Patent ID: 7453237

Claim:
A battery pack comprising: a plurality of series connected batteries; a plurality of voltage detection circuits for detecting battery voltage of each of the batteries, each of the voltage detection circuits having an output terminal connected to battery pack ground via an output resistor; a plurality of output switches that are controlled on and off by an output terminal voltage of each of the voltage detection circuits, wherein the plurality of voltage detection circuits are connected in series and their outputs are switched HIGH or LOW depending on the detected battery voltage, and when output terminal voltage of one of the voltage detection circuits is HIGH, the batteries connected to the one voltage detection circuit discharge through the associated output resistor; and at least one discharge circuit comprising a series connection of a balance resistor and a switching device, the discharge circuit being connected in parallel with the batteries of the plurality of batteries that remain un-discharged by the output resistors of the voltage detection circuits, wherein, when output terminal voltage of one of the voltage detection circuits is HIGH and the batteries that are connected to the one voltage detection circuit are discharged, the switching device of the discharge circuit connected in parallel with the un-discharged batteries for that voltage detection circuit is turned on to discharge the un-discharged batteries via the balance resistor, and wherein, when output terminal voltage of one of the voltage detection circuits is LOW, the batteries that are connected to the one voltage detection circuit are not discharged, and the switching device of the discharge circuit is turned off leaving the un-discharged batteries un-discharged to balance discharge currents of all of the batteries.