Circuit for measuring the capacity of a battery

Abstract

The discharge capacity of a secondary battery, e.g. a Ni-Cd battery, has a maximum value at room temperature and gradually decreases both with an increase and decrease of ambient temperature. The battery discharge capacity also varies inversely with the number of battery charge/discharge cycles of the battery. A circuit for measuring the remaining discharge capacity of the secondary battery comprises a capacitor (23) whose temperature and age-variation characteristics are very similar to those of the discharge capacity of the secondary battery. The current flowing through the battery and motor (13) is detected by a series resistor (17). A voltage follower (20) responds to the detected voltage and drives a resistor (24), so that capacitor (23) is discharged at a rate determined by the resistor (24) current. The capacitor voltage is compared with a predetermined value by a hysteresis comparator (29) which controls the ON/OFF state of a transistor (21) in accordance with the comparison. The comparator output pulses, i.e., the capacitor charge/discharge cycles, are counted by a counter (31). Since the capacitance variation of the capacitor corresponds to the variation in battery discharge capacity with ambient temperature and also with the time of use, the counter (31) indicates very accurately the remaining battery capacity