Hi I'm off to France in June fishing without thre caravan but goig to take the bivvy (tent), and was woundering if i take a 240/12v fridge and my 85amp liesure battery, how long the battery will last. the fridge I'm going to borrow hold about 6-8 cans of beer, not that I'm taking beer but a bit of butter to go on the toast would be nice.