DIY heated footbeds?

[North]

Timely bump - was looking at a used Hotronics setup in gear swap, but I want to keep my custom footbeds, so that one wasn't right for me.

gregorys - please do post up any more details on your project, as you get it sorted out.

I installed my Hotronics on a pair of thin Bontex shims so I could leave them in my alpine boots and swap my footbeds back and forth to touring boots. Insoles go on top of shims, and the shims never come out of my alpine boots.

[gregorys]

I power these with a couple of usb battery packs. You can use the ones with a switch or just plug them in on the lift.

Somehow I missed that.
So, you're pumping 5v into the cozy-feet elements?
A test setup I did where I used 5v [under load, not 5v quiescent] and those things get pretty hot. [Like very, very warm. ]
I was using NiMH AA cells for testing, so no great idea how LiPo cells will act - but I had to have (7)*1.2v cells to get a bit more than 5v under load. [Meaning, I had around 8.4v of battery, but under load, I'd get 5.2v or so, IIRC]

I've got a USB battery pack on the way and a buck converter [regulates voltage down] so I can adjust voltage.

At around 2.8v under load, they are toasty enough for me, but probably not warm enough for people with colder feet. I'd guess that actual 3.5-4v will be pretty nice, even for those with pretty cold feet, and in cold conditions.

So a few questions;
I'm currently using 26ga wire. I'm a computer guy, so ethernet patch cables are easy places to scrounge wire, cheaply and easily. But that's probably not going to be optimal long-term. [Especially if I'm going to suggest/recommend some parts for others.] So, what wire are you using? Even twisted, the ethernet cable doesn't really bother my feet - the cozy-feet units have the connector toward the outside of the foot so the wire isn't really running under the foot much, but something that's more rugged and _flat_ would be handy.

You're just tucking the battery into your boot/pant-leg? No other attachment?

Have you tested what happens with your battery packs if a straight short occurs? I don't know if most/all/few usb battery packs take the time to design them to fail-safe in the event of a straight short. If most don't, then I'll probably design in a cheap part that will address that case.

[gregorys]

I'll add:

I think the sweet part of using a USB battery pack is the ease. They handle their own charging, so that's solved. If you want more voltage than a regular Lipo cell gives [3.7v] you could use up to the 5v that comes from the usb battery. [Yeah, it's probably a 3.7v call internally and it's boosting the voltage to 5v, and then we'll be doing it back again, to something less than 5v, perhaps - and there is efficiency losses, etc. That's all true, but it is still better if it works "well-enough', IMO to simply live with it.] Replacing the battery is cheap and trivial. Having multiple battery packs is easy.

Good usb battery packs probably have some safeguards that a plain li-po cell won't, etc.

[gregorys]

Timely bump - was looking at a used Hotronics setup in gear swap, but I want to keep my custom footbeds, so that one wasn't right for me.

gregorys - please do post up any more details on your project, as you get it sorted out.

Yeah what "North" said. The hottronics, I thought, used a small circular heating element that you could put on top of the footbed. Now you'd be stuck with the element on that footbed, and couldn't change boots, without the footbeds, but it should work.

Without wasting too much of your time typing a response, am I missing something?

[Skeeze]

I got a Therm-IC setup about a month ago--had it shipped to an address in the UK when I was visiting there and it was way cheaper than if bought in the US--I paid about $120 total for the 1300B kit that included everything I needed.

In short, they work. I lost feeling in one of my toes for a month last year and have always dealt with cold toe issues, but those are now gone. They Bluetooth control seems gimmicky but it's nice to have. They last all day for me at a medium setting, so don't see the need for the larger (1700B) battery packs.

Additionally, they're plenty easy to swap between boots (if you use the same footbeds in both boots). Takes me about 5 minutes total, and I'm sure I'll get quicker over time. Assume Hotronics would be swappable in the same way.

[El Chupacabra]

Yeah what "North" said. The hottronics, I thought, used a small circular heating element that you could put on top of the footbed. Now you'd be stuck with the element on that footbed, and couldn't change boots, without the footbeds, but it should work.

Without wasting too much of your time typing a response, am I missing something?

I may be the one who missed it - my understanding on the Hotronic footbed/insoles (at least on the ones being sold in gear swap, from what I could tell when googling) was that they had the heating element integrated into the footbed/insole. Perhaps there are different types offered by Hotronics. Or perhaps I just misread it.

[EWG]

You can get them separately. Cheaper other places but this was the first google hit. You just stick them to your footbed. Easy.


https://www.rei.com/product/876312/h...-elements-pair

[motopsycha]

I haven’t had any problem with the 5v output. I tuck the battery in the top of my sock, above my boot. When my toes get cold I plug them in. I unplug them once they warm up.

The elements go under my insole and fish out the back of my liner. I splice on a usb plug and reinforce it with some shrink tubing. There is 8” of cable hanging out between my cuff and liner. Tuck the usb plug next to the battery until you need it. I probably have 80-100 days on this setup and the only issue is when I accidentally ripped off the usb plug. Fix was easy









QUOTE=gregorys;5869736]Somehow I missed that.
So, you're pumping 5v into the cozy-feet elements?
A test setup I did where I used 5v [under load, not 5v quiescent] and those things get pretty hot. [Like very, very warm. ]
I was using NiMH AA cells for testing, so no great idea how LiPo cells will act - but I had to have (7)*1.2v cells to get a bit more than 5v under load. [Meaning, I had around 8.4v of battery, but under load, I'd get 5.2v or so, IIRC]

I've got a USB battery pack on the way and a buck converter [regulates voltage down] so I can adjust voltage.

