When you say "boiling", do you mean:
* Making a bubbling sound
* Really hot
The first one isn't technically "boiling", it's a chemical reaction that's pretty normal during charging. It's why you have to check the battery fluid levels frequently. However, the amount of bubbling goes up the more aggressively the batteries are being charged, so it still might be an indication that something is amiss.
If they're getting really hot, that's usually a bad sign.
The charger might be fried and stuck in bulk mode or something. Need to get something like this to find out what it's really doing: https://www.amazon.com/AstroAI-Multi...s%2C164&sr=8-8