It's best to get another battery, also sound like with the three wire,you have 12 and 12 plus ground and when you select either 12 or 24 what you have is it take all 3 wires to make 24v and then on 12 it shuts off one wire. This make the wiring to motor different than normal 24v. I know that was the way of old style was. Some of these guys can draw you a diagram,don't know how to draw a picture on computer.