+1 what Allan said, those power tanks from Celestron/SW are really pricey for what you get, there are lots of other options.
A while back I bought a Fusion AGM battery, I got the 55Ah for $180! I got that capacity so that it'd have enough juice to power my scope and dew heater, even my little netbook for a long weekend without any worries. If you search for that type of battery, you'll find many capacities around, just get one to suit your requirements. Assume the scope will use 0.5-1.0A, then another 1-2A for the dew heater, and extrapolate that out for a full night, if that's all you need. It soon adds up
I use my battery even when at home, as even the best types of batteries like to be topped up at the wall regularly, so I figure it's a good way to keep on top of that.