Fried electrical system?

I took the battery to the store and talked to the store manager. He wouldn’t admit anything despite their test printouts before and after the recharge showing the battery was good. More so, he tried to BS me when I showed to him, with my own voltmeter, the reading was -12.54 V, saying “See, the battery is bad.”. That after saying the battery was good, according to their own test printout. I don’t think he even knows what polarity is. That or he’s playing really dumb.
Anyway I realized I’m losing even more time with them and left. I’ll take it with their corporate management tomorrow.
In the mean time does anyone have any hints where I should start testng what’s fried in my system?
Thanks.