Forklift batteries! They're often available cheap, used from scrap yards; sometimes with much more life in them than you'd expect. They might just need more ele

Forklift batteries! They're often available cheap, used from scrap yards; sometimes with much more life in them than you'd expect. They might just need more electrolyte. They're designed to be durable under harsh uses. They're often separated cells for easy maintenance and that makes it possible to hack them into different configurations. Hacking high energy DC systems is perhaps a little more responsibility and risk than was desired tho.