It would be reasonably easy to modify many headlights for your purpose.
Use NiMh batteries and plug the battery into the 12V (when not in use) with a suitable dropping resistor. Set the resistor so the charge rate is low.
You will overcharge the batteries but they will still give a reasonable life and are cheap to replace.
You could develop a more complex set up but at much greater cost and time.
Cheers John
