Since we've gone most of the northern winter without an answer, I'll try to provide something useful despite my lack of experience.

TL;DR: They'd need about a 14000mAh battery to heat as well as an oxidising pouch.

The above is only an estimate, based on a lot of assumptions, but it seems consistent with the reviews I've read.

Sellers of hand-warmers don't generally give ratings on their heaters, but they do give the battery capacity so it's possible to calculate the maximum power output and compare it to other heat sources.

Note: Some warmers come in pairs (one for each hand) while others are sold singly - to keep things simple I'll consider the total capacity for the pair if it's sold as a pair.

Electric

The first few electric hand warmers on Amazon claim a battery capacity around 4Ah (=4000mAh) and a heating time of "up to" 8 hours, implying their lowest setting averages half an amp. They use lithium batteries so probably run at 3.7v.

0.5A*3.7v = 1.85W. That's comparable to a bright bicycle light with a filament bulb, which doesn't seem very warm to me.

Going up the price range a bit, UK£25-30 will get one with 14Ah of battery. That's enough to give out 6.5W for 8 hours.

For comparison, the same price will buy one of the fuel burning heaters below, or around 40 of the oxidising pouches.

Iron-oxidising pouches

The heat output of these pouches is a popular chemistry homework question, but most of the answers are per gram of iron. Estimates of the iron content vary a lot, and so does the pouch size. This paper puts one brand at 50-60% iron by weight; most sites assume less than that. The first few pouches I found on Amazon are around 50g each, so I'll guess at 25g of iron.

This example gives a figure of 81.2kJ for 11g, which gives us 185kJ per pouch.

If it lasts for 10 hours (a common claim), that gives us 185000J/36000s = 5.14W. At that power, a 4Ah handwarmer would last less than 3 hours, and a 14Ah job would match the pouch's 10 hours. Either of them would be heavier than the pouch, though.

Liquid fuel

Per Google, a liquid-fueled Zippo hand-warmer will last for "up to" 12 hours (43200 seconds) on one US fluid ounce (29.5ml) of fuel. I can't find a reliable value for the energy density of lighter fuel so I'll use a commonly-available figure for petroleum: 33.5 kJ/ml. I think this will be either spot on or a slight over-estimate.

That gives us 33.5*29.5 = 988 kJ over 43200 seconds = 0.0229 kW or 22.9W. (I have an in-car electric blanket rated at 50W, so 20W+ for a hand-warmer is high but believable.)

If a 14Ah hand-warmer matched that heat output, it would last two and a quarter hours (assuming 100% efficiency). The 4Ah one would run for less than 40 minutes.

The bigger electric one would almost certainly be heavier than the fuel burner.

Conclusion

It looks like electric hand-warmers should last as long as iron oxide pouches and give a similar amount of heat, provided you get decent ones. Being rechargeable, they would probably work out cheaper than pouches if used a lot.

They can't yet match liquid fuel burners on time and heat output, at least not for the same price, and they can't match either technology on weight.

They do have one advantage over pouches and burners though: to paraphrase an old Nick Park animation, electric heaters are easily turn-off-and-on-able. Both the other kinds just keep "burning" until they run out of fuel.

posted 7 days ago

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Pastychomper‭

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