PURPOSE: To reduce self-discharge by using a plate group which consists of a positive electrode plate made of a lead-antimony alloy grid, in which antimony less than a specified quantity is included, and a negative electrode plate made of lead-calcium alloy grid, and a separator made of thermal flexible synthetic resin.

CONSTITUTION: As a positive electrode plate, a grid cast by using lead-antimony alloy, which is composed of antimony at 4weight%, arsenic at 0.2weight% and lead as the residual, is prepared. As a negative electrode plate, cold-rolling is performed to slab of lead-calcium alloy, which is composed of calcium at 0.08weight%, tin at 0.6weight% and lead as the residual, with a roll to make a thin plate, and expand processing is performed in net to form a grid. As a separator, the material mainly composed of polyethylene as thermal flexible resin is used. These plates and each separator are combined to form a plate group.