This project starts with a battery in good shape. I have used a small lawn tractor battery from Costco ($39.00). It worked OK until I managed to scavenge a 12-volt 55 amp-hour sealed battery for free (sealed batteries are ideal for inside use, no off-gassing, safe). The next step, find a 12-volt DC Power Outlet Adapter with alligator clips ($9). Normally features a 5′ long cord and a built-in fuse. In a pinch, you could use an extension cord with a designated positive and negative wire, but the DC power adapter will be safer. Clip the corresponding colored alligator clips to the battery terminals (red to red, black to black) and insert a 12-volt USB charger into the round outlet. You are finished! You have created a simple charging system. When not in use, just unhook the alligator clips for the battery terminal.
If you want to charge something bigger, like a laptop or a small LCD TV, look for a 12-volt power adapter with an AC plug on it. It can easily fit into the outlet. I sourced a 100-watt Black and Decker Power Inverter for $19.00 at a local big box store. This particular model does not have an internal fan, so it doesn’t waste power when the electronic device is not in use. It also has a light on the unit that indicates the battery charge. This micro system works great when you might need to charge the main battery bank with a small generator/charger, but the kids still want to watch a movie. It also ran my wife’s MacBook for a whole day before she replaced her aging laptop battery.
Black and Decker 12-volt power inverter: http://www.blackanddecker.com/en-us/power-tools/automotive/portable-power/120-watt-power-inverter/pi120p
12-Volt DC Power Outlet Adpater: http://www.harborfreight.com/12-volt-battery-to-lighter-socket-extension-cord-66407.html
12-Volt Sealed Battery: http://www.harborfreight.com/12-volt-10-ah-sealed-lead-acid-battery-62586.html