We had some of these that were a few years old at best and as to why they worked well they weren't the best. We bought them, new ones, and damn, they made a difference! They heat up/heat up faster (depending on ambient temperature, "room" temperature) and last longer. We mainly use them for snow activities (with ski gloves) and they work great. Your hands/fingers stay warm even on frosty days. They're expensive, but at less than $1 a pair, it's worth the investment. I recommend these heaters!
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