In a conventional battery, specific energy density is approximately constant, while power density may be varied by design. In a fuel cell, by contrast, power density is essentially constant and energy density can be varied. The zinc-air battery is an intermediate case with an electrode derived from each type. Consequently, the weight of zinc must be maximized with respect to the remainder of the battery, for maximum specific energy density, but must be minimized with respect to plate area, for maximum specific power density. This basic tradeoff implies that a design optimization of the zinc-air battery is required for each application. A design procedure is outlined and demonstrated for the case of an electric passenger vehicle.