I recently thought I had a dead battery for a drill. When I called to see
where the service center was located, they told me to bring the charger for
testing. The charger had been fried, most likely from a power surge. It then
shorted one of my two batteries. Had I left both batteries in the charger
for an extended period, both would have been fried.

Some of the independent battery stores can test both your charger and
battery.