We have tried to develop a lithium/thionyl chloride reserve battery (560W). A cell consists of 5 double-sided cathodes and anodes (toroidal shape, electrode area: 100 cm2, cell surface area: 900 cm2, current density: 22mA/cm2). We have designed a unique internal serial connection, which is capable of obtaining a desired voltage by simply stacking the cells and then tightening the internal serial connection volts. A piston-type electrolyte reservoir is designed. Our dual-piston electrolyte reservoir requires smaller space for the piston movement than a single-piston reservoir. It also renders reliable opening of the diaphragm, and appropriate electrolyte transferring speed to avoid damage to the carbon electrodes. The batteries underwent performance tests, environmental tests, and accelerated aging test.