Patent ID: 7145314

Claim:
A DC power source unit used as a power source of a tool and having a charging function for charging a battery pack used as an alternative power source of the tool, the tool having a power switch, the DC power source unit comprising: a battery pack connecting portion that connects the battery pack to charge with DC power; an adapter that supplies DC power to the tool; a power output switching circuit that switches between supplying the DC power to the tool through the adapter and charging the battery pack; a cooling device that cools the battery pack; a battery temperature detector that detects at every predetermined timing a temperature of the battery pack and outputs a temperature signal indicative of the temperature of the battery pack; a first storage device that stores the temperature signal output from the battery temperature detector; battery temperature gradient computing means for periodically computing a battery temperature gradient based on a currently detected temperature signal and an earlier detected battery temperature signal and outputting at every predetermined timing a BTG signal indicative of the battery temperature gradient; a second storage device that stores the BTG signal output from the battery temperature gradient computing means; and minimum value storing means for storing and updating a minimum value of the BTG signal amongst BTG signals ever computed by the battery temperature gradient computing means, wherein the minimum value storing means does not update the minimum value of the BTG signal for a predetermined duration of time from a time when the power output switching circuit starts supplying the DC power to the tool to a time when a predetermined period of time has expired from resumption of charge of the battery pack immediately after supplying the DC power to the tool is ended.