Nokia have filed a patent for a new type of mobile phone battery which they are currently developing. The new battery uses a Piezoelectric Kinetic Energy Harvester, so basically it will use some sort of kinetic energy.
Nokia Developing A Self Charging Mobile Phone Battery
The battery will work in a similar way to kinetic powered watches, which uses movement to charge their batteries and power the watch.
It is not clear from the patent application how far Nokia have developed the technology so far, or whether it will be used to fully keep your battery charged as Nokia has indicated that the technology may be used to partially recharge the battery.
This certainly would be a great development in mobile phone technology, even if it can only be used to partially charge a battery it would add extra battery life and cut down on electricity use through recharging.
via TFTS