Aluminium/water batteries using hydrogen evolving cathodes are one of the candidate batteries for sub-sea application. However, it is impractical to use parallel plate construction because the significant increase in the anode-cathode gap during long term anodic discharge leads to unacceptable iR losses. A conceptual design, using a dome-shaped configuration, is presented and preliminary tests on a prototype show that such a battery can ensure constant anode-cathode gap and uniform anode dissolution. The energy density of such a battery for a two year period is estimated to be 855 Wh kg-1.