Funny you should mention that
I just got home from Canada and such and about 6 vehicles here have dead batteries for some reason. They only sat un-used about 2 months but are totally dead, not just low.
Anyway I used that little Xantrex power booster plus box I have and started them all. A couple I had to jump more than once. I think when I took the battery out the other day it is only a 20amphr, and I think it is a sealed lead acid deepcycle though not sure, it could have been an AGM. I tossed a rod on the aircompressor and the inverter is acting up (probably from running in the rain a few times), but it's still a great booster pack.
Anyway, 20 amp hours does alot of starting of engines!! V6 3.3, Jeep straight 6, V8's, etc..
Always nice to have those jumper boxes handy
At one time I saw a formula for close geussing CCA to amp hours but I forget what it was now since I never use it. I don't think it was right anyway, to many varibles in the way batteries are manufactored for cars. Some are good and last forever, others die an early death, both are rated like 650CCA, both are well cared for. Has to be a difference inside the case that makes them die early and that probably effects the amp hours available also. I would be willing to bet the better built 650CCA battery that lives long would also provide far more amphours than the junk one that dies young, even though they are both rated the same CCA.