AH = Amp Hours
If you pull 1 amp per hour, how many hours will this battery run for before completely flat (not a good idea by the way!) Rule of thumb is only take them down to half charge.
e.g. a 100AH battery can provide 1 amp for 100 hours or 2 amps for 50 hours, at ~12v, or 2A for 25 hours if you are going to stick to the half charge rule!
The voltage of a battery varies depending on the charge remaining in the battery so the voltage produced by te battery can be a rough indicator of the charge available in the battery…
There is not much point writing detailed descriptions of Deep Cycle or Leisure batteries when there are so many good descriptions out there. Here’s a fairly detailed one if you want to read up.
Please be aware that some batteries can emit hydrogen when charging so the space they are stored in should be vented!
OK so I compromised and we didn’t go for a gel battery as I would have liked but on the plus side I managed to get a 110AH Sealed Battery for £65!!