Reversed battery polarity

[Joopa]

Finally a chance to use the most common Star Trek phrase about switching the polarity

I was overseas for a month and forgot to disconnect Safari battery cables (24 volts, twin battery system) before leaving. When I got back and tried to start the car it was completely dead, no interior light, clock, nothing.

Hooked up a charger which refused to start charging so I picked up a multimeter and checked voltage on each battery.
Right one (negative on ground) was down to 9 volts, left one (negative on other battery's positive) had -3.4 volts

First time ever I've seen battery polarity to reverse. Managed to get it back to life by first completely draining it out with a bulb and wires. Then disconnected all battery leads and hooked both batteries together with jumper cables connecting + to + and - to -
After that I left it to trickle charger overnight and now it's going just fine.

Must have been that connected leads drained the left battery completely and then positive voltage from right battery caused polarity to change.

[diogenese]

I have seen ( and used) a pair of very expensive deep discharge batteries that were on a bomb disposal truck in the UK swap polarity. they were connected to some sort of smart charger that wasn't so smart and did something weird to them.
They wouldn't charge up properly again but worked fine as house batteries in my camper but wired up back to front!
never seen or heard of it before till now

[Rotazuk]

The battery in my MR2 did it after being away , the alarm was slowly draining it . Only had about neg 2 volts thou . Put it on one of those cheap chargers and after a day or two it was fully charged and good as gold . That was about two years ago now and its still going fine .

Chris