It's a nice idea and would be useful for those who plan touring and camping by bicycle.
Another idea would be to have an option to charge a battery pack instead of the phone directly. This way the user can plug the battery back into the mobile when needed or use the battery pack to charge other devices, such as a GPS, MP3 player, etc.
For example, some solar powered phone chargers have their own battery pack, so the user can leave the solar charger in the Window continuously to collect and store the energy as the sun comes out. The user can then charge their phone off the battery pack from its stored power, even when cloudy, wet, at night, etc.