This is the rechargeable hand warmer that I bought for gness (and then another one for her daughter, as she was so impressed) :
Amazon.co.uk User Recommendation
It costs pennies to recharge and lasts for a good many hours, depending on which setting you choose, and also the current ambient temperature.
There are a few reviews complaining about short run times - but they tend to be people using them continuously on the highest setting in cold outdoor situations - gness uses hers to get her hands nice and warm and then intermittently to top up the warmth - she charges every couple of days in winter.
This is the third winter and it still charges fully and works as well as when first bought.