http://www.tsksupply.com/categories/Heat-Packs/
There are many types of heat packs, I always have a selection on hand and replace them when they expire. The little hand warmer ones are quite short lived. In my situation I have a battery back up good for around 8 hours maybe a touch longer. Then I have a catalytic heater and propane for 40 hours or so. After that 72 hour heat packs for my cooler holding system and enough for 9 days.
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