Abstract

In a protection circuit for protecting a plurality of chargeable electric cells by detecting overdischarge and overcharge by detection circuits for respective electric cells to control a protection switch upon detection, the detection circuit is a single chip device comprising an overcharge detection circuit and an overdischarge detection circuit to produce first and second detection signals, respectively, first and second terminals to be connected to an electric cell, third and fourth terminals for receiving first and second input signals, respectively, and fifth and sixth terminals for delivering first and second output signals, respectively. The first input signal and the first detection signal are processed by an OR gate to produce a first processed signal which is delivered to the fifth terminal. The second input signal and the second detection signal are processed by AND gate to produce a second processed signal which is delivered to the sixth terminal. In protection of a plurality of electric cells, the detection units are connected to electric cells, respectively, and connected in a cascaded form so that the first and second output signals from one stage of detection units are transmitted as first and second input signals to the next succeeding stage of the detection units in the cascaded form Accordingly, the overcharge and overdischarge detected in any one of stages are transmitted to the protection switch through the succeeding stages.