I have no technical knowledge of these but I'm sure I remember Hamwich saying how good they were - maybe worth a pm.
I bought a 13600mAH job from Amazon a couple of years ago, it cost around £55 back then. Truly excellent bit of kit. It will start a 2.2 litre diesel 3 or 4 times before needing to be recharged. You can use it as a power source for laptops, mobile phones and so on. Mine doesn't seem to be available any more but
this one has got a higher capacity for less money.
My recommendation would be to go for the biggest AH capacity you can afford. They really aren't very big, mine's about the size of a paperback book. Even in its zipped case it will fit easily into an odd corner of the car.
