DISCHARGE CHARACTERISTICS OF LI-ION BATTERY
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for a battery pack with total of nbatt batteries is the min(EODV) of all the batteries in the pack.
DISCHARGE CHARACTERISTICS OF LI-ION BATTERY On a module level, the Li-ion battery cells are connected in series and packaged to form modules varying from a 3-cell combination to a 10cell combination. The module design includes a thermal battery management system based on liquid coolant. This system is able to keep the battery temperature within an optimal temperature range, either by cooling during heavy duty driving conditions or by heating when the battery operating temperature is low. On a battery pack level, the Li-ion modules are connected in a series combination to form a 300 to 350 V battery pack system for EV application. Each Li-ion battery module is equipped with an electronic module controller. This device combines the function of monitoring the electrical and thermal module data for transmission to the central battery monitor (BMON). The BMON in turn, transmits commands to the battery. Thus the thermal loop and the electronic control devices are both integral to the Li-ion battery. This design of the module improves the safety and the reliability of the battery system. The high level of battery energy density by weight and volume is suitable for full EV applications. However, the current rate capability of the Li-ion battery system is insufficient for hybrid EV applications. The hybrid EV requires fast discharge and recharge of electric energy. These characteristics are required for acceleration and energy recovery upon regenerative braking. By reducing the electrode thickness, the power capability of the Li-ion cells can be enhanced. The use of thin film electrode for the Li-ion battery design demonstrates continuous discharge characteristics at 15C rate as shown in Figure 6–11. Under room temperature conditions, a continuous power of more than 850 W/kg at 80% DOD and a voltage level of 85% of the nominal voltage can be achieved. Due to the trade-off between energy and power density, a specific energy of 60 Whr/kg is achieved easily. If the user reduces the amount of inactive components and uses active materials with higher specific capacity, a specific energy of more than 80 Whr/kg can be achieved. The analysis of the Ragone plot for the Li-ion module, as shown in Figure 6–11 indicates that: •
Prismatic cells exhibit higher specific power and lower specific energy than the cylindrical cells at several discharge current levels.
