It's in the butt end. You must unscrew all the black screws in the lower handle. Remove the black heel. Then unplug the Red wire. Carefully pull out the battery pack. Then unplug the black wire. I got a battery tester, put black to black, red to red and check the charge. Not sure your charger is bad though. Be sure that the plug end is FULLY seated into the unit. If it's not fully down it wont recharge. Also you know it's time to replace the battery when it wont hold a charge for longer than a couple weeks.