PURPOSE: To make a charging state detectable for a long time in succession, by installing more than two sets of paired electrodes composed of a lead dioxide electrode and a lead electrode and a connection transducer which makes these paired electrodes alternately connectible to a voltage detector or a battery charger and a battery terminal or a nonconnective state.

CONSTITUTION: Paired electrodes 3 are connected to a voltage detector 6, indicating a charging state of a lead battery. On the other hand, paired electrodes 3' are connected to a terminal of the lead battery during float charging or a battery charger 7 and, after being charged, left nonconnective intact. After the elapse of a specified period of time, these paired electrodes 3' are connected to another voltage detector 5, indicating the charging state of the lead battery. Meantime, the counter part electrodes 3 are connected to the terminal of the lead battery or the charger and, after being charged, left nonconnective intact likewise. With repetition of these operations, these paired electrodes indicating the charging state of the lead battery are in no case reaching to the unstable potential due to discharge by selfdischarge and/or connection to an external detection circuit, so that the charging state of the lead battery is made possible for a very long time. Moreover, with two sets of paired electrodes emplyed, even in time of a voltage unstable period, during charging or after charging, the charging state can be detected by other fellow paired electrodes.