Like Steve, I built my astro power supply from a deep cycle marine lead-acid. It is 110 amp-hour I think. There is nothing like just loading up the power supply and going forward with no power worries.
I keep it on a battery conditioner/charger between uses. My setup includes many different output connectors, including high output USB jacks, etc. Although anyone starting out should [probably employ Anderson power pole connectors, for their main power use, I used the venerable and terrible car cigar lighter adapters. I also have a metering system that displays the voltage, current, watts, and amp-hours drawn. Its really handy to have that information, especially the amp-hours so you know where you are.
These big batteries are really overkill for the drive mechanisms but are pretty much a necessity for dew heating, TE cooling of cameras, charging lap tops, running cameras, charging phones, operating a local wireless router...all the danged electrical equipment you might need in the field. Also, since the supply is deep, I keep a 12-120v inverter just in case there is something that needs 120...I don't like to use it, but at least efficiency loss is not a disaster given the size of the power tank.
My battery has lasted over 5 years, and shows no sign of giving up anytime soon.
Instructions for building these things are all over the internet.
Wes, Southport NC
PMC-8, ES ED 127, 10" LX200GPS, Astro-Tech 8" Newt, ETX-90
Polemaster, Orion ST-80 and SAG
Electrical Engineer, Retired