This is my poor man's DIY version of the Milwaukee range of heated outerwear.

I did it for about £20 plus a few bits and pieces I had sitting about, and the batteries of course.

Materials:

A hoodie
Carbon fibre heated car seat kit
A current limiter

Tools:

Sewing machine
Soldering iron

Step 1: Alterations (optional)

At size XL, my hoodie was way too big so I cut it down a bit. To get the best heat retention, you want a fairly well fitting garment.

Step 2: Attach Heat Pads

The carbon fibre pads are pretty easy to stick on. I put the front one even with the bottom of the pocket, so it will keep my hands warm when they're inside, and I put the back one as high as possible.

With the hoodie inside out, lay one pad out where you want it, turn the corner up and begin to peel off the adhesive backing. Then while holding the pad in place, pull the backing out from under the pad and press it down onto the fabric.

I then went and sewed around the perimeter of each pad with an elastic zigzag stitch. It's fine to sew through the carbon mesh, but try and avoid the wires obviously. Oh, and don't sew through the front pocket either, I almost did that.

Step 3: Route the Wires

First I ran some tests,

Connecting the pads in series, I was able to put them straight to the battery. They got noticeably warm in about 5 minutes. In parallel however, they got warm right away, but the current draw was too much for my little alligator clips, and the batteries too for that matter. So I put in the dimmer pot I had and was able to adjust how much juice went to the pads.

To get the wires from the pads to the pocket I poked them into the elastic waistband of the hoodie. Use something pointy like a sharp pencil to create a hole in the knit fabric without cutting the threads, then you can feed the wires in. I used a cording foot on my machine to hold down the wires with a wide zigzag stitch.

Step 4: Battery Attachment

To attach the 12v lithium batteries to the garment, I took my drill apart to see what size the connectors were. Basically the same as a 1/4" spade crimp connector. I then cut a piece out of an old PCB for the spades to pass through, and used Sugru to fashion a cap for the batteries.

Step 5: Finishing Touches

To finish it up, I put the dimmer switch inline between the battery and the pads. They still draw quite a bit, but it really does keep you toasty warm! I haven't tested it extensively yet, but I get about an hour out of a 1.5ah battery on a really cold day. I find it helps to wear a jacket or at least a vest over the top of the hoodie to keep the heat in.

Future improvements:

I could cut down the size of the carbon pads to reduce power consumption, but I've got plenty of batteries.

I might make a more efficient PWM current controller, and possibly include some sort of battery gauge so I know how long I've got left.

I'll try and get some temperature readings up soon, my IR thermometer is on the blink at the moment