Infinite Power Solutions have come up with a range of tiny, thin film rechargeable batteries called Thinergy Micro Energy Cells. The technology in Thinergy lies between rechargeable batteries and ultra capacitors and has some of the benefits of both systems. The Thinergy cells can be made incredibly small, down to 12mm² and 0.2 mm thick, i.e. half the size of a postage stamp and about the same thickness. They recharge to 90% in around 8 minutes and can be deep-cycled in the order of 10,000 times, or shallow cycled 100,000 times. Best of all once the micro energy cell is fully charged it can be stored for years and still retain that charge.
Recharging the micro cell is also very easy due to it accepting any current in the range of tens of nanoamps up to milliamps. All it requires is a constant 4 volts. This is very useful if you want to build a system that recharges using low-level energy harvesting say from vibrations, walking, car travel or very low level photovoltaic systems. You can probably expect these to start popping up in smart cards, RFID tags and even implantable medical devices.