Thermal characterization and cooling of lithium batteries for electric vehicles

The maturation of the electric car market is strongly influenced by the performance of new energy storage systems. These new types of batteries are required both to increase the reliability and to maintain the same performance for a longer time. In this respect, a key factor is their operating temperature. Recently this problem has assumed a strategic importance after the inconvenience to the 787e airplane in Japan due, very probably, to an overheating of the lithium battery causing a smoke release in the cabin. The accumulators consist typically of modules constructed by assembling unitary elements (cells). The first step of this work was the thermal characterization of a single cell of the module, which consists in the detection of the thermal field by means of thermocouples and thermographic tests during electric charging and discharging phases. The purpose was to identify possible critical hot points. Downstream of this phase, a study on the optimal configuration to obtain the flattening of the temperature profile and to avoid hot points was executed. An experimental plant for cooling capacity evaluation of the batteries, using air as cooling fluid, was realized in ENEA Casaccia. The plant is designed to allow testing at different flow rate and temperatures of the cooling air, useful for the assessment of operative thermal limits in different working conditions. Another experimental facility was built to evaluate the thermal behaviour changes with water as cooling fluid. Experimental tests were carried out on the LFP (LiFePO4) batteries, under different electric working conditions. The results obtained will be used to identify the general characteristics required for a more effective heat removal.