Orginally 12v positive but there have been discussions in the past and the conclusion is, it will work on either pos or neg aslong as in series.

Now dumbies guide;
hmmm?
2 12v batteries. A and B
Wired in parrallel gives 12v - wiring is A pos to B pos, A neg toB neg.

2 12v batteries. A and B
Wired in series gives 24v - A pos to B neg, A neg to B pos.

Basically Series means going through one and then into the next.
Parrellel means side by side

I am sure I have made a dumbies guide confuzing