Heat and mass transfer modeling and investigation of multiple LiFePO4/graphite batteries in a pack at low C-rates with water-cooling

Abstract

Li-ion batteries (LIBs) are found to be deep spots in the Electric Vehicles (EVs) as well as Hybrid Electric Vehicles (HEVs) application. Combination of such high capacity LIBs in large serial-parallel configurations have been wrapped up with serious problems such as safety, durability, cost, and uniformity, which are imposing limitations on this broad application. A narrow area of these limitations, in which LIBs should operate safely and reliably, is the essential requirement of an effective control-thermal management scenario. In this paper, we examined the heat and mass transfer (temperature and mass flow rate of water) field as well as voltage profiles for a 20 Ah Graphite/LiFePO4 LIB pack at low currents of 20 A (1 C) and 40 A (2 C) with the selected ambient conditions using unique water-cooling methods with 35 °C, 25 °C, 15 °C, and 5 °C. This gives significant data information for the thermal behavior of LIB packs to design the temperature control (or thermal management) systems and develop an empirical voltage-thermal estimation model. In such manner, 3 prismatic LIBs with 20 Ah nominal capacity are arranged in a series with 4 micro-channel cooling plates placed within cells. There are six evenly placed thermocouples on the surface of each of these 3 battery cells used to extract time dependent temperatures. To make the data compatible for EV/HEV application, we developed a modified exponential-polynomial equivalent circuit model to simulate the temperature and voltage field. Outputs of the simulations are compared with the test data with specified C-rates and ambient conditions. The results demonstrate that increasing discharge currents and ambient conditions result in an increased surface temperature at 3 spots; close to the −ve electrode, close to the +ve electrode, and near the middle part of the LIB cell.