Thermal management of Li-ion batteries and its influence on electrical performance

The current development activities for hybrid (PHEV/HEV) or full electric driven vehicles (FEV/BEV) focus on Li-ion batteries as energy storage. Since the amount of storable energy inside an automotive battery pack still prevents longer distances in the electric driving mode, the available energy has to be used as extensively as possible and high power throughput and maximum utilization of the nominal capacity are key development goals. Nevertheless, safe operation is limited to a certain temperature range which makes operation of Li-ion batteries in automotive application without reducing the battery’s life span and without exceeding recommended temperatures a challenging task. The temperature is one of the key factors that influence the electrical behaviour and the ageing characteristics of Li-ion batteries. At low temperatures the output power is significantly reduced [1, 2, 3] and high charging rates cannot be applied. The deposition of metallic lithium at the anode as a result of charging at lower temperatures can cause internal short circuits and can even initiate a thermal runaway of the battery [4]. However, charging at low temperatures occurs while decelerating a cold started electric vehicle with a thermally unconditioned battery pack. Batteries in adequate thermal state on the other hand facilitate a high power output because of reduced ohmic resistances [5] and raised diffusion coefficients inside the battery cell respectively. Whereas charging at higher temperatures is more feasible, the storage of Li-ion batteries (on stock, but also in the parked car) under elevated temperatures has a detrimental effect on the battery’s long-term endurance. More precisely, the calendar life ageing, which reduces the battery’s capacity and increases its ohmic resistances, intensifies with temperature [6, 7].