To develop solid-state metal-hydride batteries, heteropolyacid hydrates, such as 12-tungstophosphoric acid hydrate (H3PW12O40·nH2O) and 12-molybdophosphoric acid hydrate (H3PMo12O40·nH2O), were used as electrolytes. The Ni/MH battery using 12-tungstophosphoric acid could be charged and discharged even at a considerably high current density of 100mAg−1, but showed very poor cycle life performance due to corrosion of the electrode materials by the heteropolyacid electrolyte. The cyclability was improved by employing manganese dioxide and zirconium-based alloy as positive and negative electrodes, respectively. The best performance was obtained on the battery using 12-molybdophosphoric acid; its operation was possible up to 50mAg−1, and/or for more than 100 cycles at 5mAg−1.