# Little project - rechargeable pocket heater?

## Main Question or Discussion Point

I work outside, and have absolutely no circulation in my hands. And I like hair-brained schemes.

So with these things in mind, I thought I might have a crack at making a little rechargeable pocket heater. Unfortunately, I know very very little about actual electrics. But hey, that's what makes hair-brained schemes more interesting isn't it? :P

Anyways, I was wondering, how would I go about getting something nice and warm using some rechargeable batteries? I've heard that resistors produce heat as a result of, well, resisting. But I don't know how much? Could one warm one's hands if one made a circuit full of resistors (and maybe a little LED so you know when the batteries are dead or dying)?

Would the batteries die too soon for this to be a realistic option? What would you suggest? I'm loath to get one of those annoying heat pack things as I find they only last about half an hour, or get *too* hot (one virtue of an electrical appliance - it has an off button).

## Answers and Replies

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f95toli
Science Advisor
Gold Member
Would the batteries die too soon for this to be a realistic option?
Yes, this is why hand-warmers tend to use "direct" chemical reactions instead. There simply isn't enough energy in a small battery for electric heating to be useful (unless you are willing to carry around a car battery).

mgb_phys
Science Advisor
Homework Helper
You can buy battery heated socks that just use a D cell.
Otherise 4 AA rechargeables should give you 5V and around 1000mAH of capacity.
Assume you want power for say 5Hours you can take 200mA.
R=V/I so 5/0.2 = 50 Ohm resistor Power = I V = 0.2 * 5 = 1 Watt

Probably not worth it even inside gloves, of course you can just half the resistor to give twice the power for half the time, I probably wouldn't use more than 400mA - you risk damaging the batteries.

Danger
Gold Member
W bought me an electrically heated vest for Xmas. It uses two battery packs of 4 AA cells each (individually switchable), gets nicely warm, and goes for several hours. The heating elements appear to be high-resistance wires that are looped up and down through the fabric, but I'm not going to take it apart to check for specifics.