I made these insoles from parts I ordered from DX.com. Most of their stuff comes from China and takes a really long time. For a little more money the parts can be ordered from Amazon and take less time. I used a few things I had on hand already, namely a pair of insoles, some spray adhesive, and some fabric to cover the insoles with.
I made them for my wife and she has used them several times with really good luck. The run-time is over 2 hours. The weather here has been pretty warm for winter so I can't say how they perform in really cold weather. There are plenty of modifications that could be made to these to make them better, and I'll probably make some changes to them for next winter, however I wanted to get them ready to go right away and they've worked fine so far.
The parts I ordered are...
Stylish USB Heated Warm Gloves - Coffee (Pair) - Free Shipping - DealExtreme
Cheap UltraFire 18650 3.7V "3600mAh" Batteries (Pair)
Universal Convenient 5V USB Adapter Power Bank w/ Torch Light - Black (2 x 18650) - Free Shipping - DealExtreme
There were no reviews on the battery case when I bought it. I had the same experience as the reviewers that the batteries didn't fit well. I bent some stuff and it worked just fine. I like the pack because the batteries can be charged and used while in the pack so they never need to be removed. I also like the little flashlight that it has built in, just in-case.
Gloves with heat elements removed.
Elements glued to the insoles. They are on the top of the insoles to be close to the feet.
I ran the wires through a small hole I cut in the insole and cut pieces of fabric to glue over the top of the element. I used the spray adhesive to attach the fabric, and the wires that plug into the battery pack slip out through the tongue of the shoe. The wires would need to be extended for a pair of shoes with a sealed tongue. I'll probably extend the wires and run them under the insole to the back of the boot next time.
I split the cable all the way up and the pack gets tucked in a pocket with the wires running down each leg. I wrap tape around the plugs so snow doesn't foul anything and they don't come un-plugged.
Not sure if this will be useful for anyone else, but for $20 I figured I'd give it a shot, and my wife has been happy with them. If winter ever actually arrives in Southwestern Colorado I'll probably make a pair for myself.
Mtbr's 2016 Winter Biking GearReviews and Roundups
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Thread: DIY Heated Insoles for $20