the 2 batteries are hooked together to get 12volts. any 12 volt battery will work eg. car battery( too heavy)
motorcyle or garden tractor, computer batteries, cordless drill (12v). I bought a skill cordless drill, has 2
batteries, and charger. I used a pcv pipe collar, drilled and tapped 2 holes, put bolts in to hold wires from
fishfinder. Runs all day no problem.