Li-Ion Battery-Charger Solutions for JEITA Compliance

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Power Management Texas Instruments Incorporated Li-ion battery-charger solutions for JEITA compliance By Jinrong Qian Sector Manager, Battery Charge Management – Advanced Portable Introduction from the battery casing or from electrode materials. If Lithium-ion (Li-ion) batteries tend to become dangerous these shards get between the battery’s electrode and sepa- when they are overcharged at high temperatures. Safely rator, battery cycling in the negative electrode can eventu- charging these batteries has become one of the most ally cause the shards to puncture the separator. This important design specifications in battery-powered porta- results in a microshort between the positive and negative ble equipment. Progress has been made in establishing electrodes, producing high heat that may ultimately result industry standards such as the Japan Electronics and in fire and/or an explosion. High temperatures, fire, and explosions are all results of Information Technology Industries Association (JEITA) thermal runaway—a condition whereby a battery enters guidelines for improving battery-charging safety. This into an uncontrollable reaction. Thermal runaway is a article addresses safety requirements and battery-charger process in which the internal temperature of a battery solutions that meet these requirements in both notebook with LiCoO2 as the cathode material and graphite as the and single-cell handheld applications. anode material reaches approximately 175°C. This is an irreversible and highly exothermic reaction that can cause Battery-charger safety and the JEITA guidelines a fire, usually when the battery is charging. Widely used in consumer electronics from cell phones to Figure 1 shows the charge current and charge voltage laptops, Li-ion batteries have the highest volumetric and over temperature commonly used in the older Li-ion- gravimetric energy densities among the rechargeable battery-charging systems that are prone to thermal run- batteries, with no memory effect. They also have a self- away. Both the battery charge current and charge voltage discharge rate that is 10 times lower than that of NiMH are constant over the cell temperature from 0 to 45°C. batteries, and they can provide the instant power required High cell temperatures not only speed up battery aging by the system; but are they safe? but also increase the risk of battery failure. Everyone in the industry has seen pictures of exploding To improve the safety of charging Li-ion batteries, laptops and heard about the massive and unprecedented JEITA and the Battery Association of Japan released new recalls of Li-ion batteries due to cell safety concerns. Such safety guidelines on April 20, 2007. Their guidelines battery explosions or fires originated within the manufac- emphasized the importance of avoiding a high charge turing process. Batteries contain several metal parts that current and high charge voltage at certain low and high can sometimes result in undesirable metal impurities within temperature ranges. According to JEITA, problems in the the cell. These impurities are typically sharp metal shards Li-ion batteries occur at high charge voltages and high cell

Figure 1. Upper-limit charge current and charge voltage in older Li-ion-battery-charging systems

Upper-Limit Charge Current: 1C

Charge Current

Upper-Limit Voltage: 4.25 V (4.2 V Typical) Charge Voltage No Charge No Charge

TI T4 (0°C) Temperature (40 to 45°C)

High-Performance Analog Products www.ti.com/aaj 1Q 2010 Analog Applications Journal Texas Instruments Incorporated Power Management temperatures. Figure 2 shows the JEITA guidelines for the charge current and charge voltage Figure 2. JEITA guidelines for charging Li-ion batteries in notebook applications over cell temperature for batteries used in notebook applications. These batteries have LiCoO2 as the cathode active material and graphite as the anode active material. Upper-Limit Charge Current In the standard charging temperature range Charge from T2 to T3, a Li-ion cell can be charged in Current the optimal conditions of the upper-limited charge voltage and the upper-limited charge current recommended by the cell’s manufac Upper-Limit Voltage: 4.25 V turer for safety. (4.2 V Typical) Charge 4.20 V Charging at low temperatures Voltage If the cell’s surface temperature becomes lower than T2 during charging, the lithium ions could No Charge No Charge each gain one electron and become metallic lithium. This metallic lithium is likely to deposit on T1 T2 T3 T4 (10°C) Temperature (45°C) the anode, because at low temperatures the transfer rate decreases and the penetration of lithium ions into the negative electrode carbon slows down. Such metallic lithium could easily react with Battery-charger solutions for meeting electrolyte, causing permanent loss of the lithium ions, JEITA guidelines which degrades the battery faster. In addition, the chemi- The smart battery pack, which includes a fuel gauge, cal reaction between metallic lithium and the electrolyte analog front end, and second-level protector, is commonly generates a lot of heat, which could lead to thermal run- used in notebook applications. The fuel gauge provides the away. Therefore, the charge current and charge voltage battery’s cell voltage, charge and discharge current, cell are reduced at low cell temperatures. If the temperature is temperature, remaining capacity, and run time to the further reduced to T1 (0°C as an example), the system system through SMBus for optimizing the system perform should not allow charging. ance. The bq20z45 and bq20z40 fuel gauges with Charging at high temperatures Impedance Track™ technology, recently developed by If the cell’s surface temperature rises above T3 (45°C as an example) during charging, the cathode material, Figure 3. JEITA guidelines for charging Li-ion batteries in single-cell handheld applications LiCoO2, starts to become more active and can chemically react with the electrolyte when the cell voltage is high. If the cell temperature is Maximum Charge Current: 1C further increased to T4, the system should prohibit charging. If the cell 0.5C temperature reaches 175°C with a cell Charge Current voltage of 4.3 V, thermal runaway may occur and the battery may explode. Similarly, Figure 3 shows the Maximum Charge Voltage: 4.25 V JEITA guidelines for charging Li-ion (4.2 V Typical) batteries in single-cell handheld applications, where the charge 4.15 V Maximum current and charge voltage are also Charge 4.10 V Maximum functions of the cell temperature. Voltage The maximum charge voltage of 4.25 V includes the battery charger’s full tolerance. The battery can be T1 T3 T5 charged at up to 60°C with a reduced (0°C) Temperature (45°C) (60°C) charge voltage for safety. T2 T4 (10°C) (50°C)

