Don't laugh! No, I didn't do that myself. But I was stupid enough (?) to buy a motorized electric wheelbarrow thingy at a yard sale (cheap). 2 motorized main front wheels (with separate motors) and one swiveling rear wheel. It seemed to only go in reverse, and strongly so - but that was in the fast-forward switch setting. Also, the charger they said went with it was clearly not the original, as it says "Linksys" on it (tiny switching supply, 12V, 0.25A or so).
After taking it home, I found that it runs on a 12V SLA battery of about 20 AH size. Checked the battery voltage. About -12V. That is, the polarity is reversed! The positive side of the battery was connected to the red wire though. Checked the polarity of the "charger" as is appears on the wires leading to the battery, and sure enough, it's reversed: negative to the red wire.
So, if the 12V SLA battery was charged in reverse, all the way to -12 volts, should I:
* keep using it that way, or
* discharge it to zero and then charge it the right way, or
* throw it away?
This wheelbarrow is an old model and the control circuit is all electromechanical relays, 6 of them. So the good news is that probably nothing got fried from the reverse polarity battery. The bad news is that something is not right with the relays, in particular, in the normal-forward switch setting I hear a click but the motors do not turn. May have to replace a relay or two?