The provision of a mode in silver zinc batteries where a user can access extra capacity as an emergency reserve for times when extra capacity is needed. While this temporarily increases capacity, it does not detrimentally affect cycle life over the longer term, and it permits a silver zinc battery to essentially mimic the long term capacity and cycle life characteristics of a lithium ion battery while still affording inherent advantages associated with silver zinc batteries. In a variant embodiment, this ability to temporarily increase capacity is optimally employed at the end of a battery life cycle in a controlled “roll-off” that accords additional cycles of battery service life. In another variant embodiment, the general capability to control capacity is employed to gradually decrease the available capacity of a battery over the life of the battery, to thereby extend the battery service life.