Hi there!

I am new to electronics, and am trying to create some DIY ski boot heaters using AA batteries and 30AWG silver-plated copper wire with TFE insulation.

My problem is, the batteries themselves are heating up, leaving the wire only warm at best. Here is what I have done so far:

The resistance of the wire is 0.1 ohms per foot, so I calculated that 7.5 feet of the wire would be 0.75 ohms resistance, and with 6 volts from 4 AA batteries, would draw 8 amps. This should give me 48 watts of power, similar to other heated clothing options online.

However, this setup only made the wire slightly warm, and the batteries - Amazon Basics brand - became too hot to touch. I then tried Duracell Quantum AA batteries, and they didn't get as hot, but the wire was still barely warm. I also tried reducing the wire length to only 1 foot, but that didn't seem to change anything.

Any thoughts on where I am going wrong? My only idea at this point is to increase the length of the wire so I am sure the wire's resistance is higher than that of the batteries, and possibly to increase the battery power (number of AAs or use a C or D) I am using.

Thanks for the help!

 

Hi there!

I am new to electronics, and am trying to create some DIY ski boot heaters using AA batteries and 30AWG silver-plated copper wire with TFE insulation.

My problem is, the batteries themselves are heating up, leaving the wire only warm at best. Here is what I have done so far:

The resistance of the wire is 0.1 ohms per foot, so I calculated that 7.5 feet of the wire would be 0.75 ohms resistance, and with 6 volts from 4 AA batteries, would draw 8 amps. This should give me 48 watts of power, similar to other heated clothing options online.

However, this setup only made the wire slightly warm, and the batteries - Amazon Basics brand - became too hot to touch. I then tried Duracell Quantum AA batteries, and they didn't get as hot, but the wire was still barely warm. I also tried reducing the wire length to only 1 foot, but that didn't seem to change anything.

Any thoughts on where I am going wrong? My only idea at this point is to increase the length of the wire so I am sure the wire's resistance is higher than that of the batteries, and possibly to increase the battery power (number of AAs or use a C or D) I am using.

Thanks for the help!

The internal resistance of the 'AA' is about 1.5 ohms new. 4 in series is 6 ohms. The current at startup will be less than 900mA, not 8A and will drop rapidly as the cell voltage drops. The batteries will initially dissipate 4.7W and your wire will only dissipate 0.6W. 'AA' batteries are not going work in this situation unless you put the batteries in your boots -- and then only for a short time.

 

Oops...I shifted a decimal place:

Internal resistance of 4 'AA' = 4 * 0.15 = .6 ohm

Initial current: 4.4A

Initial power dissipated by batteries: 11.8 W

Initial power dissipated by wire: 14.5 W

In any case, power to the wire will drop quickly and won't be available for long.

Edit: And, internal resistance increases at lower temperatures. If you don't mind your boots catching fire or exploding, try lithium cells.

 

Your batteries are way too small.
AA batteries have a capacity of perhaps 2 ampere-hour so, even if they could supply the current (which they can't), they will only last a few minutes.
Even large D types would last much less than an hour at 8A of current.

So what type of batteries does these "heated clothes" use?
48W for any length of time requires a high capacity battery.
Likely they are some high energy lithium ion types.
That's what you need.
Look up Li-ion batteries to see their voltage and ampere-hour capacity and determine what you need.

 

Thanks to both of you for the quick replies! I can see now that I will need some different batteries to make this work...

The heated clothing I was looking at was for motorcyclists, so would be plugged into a 12V battery - which obviously wouldn't run down. I was hoping to replicate the idea for a few hours, and with less length of wire, but I guess I'm dealing with batteries that won't last long enough and likely wouldn't have enough power anyway. :/

I found several people using AA batteries for ski boots, but they were using purchased heating elements instead of wire, so that probably changed the amount of power required. Here's the forum, if you're interested: http://www.epicski.com/t/100607/diy-boot-heater

I'll research some batteries and post what I find here...

 

Any thoughts on this Li-ion battery? http://www.batteryjunction.com/nitecore-nl189.html

If I understand it right, using two of these in series would give me 7.4 volts at 3.4 amps for an hour. With the wire at 0.75 ohms resistance, this should work out to around 65 watts of power. I'm guessing that would be too hot, so if I reduced the amount of power used, I could get more heating time out of the setup.

