Experimental Study Of Electro-thermal Properties Of Parallel Connected Battery Module

Electric vehicles(EVs), using lithium ion batteries(LIBs) as their power sources instead of conventional fossil fuels, have been considered as a replacement for internalcombustion engines vehicles in the long term due to their high-efficiency and lowemission rate. The diffusion of EVs can help to solve significant problems such as global warming and the air pollution. One of the key problems with the LIBs is the decrease of their power and capacity with aging. The researchers showed that the temperature, state of charge(SoC) and charge/discharge rate are the main factors affecting the battery aging process. The used LIB packs in EVs consist of battery modules, which each battery module in turn consists of parallel, series or mixed connected cells to provide sufficient energy and power.Due to the internal cell-to-cell variations, as well as the difference in their locations, the cells are experiencing different load and thermal conditions, which leads to different levels of aging inside the parallel connected battery module. Direct relation between aging and resistance leads to resistance mismatching. The mismatched resistance cells follow non-uniform current and temperature distributions inside the battery module. The cell with lower resistance takes more current during battery module charge/discharge cycle, thus it gets the higher SoC/DoD earlier than the others, which leads to SoC/DoD difference. The current amount of each cell impacts its temperature increasing, and the different temperature rising rate leads to the temperature non-uniformity inside the battery module.In this study, for a two parallel connected cells the effect of cell-to-cell variations was examined by considering their resistance variations, initial imbalance(transient) effect and thermal conditions.Firstly, the capacity and internal resistance of single cells were measured. Due to the importance of the internal resistance to the current distributions in parallel connected battery module, this study applied and compared five different methods for its measurement.Secondly, the battery module was subjected to different operating conditions, such as different load current and different temperatures, to examine the effect of inner-cell resistance difference, the rest time after the initial SOC imbalance, and the thermal environment of the cell.It was found out that the current distribution in a parallel connected battery module is affected by three main factors:(1) the resistance difference between the sub-branch, which includes internal resistance of the cell and the contact resistance,(2) the initial open circuit voltage(OCV) difference between the cells, which depends on its SoC and temperature, and(3) the individual cell’s temperature, which is affected by its position in a pack.To minimize the current distribution and SoC differences between the parallel connected cells, we examined the effect of the following factors:(1) The resistance of connection wire: By replacing the copper wire with the silver wire of the same length, the SoC and current distribution differences was reduced during the discharge/charge cycle.(2) The initial OCV difference between the cells: By increasing the length of the rest time to 13 hours after each charging/discharging, current and SoC difference was reduced during the next discharge/charge cycle.(3) The large current difference resulted from partial wrapping can be reduced by improving the uniformity of the thermal environment of the cells.