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Texas Instruments (TI), include a series of flash-memory guidelines for notebook applications. This SMBus- constants for flexibly programming the battery’s charge controlled battery charger with a synchronous switching current and charge voltage based on the JEITA guidelines. buck converter can support Li-ion batteries with one to The temperature thresholds are user-programmable and four cells and a charge current of up to 8 A. The dynamic provide flexibility for meeting different specifications with power-management function allows charging the battery different applications. The fuel gauge usually broadcasts and powering the system simultaneously without increas- the charge current and voltage information to the smart ing the adapter’s power rating. battery charger or keyboard controller for periodically The battery pack in single-cell portable devices usually setting the proper charge current and voltage. An SMBus- has the cell and a safety protector but uses the charger controlled battery charger, such as the TI bq24745, can be instead of a fuel gauge to monitor the cell temperature used as a slave device to get the charge voltage and cur- and adjust the charge voltage and current. TI’s bq24050 rent information from a smart battery pack with either the single-cell linear battery charger was designed to meet the bq20z40 or the bq20z45 fuel gauge. JEITA specifications for handheld devices. It reduces the Figure 4 shows a schematic of a smart battery charger charge current by half when the cell temperature is with a smart battery pack that complies with the JEITA between 0°C and 10°C, and reduces the charge voltage to

Figure 4. Smart battery charger bq24745 with fuel gauge bq20z40 or bq20z45

RAC D1 Adapter 10 m System Load R1 0.1 µF 10  1 µF 430 k

CSSN DCIN R2 CSSP 66.5 k ACIN bq24745 C8 ACOK 1 µF 10 k VDDP VREF C6 C4 R7 BOOT Q3 10 µF 1 µF 200 k ICREF UGATE RSN R8 L To Smart 5.6 µH 10 m R9 50 k 0.1 µF Battery Pack 1.4 M GND PHASE C3 ICOUT LGATE Q4 10 µF R4 10 k PGND +3.3 V VDDSMB CSOP R6 R11 C7 10 k Keyboard Controller or 10 k 0.1 µF Smart Battery Pack with CE CSON bq20z40 or bq20z45 SDA VFB SMBus SCL 0.1 µF R9 7.5 k EAO VICM R10 C21 C23 C5 C22 20 k R11 2 nF 51 pF 100 pF 130 pF 200 k EAI FBO

High-Performance Analog Products www.ti.com/aaj 1Q 2010 Analog Applications Journal Texas Instruments Incorporated Power Management

4.06 V when the cell temperature is between 45°C and temperature ranges as JEITA recommends can significantly 60°C. Figure 5 shows a typical application circuit with the improve the safety of charging these batteries. Both bq24050 linear charger. The charger monitors the battery’s switch-mode and linear battery-charger solutions that cell temperature via the thermistor (TS) pin and adjusts comply with JEITA guidelines have been presented. the charge current and voltage when the monitored temperature reaches the threshold. Related Web sites power.ti.com Conclusion www.ti.com/sc/device/partnumber Charging Li-ion batteries safely is critical and has become Replace partnumber with bq24050, one of the key specifications for charger design. Reducing bq24745, or bq24747 the charge current and voltage at lower and higher

Figure 5. Typical single-cell application circuit with JEITA- compliant linear battery charger

bq24050

Adapter IN Q1 OUT

C1 CHG VSS R2 103AT 1 µF ISET TS

R1 PRETERM ISET2 Host

USB Port LDO/CE D+ bq24050 D– VBUS ISET/100/500

D+ D+

D– D–

GND GND

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