If you have access to cheap deep-cycle 'leisure' batteries, you're gonna be ok!

A thought: if you are going to be carrying 2 batteries in any case, you could consider wiring them in parallel - they'll then last twice as long without having to swap them over, but - more importantly - each will only suffer half the discharge of a single one.

You'd only have to join the second one with low-amp wires - fit a switch in the line and turn it on after you've fired up the car using the starter motor!

DA,
would I be right in guessing that you've never owned a motorcycle, which might explain why you haven't got hold of the concept of a reserve supply of fuel (or in Brooky's case, a reserve supply of sparks)?

Haha! Quite apart from which, I haven't got a starter motor! Everybody should try hand-cranking a 990cc V twin at least once. Get it wrong and at best it will hurt a lot. At worst it will break your wrist and knock your knee cap off if you're standing too close. That kind of potential carnage tends to concentrate the mind somewhat! It also makes me think I'd quite like a starter motor.

Brooky.

... in which case you could run your low-amp wire to wherever your spare battery is stored on the car and it would - sh!t, gotta run - Ken's back...