Both 12 or 24 volt opinions made sense to me so I used my volt meter to try both ways and to my surprise it showed 12 volts both ways. I measured all three batteries and got 12 volts even when I tested the two trolling motor batteries hooked together. I tested the trolling motor power plug at the front of the boat and it showed 24 volts. I'm still scratching my head, but those were my results.