4 Blown batteries (pics) How??

OK, these are not exactly RV batteries but a battery is a battery.

These 4 small 12V batteries are in a box, hooked up in series to provide 48 volts to run my wifes Gio electric scooter.

She went to use it yesterday and it would barely power up. She put it on the charger overnight, as she usually does to charge the thing and the next day it still would not go.

My son has one too, so we swapped batteries and all was well. The scooter ran fine.

Then we pulled the battery pack out and removed the lid. This is what we found:







All 4 batteries are severely bulged and melted. Needless to say they are toast. (pardon the unintended pun)

I am wondering what the heck went wrong here.

The charger is a typical little charge box that you plug into the scooter to charge the batteries up. It likely has a sensor to shut off the charge when the batteries are full.

About a year ago, the charger ceased functioning. I took it apart and found that there were 2 connections internally that had broken off. Cheap Chinese junk! I soldered them back on and all was well until this happened.

Any ideas on this?

I am thinking maybe one battery packed it in so the charger wouldn't shut off and severely overcharged them.

Or maybe the charger malfunctioned (cheap Chinese junk) and applied full voltage for too long, thus overheating them.

I haven't tested the charger - I don't really know how I could.... I could check the output and thats about it.

I am amazed they never exploded!