereusable = iovodt. (1) where Eb, Es, Hb and Hs are the amount of stored energy in the tcuto f f battery, amount of stored energy in the supercapacitor, volume where io, vo, and tend, denote the load current, load voltage, of the battery and supercapacitor, respectively. Finally, we get and service life of the hybrid system, respectively. the deliverable energy density which is given by Next, we carefully control the charging current and the ρdeliver = ηsystem · ρhybrid. (5) terminal voltage of the supercapacitor to achieve efficient and stable operation. The supercapacitor terminal voltage must be Based on definition of the system efficiency, delivered en- maintained within a proper range in order to meet the load ergy, volumetric energy density, and deliverable energy den- power demand. If supercapacitor terminal voltage is too high, sity, We design the constant-current hybrid system to enhance excessive power will be transferred from the battery to the the deliverable energy density while achieving stable opera- supercapacitor. Therefore, the terminal voltage continuously tion of the system. Key parameters of the supercapacitors in rises until some other circuit element pinches off the voltage the proposed system are the voltage rating and capacitance. rise at that terminal. On the other hand, the load demand may These parameter are directly related to the energy transfer ef- not be met when supercapacitor terminal voltage is too low. ficiency and energy density of the proposed system. Capaci- We should have steady-state operation during interval tance value of the supercapacitor affects the efficiency due to b in Fig. 4 with periodic pulsed load and constant charg- its filtering effect on the pulsed load. Moreover, the volume ing current. We need to make the supercapacitor voltage of the supercapacitor is determined by its capacitance value at the start time of the high current time period equal to and voltage rating. The value of charging current results in the voltage at the end time of the low current time period different requirement for the voltage rating for the supercapac- (vss = vs(ti) = vs(ti+1)). Different charging currents cause the itor because different amount of charging current results in a proposed hybrid architecture to operate at different steady different steady state. These two design parameters, i. e., the states for the same pulsed load. We determine not only the supercapacitor capacitance and charging current, also strongly capacitance but also the charging current since the charging influence the efficiency of the charger and regulator. The max- current and the supercapacitor terminal voltage affect the imum ratings of the switching converters, battery and super- efficiency of the switching converters. capacitor should be considered as constraints. The amount of energy delivered to the load is given by V. DESIGN SPACE EXPLORATION BY SIMULATION

Z tcuto f f Z tcuto f f A. Simulation Models E = i v dt = η i v dt, (2) ss o o reg h s We model the supercapacitor by connection of circuit tprechg tprechg elements. The equivalent circuit model incorporates a trans- where ηreg, ih and vs are the power conversion efficiency mission line behavior, a parasitic inductor model, a charge of the voltage regulator in the system, input current of the redistribution element, and a self-discharging current model output regulator, and input voltage of the output regulator, [9]. We have simplified the model of [9] to the circuit model respectively. Finally, energy delivered to the load, Edeliver = shown in Fig. 5 for fast simulation while preserving accuracy Eresuable + Ess. As a result, the overall energy efficiency from under the actual operating condition of the hybrid system. TABLE I Energy density Efficiency Energy density Efficiency EXTRACTED SIMULATION PARAMETERS. (Wh/L) (%) (Wh/L) (%) 160160 100100 160160 100 b11 -0.67 b12 -16.21 b13 -0.03 140160 90 140160 100 90 100 160 100160 100 b14 1.28 b15 -0.40 b16 7.55 120120 8080 120120 8080 b21 0.10 b22 -4.32 b23 0.34 100140 140 70 10014090140 90 70 90 90 Battery b31 0.15 b32 -19.60 b33 0.19 8080 120 6060 8080 80120 6060 80 120 160 120 100 80 80 b41 -72.39 b42 -40.83 b43 102.80 160 100 60 100 50 60 70100 50 70 b51 2.07 b52 -190.41 b53 0.20 140 ρhybrid 90 ρhybrid 4040100 4040 4040100 14070 4040 90 70 Energy density (Wh/L) b61 -695.30 b62 -110.63 b63 611.50 80 ηsystem Energy density (Wh/L) 60 80 ηsystem 60 20 120 30 20 80 30 ρdeliverable 120 ρdeliverable 80 Super- Ls 0.93 uH Racc 34 mΩ Cacc 0.8 F 0 80 60 0 8050 60 60 50 60 0 100 2020 0 70100 2020 70 capacitor R1 68 mΩ C1 (7.2 +0.616 · vs)F 00 1 2 3 44 00 1 2 3 44 60 40Capacitance (F) 6040 Capacitance40 50 (F) 40 50

R 25 mΩ R 25 mΩ R 39 mΩ Energy density (Wh/L) Capacitance (F) Capacitance (F) sw1 sw2 L 80 Energy density (Wh/L) 60 80 60 Converter R 100 mΩ I 4 mA Q 60 nF 20 30 20 30 C ctrl sw1 40 60 40 50 6040 50 40 Q 60 nF f 500 kHz (a) Parallel connection (b) Constant-current architecture Energy density (Wh/L) sw2 s 0 Energy density (Wh/L) 20 0 20 Fig. 7.20 System040 efﬁciency,1 energy2 density,3 and4 deliverable4004030 energy1 density2 of 3 4 40

