Patent ID: 8384349

Claim:
A battery monitoring system for electric power tool comprising a battery pack for electric power tool, a battery charger for the electric power tool, and an auxiliary power source, wherein the battery pack includes: a battery that is used as a driving power source for electric power tool and has at least one battery cell; and a monitoring circuit that is operated by electric power supplied from the battery and monitors a status of the battery, the battery charger is configured such that the battery charger is detachably attached to the battery pack, and the battery charger generates and outputs charging power for charging the battery, and the auxiliary power source is provided inside or separately from the battery pack and outputs electric power allowing the monitoring circuit to operate, wherein the monitoring circuit is configured to be operational by the electric power supplied from the auxiliary power source when the monitoring circuit becomes unoperational with the electric power of the battery due to a decrease in a voltage of the battery, and includes: a chargeability determination unit that determines whether or not the battery is in a chargeable state by monitoring the battery; and a signal output unit that outputs, as one of the monitored items, a charge permission signal to the battery charger when the battery is determined to be in a chargeable state by the chargeability determination unit, the charge permission signal indicating that the battery is in a chargeable state, wherein the battery charger outputs the charging power to the battery pack when the charge permission signal is inputted to the battery charger from the battery pack.