I used to raid scrap yards for batteries out of cars when I needed them so I would avoid a core charge at the auto store. The scrap yards would just let me have them so they wouldn't have the lead and acid hazmat.

If you get a few to wire correctly they should be able to give you the power you need if you just put in a regulator. Most batteries are "dead" because they just can't produce the needed amperage or pressure to turn a starter. Much like how you could run a radio for a crazy amount of hours even if your car won't start.