Heres a couple examples but these batteries are available in many sizes.
Lawn batteries from what I can figure are usually around 15 AH batteries. These batteries are twice that which will give you a lot of extra power for running pumps. Still have to charge when needed.
I can't figure why mine isn't charging. I replaced the stator, flywheel and set the air gap as best as I could. Machine runs fine but I am only getting 2 to 3 volts AC out of the stator at 3000RPM. I should get ten times that.