I agree with Alan,

Pilgrim uses 2*12Ah SLA batteries so I can have 24v for speed (series) or 12v for endurance (Parallel).

If you are using just the lights head movement ETC then you can expect 1 * 12Ah SLA should last a long time.
If the current load is low they seem to go on discharging close to the 8-12Ah, so 1 amp load 8 hrs use.
Cheep to buy and easy to charge, when I use Pilgrim I have one set on charge and the other in the robot plus a spare set.
The only thing I would say is make the battery easy to remove, so swaps are no big deal.