I'm having some problems with my PC's uninterpretable power supply. It came with two 12v 9Ah batteries that aren't charging anymore so I've decided to replace them with higher capacity batteries that are 12v 12Ah. The old batteries were connected in serial (plus to minus) but if I try to connect the new ones the same way it produces a sparks when connecting the last UPS wire to one of the batteries. The spark is about as big as the one that ignites kitchen gas burners, not too big but enough to startle me like a movie jumpscare. I thought the capacity doesn't matter as long as both batteries are 12v. Also they are bigger in size and don't fit the UPS case so I have to use them externally. I know nothing about electricity and, to be honest, it scares me. Any help will be appreciated.