So the battery in the back was overcharged? How's that possible if the amp/amps were used to run off that? Shouldnt the front one be more vulnerable to exploding?

2x on the relay, if buying the same battery or buying two batteries is not an option.

WRX how did you go about finding the right belt size for the 2 alts? I thought about doing that when I can get hold of brackets and a new alternator.