Cycle life is a performance requirement combining energy and time (Table ). The term secondary battery refers to the class of batteries whose chemical reaction is reversible such that energy may be stored again and then subsequently released . Cycle life can be expressed as the number of completed cycles before the battery capacity or energy drops to some percentage of its initial capacity or energy , e.g. 80%. Cycle life depends on factors such as depth-of-discharge (DoD) , discharge and charge rates, time between cycles, and ambient conditions (e.g. temperature) .