Modeling Study of the Battery Pack for the Electric Conversion of a Commercial Vehicle

Many aspects of battery electric vehicles are very challenging from the engineering point of view in terms of safety, weight, range, and drivability. Commercial vehicle engines are often subjected to high loads even at low speeds and this can lead to an intense increment of the battery pack temperature and stress of the cooling system. For these reasons the optimal design of the battery pack and the relative cooling system is essential. The present study deals with the challenge of designing a battery pack that satisfies both the conditions of lowest weight and efficient temperature control. The trade-off between the battery pack size and the electrical stress on the cells is considered. The electric system has the aim to substitute a 3.0 liters compression ignition engine mainly for commercial vehicles. The curve of delivered power during the homologation cycle WLTC is experimentally recorded with the engine at the test bench and then used to obtain the discharge current profile for the battery pack. The battery pack is conceived as made of Li-polymer pouch type cells, whose shape provides easy stackability and high volumetric efficiency. A single cell is experimentally investigated via infrared imaging during a discharge cycle and the following resting phase for the estimation of the cell thermal parameters. Then, an equivalent circuit model of the battery is set up using the experimental results of a multi-step test cycle. The electrical and thermal characteristics of the battery under investigation are finally used to model different packs with liquid cooling. Several configurations of the battery pack in terms of weight and generated heat are analyzed with the vehicle performing both discharge and charge cycles. The final configuration of the battery pack for the vehicle under investigation is 20p100s, that is 100 series branches of 20 parallel cells, for a total of 2000 cells, the maximum voltage of 420 V and the nominal voltage of 380 V. Total weight of 150 kg. 1h 51min of charge time with Fast Charge mode. For the cooling system, a propylene/water solution in volume fraction of 10-90 is found as cooling liquid and a pump speed is set to 5000 rpm. These results and the presented methodology can help for the future design and dimensioning of battery packs by a low-cost analysis.