I am making a sock warmer. In the future, I plan to wear the sock, and I do NOT want it to explode.
I can understand why you would want to make versus buy a sock warmer, especially after looking at this
Google search result. These things are all over the map in terms of quality and cost and battery lifetime. If you are going to use these socks on the road to keep your feet warm, then using power from the vehicle to recharge the battery should be an option. Batteries are heavy, so if this is for outdoor use while walking or sitting (in a tree stand while hunting, for example) a well-insulated pair of boots or thermally insulated outer socks is advisable to keep power consumption to a minimum and thereby prolong the available working time from battery power. You also need some way to keep the battery warm for outdoor use. Battery capacity drops rapidly with decreasing temperature because the internal chemistry slows down. A belly belt with the batteries mounted in custom pockets on the belt, along with an insulating outer garment will allow your body torso heat to keep the batteries warm and operating at full capacity.
I am diabetic and my feet get cold while sleeping because of poor blood circulation. Insulated socks help some, as does my heated waterbed. but a pair of heated socks that I can plug in beside the bed would be a wonderful thing to have... if I didn't get tangled up in the power cord. A heating pad sometimes helps, but it is usually nowhere near my feet come the next morning.
The battery in my vape probably only has sufficient capacity to provide less than a watt overnight, but it re-charges fairly quickly from a 1.2 A USB charger. You should probably consider purchasing a pair of batteries, so one is always charging, but also purchase a separate charger. If you size the resistance wire properly, say two to three ohms at 3 V, you should be able to connect the heater directly across the battery terminals without having to use a discharge regulator and draw 1 to 1.5 A from it to provide 3 to 4.5 watts for about an hour, but probably less. Even better IMO would be more resistance and two batteries in series to provide at least 6 V, The power available goes up as the square of the voltage (P = E² x R), so 6 V will give four times more power than 3 V for the same resistance. That means you can increase the resistance by a factor of four (use smaller diameter wire) and still get the same heating effect. I am assuming you intend to hand-thread the wire through the toe-end of a commercial pair of socks. Smaller wire means higher resistance and better overall coverage of the heated area. Some experimenting is required.
You will need to purchase stranded
insulated heater wire. Here is a
Google result page with sources for that kind of wire. Also
see this page for one manufacturer.
I think even better for your purpose would be Litz wire, which consists of multiple strands of very small gauge, independently insulated, copper wires. Litz wire is not normally used as heater wire (the copper wires have low resistivity), but it is much more flexible than a single wire or a stranded wire of equivalent diameter. Perhaps you could separate the strands at each end and just use only enough of them to obtain the resistance you need. I am not aware of anyone who weaves Litz wire using heater wire, but it never hurts to ask around. Another possibility to increase flexibility is a layup of very fine strands of heater wire under a thin conformal insulation coating.
To give you some idea of what you are working with, my vape box power supply drives a 0.52 ohm kanthal coil at 20,0 watts. The box is rated to deliver up to 60 watts, depending on what coil is used, but all I need is enough power to simulate a cigarette, not fill the room with clouds of white vapor as some are wont to do. Even so, the box has circuitry inside to regulate the power delivered and safely recharge the battery. The battery does get warm while in use at 20 watts, but the duty cycle is low. Your application is closer to 100% duty cycle and may require sensing the heater resistance as a way to measure sock temperature, or you can include a thermistor (or several) to get a measure of average sock temperature. Use that information to control a pulse-width power modulator to optimize battery life if you want to go that far.
Neat project. Please report back on your results, what heater wire you chose, what battery or batteries you used, how you delivered power to the socks, how long the batteries lasted and at what environmental temperature. If you decide to implement temperature control and need some help with that, please ask here. I am sure there are dozens of people here who are willing to help.
Bottom line is this: a pair of heated socks will draw current similar to a low-end box-mod for vaping and use a "18650" lithium ion battery delivering about 3.7 V fully charged with about 3000 mAh capacity. That's theoretically 3 A continuously for one hour, but don't expect that and don't continuously discharge at that high rate. The vape sites do not recommend using two batteries in series (probably because they can't safely be recharged in seires), but as long as you use a separate charger for each battery, I don't see anything wrong with using two of them in series. It would allow you to use less current for the same heating effect, but your mileage (or kilometers) may differ.
The 18650 battery with IMR chemistry is commonly used, but these require protection circuitry to prevent excessive current draw leading to possible explosion or meltdown. And they require special circuitry to safely recharge. Do some research on vape box-mod batteries and find out what is safe and how to use them.
Here is a website to get you started, but they ask that you to click-to-verify that you are over 21 before allowing access to their site. I did that. So far, so good.
