To me, it sounds like you basically want a way to generate power. Generators that turn physical motion into electricity (such as hand-cranks) are one option. Solar power (as mentioned by highfigh) is another option, although that would be non-ideal if it's overcast due to stormy weather or at night. I also like highfigh's suggestion of a car battery, especially if she's going to be stationary at her house.
Heck, if she's going to be at her house, have you considered a gas-powered generator? You mentioned that it must run on batteries, but maybe that's a constraint that you can lift?