Energy density (Wh/L) 20 30 the parallel connection andCapacitance the constant-current (F) architectureEnergy density (Wh/L) with pulsed load. We import an equivalent circuit model of the Li-ion battery 20 30 20 Capacitance (F) 30 architecture0 achieves higher deliverable0 energy20 density with 20 from [10] as illustrated in Fig. 6. We can describe the behavior 0 0 1 2 3 0 20 40 1 2 3 20 4 of a Li-ion battery with the equivalent circuit and the following small capacitance0 because1 2 the conversion3 4 efﬁciency0 1 is less2 3 4 CapacitanceCapacitance (F) Capacitance (F) non-linear equations: affected by the capacitance. Capacitance (F)

b12vSOC 3 2 ONCLUSIONS vOC = b11e + b13vSOC + b14vSOC + b15vSOC + b16, VI.C b22vSOC b32vSOC The battery-supercapacitor hybrid can reduce the loss due Rs = b21e + b23,Rts = b31e + b33, to the rate capacity effect for the modern portable electronics C = b eb42vSOC + b ,R = b eb52vSOC + b , ts 41 43 tl 51 53 which have a highly fluctuating load profile. Conventional b62vSOC Ctl = b61e + b63,Cb = 3600 ·Cinit , (6) parallel connection battery-supercapacitor hybrid needs a large supercapacitor so as to reduce the battery’s peak dis- where b are empirically-extracted regression coefficients, i j charging current enough to relieve the rate capacity effect while C denotes the nominal energy capacity of the battery. init on the battery. However, the large supercapacitor drastically All circuit model component values, such as value of R , R , s ts degrades the overall available energy density which is one of etc., are calculated from these equations. the most critical constraints for the power source of portable Switching converters are used to transfer power between two electronics. By isolating the battery from supercapacitor, different voltage levels. Batteries and supercapacitors, which proposed constant-current architecture effectively relieves have variable terminal voltages that are set according to their the rate capacity effect with smaller supercapacitor than the state of charge, are commonly paired with switching converters parallel connection. to supply a regulated current or a regulated voltage level to the load. The switching converter efficiency is determined by REFERENCES the converter loss, which comprises the conduction loss, gate- [1] F. Simjee and P. Chou, “Everlast: Long-life, supercapacitor-operated drive loss, and controller power dissipation. We import a power wireless sensor node,” in ISLPED, 2006. [2] S. Pay and Y. Baghzouz, “Effectiveness of battery-supercapacitor com- model of the switching converter from [11]. bination in electric vehicles,” in IEEE Bologna Power Tech Conference, We obtain the discharge characteristics of Li-ion battery 2003. by measuring and extracting the regression coefficients for [3] P. Thounthonga, S. Raelb, and B. Davatb, “Energy management of fuel cell/battery/supercapacitor hybrid power source for vehicle applications,” (6). The parameters for the supercapacitor and switching J. of Power Sources, 2009. converters are measured or obtained from the datasheets. [4] T. B. Atwater, P. J. Cygan, and F. C. Leung, “Man portable power Table I shows the parameters for the GP1051L35 Li-ion cell needs of the 21st century: I. Applications for the dismounted soldier. II. Enhanced capabilities through the use of hybrid power sources,” J. of 2-cell series battery pack of 350 mAh capacity, NessCap Power Sources, 2000. supercapacitor ESHSR0010C0-002R7 of 10 F capacitance [5] C. E. Holland, J. W. Weidner, R. A. Dougal, and R. E. White, [12], and Linear Technology LTM4607 converter. “Experimental characterization of hybrid power systems under pulse current loads,” J. of Power Sources, 2002. B. Simulation Results [6] R. Dougal, S. Liu, and R. White, “Power and life extension of battery- ultracapacitor hybrids,” IEEE TCPT, 2002. We obtain the energy efficiency and deliverable energy [7] G. Sikha and B. N. Popov, “Performance optimization of a battery- capacitor hybrid system,” J. of Power Sources, 2004. density by simulation using the extracted parameters. We use [8] Gold Peak Industries, “GP batteries datasheet: Model GP1051L35.” 10 s period, 10C discharge rate, and a pulse current with 10% [9] W. Lajnef, J. M. Vinassa, O. Briat, S. Azzopardi, and E. Woirgard, duty cycle as a pulsed load. The energy efficiency, ηsystem, “Characterization methods and modelling of ultracapacitors for use as peak power sources,” J. of Power Sources, 2007. increases when the capacitance of the supercapacitor increases [10] M. Chen and G. Rincon-Mora, “Accurate electrical battery model as depicted in Fig. 7(a). However, the overall volumetric capable of predicting runtime and I-V performance,” IEEE T. on Energy energy density of the system decreases as the capacitance Conversion, 2006. [11] Y. Choi, N. Chang, and T. Kim, “DC–DC converter-aware power increasing due to the fact that the energy density of the management for low-power embedded systems,” IEEE TCAD, 2007. supercapacitor is much lower than that of the battery. As [12] NessCap, “NessCap ultracapacitor datasheet: ESHSR-0010C0-002R7,” shown in Figs. 7(a) and 7(b), the constant-current hybrid 2003.