I've found a cheap car battery will do fine *as long as you don't run it down too far*. Car batteries are designed for rapid discharge, but being kept well topped up, while leisure batteries are designed for slow discharge but being fully drained. In practice, that means a car battery is fine for one racer, but you're probably pushing it for two. Still, if you're short on cash you an always get a car battery, run it until it's busted, and get a leisure battery later.
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