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

Many aspects of battery electric vehicles are very challengingfrom the engineering point of view in termsof safety, weight, range, and drivability. Commercialvehicle engines are often subjected to high loads even at lowspeeds and this can lead to an intense increment of the batterypack temperature and stress of the cooling system. For thesereasons the optimal design of the battery pack and the relativecooling system is essential. The present study deals with thechallenge of designing a battery pack that satisfies both theconditions of lowest weight and efficient temperature control.The trade-off between the battery pack size and the electricalstress on the cells is considered. The electric system has theaim to substitute a 3.0 liters compression ignition enginemainly for commercial vehicles. The curve of delivered powerduring the homologation cycle WLTC is experimentallyrecorded with the engine at the test bench and then used toobtain the discharge current profile for the battery pack. Thebattery pack is conceived as made of Li-polymer pouch typecells, whose shape provides easy stackability and highvolumetric efficiency. A single cell is experimentally investigatedvia infrared imaging during a discharge cycle and thefollowing resting phase for the estimation of the cell thermalparameters. Then, an equivalent circuit model of the batteryis set up using the experimental results of a multi-step testcycle. The electrical and thermal characteristics of the batteryunder investigation are finally used to model different packswith liquid cooling. Several configurations of the battery packin terms of weight and generated heat are analyzed with thevehicle performing both discharge and charge cycles. The finalconfiguration of the battery pack for the vehicle under investigationis 20p100s, that is 100 series branches of 20 parallelcells, for a total of 2000 cells, the maximum voltage of 420 Vand the nominal voltage of 380 V. Total weight of 150 kg. 1h51min of charge time with Fast Charge mode. For the coolingsystem, a propylene/water solution in volume fraction of 10-90is found as cooling liquid and a pump speed is set to 5000 rpm.These results and the presented methodology can help for thefuture design and dimensioning of battery packs by alow-cost analysis.