Secondary battery cell protection circuit

Abstract

A secondary battery cell protection circuit protects a charge control FET, a discharge control FET, and a secondary battery cell from destruction due to use of a charger of greater than rated voltage to charge the secondary battery cell. The secondary battery cell protection circuit has a charger voltage detection circuit, a first charge circuit control circuit for cutting off a charge circuit when the charger voltage detection circuit detects a charger voltage in excess of a predetermined value, and a second charge control circuit for cutting off a charge circuit when the over-charge detection circuit detects a secondary battery cell charge voltage in excess of a predetermined value. When the charge control FET is OFF, a load voltage proportionate to the charge voltage of the charger appears at a −B terminal and is applied to the emitter of a transistor, activating the transistor. A current proportionate to the charge voltage of the charger then flows to the transistor. If at this time the charger voltage is above the rated voltage, then a resistor is set so that a voltage drop of the resistor exceeds a base voltage. As a result, when the charger voltage is above the rated voltage the charge voltage of the secondary battery cell declines thereafter and a charge control FET drive transistor remains in an OFF state even after the charge voltage is at or below a release charge voltage.

Dependencies are inferred from the claim text.