Thanks for the help!

 

using two of these in series would give me 7.4 volts at 3.4 amps for an hour

The Ah rating is generally tested at a much lower current (typically about 10% of the Ah rating or 0.34A) so at 3.4A, so you are likely to get significantly less than an hour (perhaps 30-45 minutes).
I would test the heater to see how much current you really need.

Ideally you would want a PWM controller to efficiently adjust the current to what you need, depending upon the ambient temperature.
Here's an example of such a controller. The pot allows adjustment of the current from full off to full on.
Just connect the PWM output to the gate of an appropriate logic-level N-MOSFET (20V, 10A) for controlling the heater current.
I can give your further help on that if you are interested.

 

Hmmm... that makes sense about the Ah rating. I am now trying some different batteries in an effort to get a better idea of what I will need for the heater.

I attached 6 feet of the wire to a 9V battery, but though the battery felt very slightly warm, the wire didn't heat up at all. I then attached 3 Lithium 3V 1500 mAh batteries together in series with a (rather unprofessional combination of aluminum foil and electrical tape, but that arrangement wouldn't heat either the wire or the batteries.

Can you see where I am going wrong? Am I losing power in the poor connections to the wire? I had thought the 3V Lithium batteries would produce some sort of result in the wire...

That PWM controller looks really interesting! Once I get a basic on/off version of the heater to work I'd like to try the adjuster.

 

rather unprofessional combination of aluminum foil and electrical tape, but that arrangement wouldn't heat either the wire or the batteries.

Can you see where I am going wrong? Am I losing power in the poor connections to the wire? I had thought the 3V Lithium batteries would produce some sort of result in the wire...

The lithium batteries should do something.
Don't you have some clip leads to connect the batteries together?
Else you should solder the wires.
Do you have a meter to measure the current?

 

Any thoughts on this Li-ion battery? http://www.batteryjunction.com/nitecore-nl189.html

If I understand it right, using two of these in series would give me 7.4 volts at 3.4 amps for an hour. With the wire at 0.75 ohms resistance, this should work out to around 65 watts of power. I'm guessing that would be too hot, so if I reduced the amount of power used, I could get more heating time out of the setup.

Thanks for the help!

Yikes! $17.95 is way over priced for a 18650 cell. Quality Samsung cells are available all day long for $5 - $6 USD.

 

You'll be better off using longer, thinner wires for this project. You'll increase the load resistance (Such that less power will be dissipated in the batteries, and more in the wire), and you'll help to increase surface area to get more uniform heating.

Also remember that electrical heating is "expensive". We use "crude" energy sources for heat whenever possible for this reason.
To give you perspective, a tea candle (those tiny candles that hold a relatively small flame) produce about 25 watts of heat, and will burn for about 4 hours. Your body heat is equivalent to about 3 such candles, and "burns" 24/7.

Not that I think heating with candles makes sense here, of course. Just don't expect to be able to put a batter in your pocket and stay toasty all day without bundling up. The most important thing is to trap the heat you've already got coming to you.

 

Thanks for the help! After working to improve the wire connections in my experiment with the li-ion batteries I have on hand, I was able to use two 3V batteries in series to generate fairly significant heat in the wire.

I just ordered a couple 3.7V 3500mAh protected li-ion batteries - Obtronic brand - and will see what I can do with them when they arrive in a few days. If all goes well, I should be able to connect them to the right length of wire and have a decent level of heat (I really just need enough to keep my toes from freezing quite so much on the slopes). Then I can start working on the ability to adjust the temperature, etc.

I'll be sure to post an update on the project later!

 

Here's a slightly simpler PWM circuit for adjusting the current.
There are also similar 555 timer based circuits for the same purpose.

 

Would a USB heater patch work for you? It's designed to heat up but not too hot. Here's one.

 

http://www.hammacher.com/product/71850
2 x D for "12 hours" on low and 10 hours on high..

TONS of other sock heaters and even plenty of ski boot heater manufacturers to "study" from

IMO you need to figure out the thermal needs first then select the appropriate power source..