Modding a charger into a power supply + charger?
You've not said anything about current requirements or available space/weight constraints, but I'd suggest looking at 18650 size cells. They're now readily available on the Internet, have standard chargers available, decent capacity, and a nominal voltage of 3.7V.

Stack four of them in series and you have a nominal 14.8V pack. You can get preassembled packs and chargers designed for use with R/C models, where this configuration will be described as "4S".