Parametric study of limiting cell design variables in a lithium battery pack

The influence of design parameters at cell level on performance at battery pack level is analyzed, in order to find the main causes of cell voltage unbalances and the consequent loss of battery pack capacity. The study parameters are electrode thicknesses, electrode porosities and electrolyte salt concentration. Three battery packs are defined with a 20 percent range variation in the values of the named design parameters. The configuration of the analyzed pack consists of six lithium cells, consisting of graphite anode and manganese oxide cathode, connected in series. For this analysis, a mathematical model with physical and phenomenological basis is applied in an optimization environment. The proposed optimization framework consists of maximizing the pack capacity by operating in simple steady-state charge-discharge cycles. The limiting cells are easily identified from the active bounds in key variables defined by the model. The most influential design parameter at the pack level, for the considered cell chemistry, turns out to be electrode porosity, since the pack with cells of different values of this parameter presents a capacity reduction of up to 15 percent when is compared to the pack of uniform cell designs.