[Tannaton]

If you really do need an additional battery, I guess you are looking at a split charge system which is commonly used for caravans, except the auxiliary battery will be in the car. There are a lot of pre-wired kits available to do this on e-bay, which will have the voltage sensing relay and fuses etc. All I would say is don't get a cheap one from China.

But do you really need a extra battery? If you run the fridge whilst the car is driving then that will always be fine, when you are stationary and the car is locked on sunny days when the internal temperature of the car gets high, most fridges wont cool anyway and you may be better off with a larger coolbox and lots of gel packs.

But if you do fit a second battery, there is a lot of room in under the boot floor besides the main battery, even if you can't find a large battery to fit you may be able to fit a number of smaller batteries - but stick with one larger one if you can as they are more efficient and cheaper than several smaller ones.