Check out these Sanyo Rechargeable Hand Warmers. I think I'll be buying at least one of these in the 2-sided warmers. It'll help to last longer out in the cold for sure. I'll be swapping rechargeable batteries with three times the mAh of the Sanyo eneloop batteries that come with it though. The eneloop are only 1000mAh or so and there are batteries with over 2500 mAh and over in easily recognisable brands. The hand warmer should last three times longer per charge. That's a minimum of 3 hours on the highest warmth. Yeeayyeeeah!