Number 1 rule when working with batteries is: Disconnect the ground (-) FIRST and reconnect it LAST. That way that problem cannot happen.
One of my first calls as a paramedic was to a mechanic who had essentially blown off his ring ginger (burns) when his wedding ring touched the chassis while that hand was holding the wrench handle as the working end of the wrench was on the + terminal. Both the finger AND the ring were damaged so badly neither could be salvaged.
I also remove my metal strap wrist watch. (My ring will no longer come off!)
