I am typically not a fan of the bulky external back up batteries because, well they are bulky. Not too mention they require you to bring your charging cable along for the ride as well. But if I lived in a place with cold winters I might consider this double duty battery and hand warmer.
The EnergyFlux Rechargeable Hand Warmer / External Back Up Battery Charger is a cool gadget. You can expect around 4-hours of heating time but only 1-amp of output charging power. If you live in Alaska you can get your hand warmer / backup battery here. If you live in California then you should still stick with the EagleTec ONTOP solution here.