Analysis of Discharge Techniques for Multiple Battery Systems

We consider the problem of scheduling multiple identical batteries for discharge in portable electronic systems. Unlike previous work reporting some experimental data to suggest which scheduling schemes are better than others, we arrive at our general conclusions formally, based on the analysis of an accurate high-level model of battery behavior. Our analytical results show that: (1) the lifetime of a parallel discharge schedule is equal to that of an equivalent monolithic battery, (2) the lifetime of a parallel discharge schedule is no less than that of a sequential discharge schedule, and (3) the lifetime of a switched discharge schedule approaches that of an equivalent monolithic battery as the switching frequency increases. We also derive bounds on the lifetime of a single battery under a constant-rate load, and then extend them to multiple battery systems. Using a low-level battery simulator, we verify our analytical findings with numerical data. For the simulated cases, the parallel discharge schedule resulted in up to 72% higher lifetimes than the sequential discharge schedule but fell short of the lifetime upper bound by up to 29%.