One of my biggest issues when riding is getting cold, like really cold, like "OMG it's the ICE AGE We're all going to DIE!" cold. So one of the first things I bought was a Gerbing heated jacket liner and that is by far one of my most favorite pieces of gear. However, as there are a bazillion reviews for that, thought I would talk about the next piece of heated gear I bought.
My feet were always getting cold. After buying numerous wool, fleece, neoprene, or whatever socks, double layering, triple layering, and finding nothing was working, I looked into power. However, I was worried by bike didn't have enough oomph to power both my jacket and socks, so I needed batteries. The trouble with batteries is I like long days and have no trouble riding at night. Thanks to Google Ads, I came upon Volt. Reviews were scarce, the best I could find was a fairly positive review on their gloves. I took the risk and ordered a pair.
Originally I just used the standard battery that came with them and liked the heat, just not the time that it lasted. The battery pack rests above my touring boot, so all I have to do it unzip the bottom of my overpants and I can push the button to turn them on. A little LED shows blinky green, green, orange, red for the four heat settings. I few times I had plugged them in without turning them on, but moving around caused the sock or my calf to push in on the little button and turned it on accidentally. Whooo HOT FOOT! Since then, I've taken to leaving it unplugged until I need it. I find it to be plenty of heat, but again the battery only lasted 3-4 hours (I used high settings, again cold).
After a few late night rides where the time just did not last long enough, I ordered the expanded batteries. I finally put them to the ultimate test, a 24 hour ride in September. As the sun went down, I turned them on low-low and it kept the edge off as the temps dropped. Towards the middle of the night, I began noticing it wasn't enough so turned them onto medium for a few hours, then just before the dawn when temps were the lowest, I clicked it up to high. Even after almost 10 hours of being on a variety of settings, they still had power and had kept my feet warm throughout the night.
My legs are too short to wear the sock and the little battery pack band around my leg, so instead I wear the extended battery around my boot. the overpants cover it and I've not had an issue in the dozen or so rides I've done with them on. However, none of those were in heavy rain so cannot say how water proof that position is.
The only complaint is the position of the wire going from the battery, down the back of your leg, and over your heel to the foot bed. Until I figured out how to shift it off to the side, I feared rubbing a blister into my heel.
if curious about my first impressions and some pictures of my oh so lovely leg (ugh), you can find it here: https://www.ninjette.org/forums/showthread.php?t=212834
My riding gear budget might not allow me to also seriously consider heated gear this year but that by no means discounts the info you have taken the time to share. I'm more focused for now on staying cool while riding 
with the ride out west this Summer.
