Being in a yak you would want a small size and light......I've just been looking into Lithium Ion batties to run a fridge in my car.......Li batteries seem to be getting much cheaper and more available with better run down rates and shorter charge rates.........Pretty much a 40ah Li will do what a 80ah AGM will do at 1/4 the weight......Still not cheap but definatly better technology.........40ah LI can be bought for around the $350 mark if you shop around on the net and would weigh about 6kg compared to a 80ah agm that will weigh 25kg and the Li will have a much longer service life......in the thousands for re-charging.
Google LiFePo4.......check it out.
Dan