I have two of these:
http://www.supercheapauto.com.au/onl...d=283446#Cross
(total 34AH).
I use a cigarette lighter-to-mains adapter on one of them to power e.g. laptops (with a powerboard).
I have one of the jump starter batteries plugged into the charging port of the other one, and have all the telescope stuff powered by the jump starter that is being charged by the other. (Does that make sense? It doesn't sound sensical in my head but it is late...).
This gives me enough juice to run the following for eight hours (exact time unknown, 8 hrs is the most I have done):
-2 laptops (one for tethered camera control, the other for running Deep Sky Stacker while on-site to save processing at home).
-An iPad
-An iPhone
-A SkyFi WiFi adapter
-HEQ5 Pro mount
-Camera
Additionally, this gives me enough power for the laptop on the way home to do DSS stuff while I'm driving. Also, if I wanted to, I could charge the batteries on the way home via the car battery/cigarette lighter... and hence get even more power. It takes about eight hours to charge one of each of them through mains power.
Unfortunately, the plugs on them aren't that durable. I bought one just after Christmas and the plug itself came out of the casing on first use. I returned it to Supercheap and got a replacement, and that one has been perfectly fine.
I bought a second one about three weeks ago. The plug on that also came out on first use (and I am going to get it replaced this coming Monday); hopefully the second replacement doesn't have issues.
If you're willing to risk getting a dodgy one and that you may have to get a replacement at no additional cost other than your time, then I can definitely recommend these.