When you use batteries for Christmas lights, and you just remember them, then you are in trouble. The rechargeable NiMH batteries don't like being abandoned in candles and lights. They drain too much and then the charger just blinks.
I was using a car battery, but just one second, and you feel that the battery is about to blow up. Do not do this at home, kiddies. I just found that an old 12V charger works just as well. Hold pos to pos, etc, for 6 seconds, and then they work on the charger. I never found there was a problem afterwards.
I just use rubber gloves to hold the wires to the batteries. All safe and dandy. I think you are just putting a few electrons in the anode, to make the battery conductive again. Then the charger can take over.