Method for protecting an excess discharge of a battery

Abstract

An overcharge or excess discharge of a battery cell can be avoided by an electronic switch formed of a single transistor. A positive electrode of a battery cell (3) is connected to a positive connection terminal (1). A negative electrode is grounded and connected through a transistor (4) to a negative connection terminal (2). When a terminal voltage of the battery cell (3) is a detected to be a predetermined voltage or greater, an output from a detecting circuit (5) is supplied through a series circuit of two resistors (6), (7) and a positive temperature coefficient resistor (PTC) (8) to the base of the single transistor (4) and supplied to power supply terminals of two comparators (9), (10). Further, voltage-divided outputs of two resistors (11), (12) and the negative connection terminal (2) are respectively connected through two resistors (13),(14) to positive and negative input terminals of the comparators (9), (10). The outputs of the comparators (9), (10) are connected through two transistors (15), (16) to a junction between the two resistors (6) and (7). The output from one of the comparators (9) is grounded through a fourth transistor (17) and the base of the fourth transistor (17) is grounded and connected to the connection terminal (2) through two additional resistors (18), (19).