An energy conversion system, comprising: a reservoir container including at least two chambers of inversely variable volume for respectively storing a quantity of fuel and receiving a quantity of exhaust; a means for decreasing the volume of the first chamber while concurrently increasing the volume of the second chamber; at least one energy conversion device; first means for communicating fuel between the at least one energy conversion device and a first of the chambers in the reservoir container; and second means for communicating exhaust between the at least one energy conversion device and a second of the chambers in the reservoir container. The reservoir container may be transported to a recharging/refilling station or recharged in-situ. A particular application for metal-air fuel cell power systems is shown and described.