EVS30 Symposium.pdf

The EVS30 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium was held in Stuttgart, Germany, from October 9-11, 2017. Researchers Andrew Burke and Jingyuan Zhao presented a study on the cycle life of lithium-ion batteries in combination with supercapacitors. The study aimed to quantify the effect of load-leveling on the cycle life of lithium-ion batteries.

Modules of LiNiCoAl and LiFePO4 cells were cycled at constant current and on a dynamic pulse discharge/charge profile. The average current for both discharge profiles was C/2. The modules were cycled for about 700 cycles over six months, and their degradation was tracked in terms of Ah capacity and resistance.

The results showed that, for both lithium chemistries, the modules degraded more rapidly with constant current cycling than with the dynamic pulse profile. This was unexpected, as current thinking suggests that reducing high current pulses should increase cycle life. The study highlights the complexity of factors affecting cycle life, including chemistry, size, temperature, charge algorithm, and discharge profile.

The researchers used 18650 cells from Panasonic (LiNiCoAl) and K2 Energy (LiFePO4) and tested them using a 6-channel Arbin battery tester. The initial Ah capacity and resistances of the modules were measured, and the cells were selected to minimize differences in the modules. The testing results showed that the K2 module used in the dynamic pulse cycling tests had a significantly lower initial Ah capacity than the module used for constant current testing.

The study's findings have implications for the use of lithium-ion batteries in plug-in electric vehicles, particularly in combination with supercapacitors. The results suggest that load-leveling may not always increase cycle life, and further research is needed to understand the complex factors affecting battery degradation.