I have been increasingly frustrated by my power tanks lately, mainly in that I have to use 2-3 of them just to get a full night of imaging with my scope, ccd, filter wheel, auto guider, and dew heaters all running. This, and the largest one I have was 17Ah and after multiple moves (and consequently being drained of power for months at a time during the move) they do not like to hold a charge anymore. My solution: build a power box.
I have done a full writeup of my requirements, calculations, parts, and the build which can be found in this pdf document: http://www.eprisephoto.com/power-box-project.pdf
In short, I have made a power box with a 140Ah 12v deep cycle battery, 6 cigarette lighter-type power sockets, a 7 port USB hub, and two 120v GFCI outlets which are hooked to a 450W power inverter.
I have red lights that shine to the side and down to help with navigating around the scope without tripping over anything, a red LED voltmeter on the front panel and a Battery Tender charger built in as well.
The most difficult part of the build was planning (as is typical with projects) partly as I had not done any wiring projects in quite a while and wanted to be thorough planning out the wiring and parts to map sure it will be safe, all work, and as inexpensive as possible. I went with Anderson PowerPoles for the wiring and distribution box so I wouldn't have to do any soldering and so it would make it simpler to fuse and exchange parts as necessary. Although using these did increase my costs it made the wiring portion significantly easier and, to me, well worth the extra expense. Other than these I did a lot of shopping around and made good use of eBay and other internet sites to find parts. The battery was one of the few things I had to source locally but managed to get it delivered for free which was very nice as a 140Ah battery is quite heavy.
It took a few weeks to get all the parts in, build the box, and wire everything but it is all together and running. I learned a good bit between all of the planning stages and testing everything as I was wiring it all together but it was very fun and nice to have a home-built power box!
I was told it looks like a squat little pig so it has affectionately been named the "AstroPig 140"
Feel free to download my pdf and read more in-depth about the build.