REI Garage offers the Celestron National Park Foundation Rechargeable Power Pack/Handwarmer for $20.73. Opt for in-store pickup to dodge the $5.99 shipping charge. That's the lowest price we could find by $9. The 4,400mAh rechargeable lithium battery provides up to 6 hours of continuous heat on a full charge. It includes a USB charging cable, carry case, and carabiner.