potential problem???

[Alastairm]

over the last couple of runs in the Cerbera i've noticed when it is idiling for a minute or so when warm that the revs tend to dip a little-like it's trying to cut out but does not. It's particularly apparent with the lights on, when i switch them off the revs rise-could this be a sign of the batter on the way out?? if so is it easy to switch batteries and is there anything that i need to look out for? Plus which battery do i need???

Thanks in Advance.