System and method for controlling charging of secondary battery that

Abstract

In a system for charging a rechargeable battery with constant voltage, a power supply supplies a voltage increased with time. An overvoltage generating circuit outputs an overvoltage signal for a first predetermined period when a supplied voltage to the rechargeable battery exceed a reference voltage. In response to the overvoltage signal from the overvoltage generating circuit, a multivibrator gradually drops the voltage from the power supply for a second predetermined period being longer than the first period. A sample/hold circuit holds the voltage from the power supply when a control signal is not supplied, and samples the voltage from the power supply when the control signal is supplied. A comparator compares the voltage from the power supply with a held voltage of the sample/hold circuit, and generates a signal when the voltage from the power supply is higher than the held voltage of the sample/hold circuit. A logical sum circuit supplies the control signal to the sample/hold circuit in response to the signal from the comparator or the overvoltage signal from the overvoltage generating circuit. A voltage follower supplies the held or the sampled voltage from the sample/hold circuit to the rechargeable battery.