Total #rookiemove on my part - it was late, dark, I'd shovelled snow for an hour then decided to try and jump my car that had been sitting in the garage for at least a couple months without being started.
This is a 2008 Mazda 3, afaik the battery is 10 years old. I haven't driven the car for a couple months and it wouldn't turn over. I used jumper cables attached as follows, in order:
- Mazda 3: correct polarity directly on battery terminals
- Running car: negative clamp on positive terminal, positive clamp on remote clamping tab
With lots of sparks on the final clamp, I attached the cables for ~5 seconds before I noticed. I didn't attempt to start the Mazda with incorrect polarity. I corrected my mistake and at this point I didn't hear the starter on attempted ignition.
Fortunately the running car has continued to run without issue. I returned and checked every fuse on the Mazda 3, all intact. I don't know how to test the relays. My multimeter read 3.2VDC on the battery. The little security light in the console was still blinking, and the cabin light came on very dim.
At this point should I buy a charger and try to charge the battery? Would it be dangerous to do so indoors (due to possible damage?) Should I replace the battery? Are there any tests that can be run on the battery? Should I verify other components?
Also: how should I maintain my battery if I won't be driving this vehicle often?