I guess nobody has done this before.
I would use a 12 volt sealed lead-acid battery(power-sonic makes some nice ones
Sealed Lead Acid | Powersport | NiCd and NiMH | SLA Chargers | Power-Sonic Batteries) and a DC-DC converter similar to this.
12v power supply and battery index ( Price $18.50 each
Retail or Wholesale. The PST-DC292 works fine in either 12 volt or 24 volt vehicles.
PST-DC292). that link acted up on me).
It supplies 2.2 amps at 6.3 volts which should be enough. Even comes with bunch of different adapters.
Could also hook up one of those solar battery chargers they use in boats and RV`s.
Just a matter of wiring everything up. I take it that you can solder and have a basic grasp on electronics.
There is something like this also.
www.songbirdgarden.com/store/scripts/prodView.asp?idproduct=7080