At around 2.8v under load, they are toasty enough for me, but probably not warm enough for people with colder feet. I'd guess that actual 3.5-4v will be pretty nice, even for those with pretty cold feet, and in cold conditions.

So a few questions;
I'm currently using 26ga wire. I'm a computer guy, so ethernet patch cables are easy places to scrounge wire, cheaply and easily. But that's probably not going to be optimal long-term. [Especially if I'm going to suggest/recommend some parts for others.] So, what wire are you using? Even twisted, the ethernet cable doesn't really bother my feet - the cozy-feet units have the connector toward the outside of the foot so the wire isn't really running under the foot much, but something that's more rugged and _flat_ would be handy.

You're just tucking the battery into your boot/pant-leg? No other attachment?

Have you tested what happens with your battery packs if a straight short occurs? I don't know if most/all/few usb battery packs take the time to design them to fail-safe in the event of a straight short. If most don't, then I'll probably design in a cheap part that will address that case.[/QUOTE]

[gregorys]

Some preliminary observations.

1) The all-in-one "Warmfits" I mention above are not bad.
Here's a link on Amazon.
https://www.amazon.com/Warmfits-Batt.../dp/B07JLT5Y1S
[I'd guess, after doing some looking, that there are many dozens of identical products - all likely sourced in China. So, you'll probably get the same thing from nearly any vendor, as long is it's very similar to this - you don't have to buy this specific one. However, I can only speak to the item I have in hand. Yours may well be different - even if you do buy this specific item. This stuff changes all the time.]

I have not yet cut them down and run them all day in a boot - but my guess is given my "carpet testing" they'll do pretty well. For simple setups, where you don't want to go through a more DIY "drama" - I think these are a good choice. Pretty good steady heat. Not super warm, but reasonable.

The downsides are:
Batteries - mostly all the issues I see someone having with these is related to batteries. You'll be using these batteries during ski season only. Letting them sit for extended periods of time, in the off-season, will probably shorten their life. How deep do they discharge? Too deep, and they'll suffer early failure. Do they have short protection? I'd hate to have the battery massively overheat while attached to your ankle. Bad for both the battery and your ankle. Probably your boots too.

As noted previously, you can't buy the battery separately - so you'll be buying a whole new setup if the battery fails. But then again, it's only $30, and by skipping a burger and fries on the hill, you can probably pay for the whole thing anyway.

For something that will probably work pretty well, provide modest warmth for 5-6 hours, cheap and low drama - this is a great option. If you are a mid-DIY person, sourcing some LiPo cells as replacements certainly wouldn't be too hard. [The added benefit of sourcing additional/replacement cells is that you could go bigger [more mAh] or have an additional set to swap in, when the first set run out of juice.]

Some technical details. The footbeds show a resistance of about 5.6/5.7 ohms. The Cozyfeets I'll talk about next have about 3.5 Ohms. That means that with equal voltage, the Cozyfeet's are going to put out a lot more heat. [Which I'll talk about, is not always a good thing.]

[continued in next post]

[gregorys]

2) The Cozy-feet beds are, IMO, quite good. I think they offer a lot of great options.
I have run these for a couple of days in a couple of different ways.

They'll probably provide the best steady-on heat using regular LiPo/Li-Ion 3.6v cells. [Especially if you keep voltage losses as low as possible.]

However, I'd rather use them with a USB battery pack. The battery pack provides some real advantages.
- Very cheap, per amp hour of capacity.
- Less difficulty knowing if it's good quality or not. Buy from a reasonable vendor and you'll probably get reasonable cells and the quality of the rest of it, will probably be reasonable.
- No special charger for the battery pack. Plug it into a micro-USB and you're charging.
- Charge/discharge indicators - you'll know when it's full or empty, easily.
- Not related to the boot heater project - but the USB battery pack has some significant utility in the off-season for me. Plus the batteries won't lie dormant for a long time because I'll use it some of the time outside of skiing. And, you could charge your phone on the hill if you're desperate, and are willing to trade potential cold feet for it.

The downside of the USB battery pack is the output voltage. The one I'm testing with is a 13ah [@3.6v] Anker (I like their stuff, and it was a reasonable price. There are certainly ones that will get you better a better cost to Ah ratio.)

In short, the higher the voltage, applied to the heating element, the greater the amount of heat provided.

The output voltage is going to depend on the awg of the cable and usb connector etc. Using a cheezy USB cable with 28awg wire, I get about 4.7v on a <6 inch lead. I use some pin connectors which are often used in RC hobby stuff, to couple/uncouple the cables at the waist and ankles. The cable running down each leg is 26awg.

All said, I get about 4.2v at the ankle.
[I should have measured current, with my final setup, but haven't yet.] [Added 2020-01-26. I finally did measure: ~0.9amps. At around 4.2v - that's ~3.8 watts - per foot/heating element.]

I skied on Wed and this setup - using the 4.2v USB fed battery - well it's quite hot. It's not hot enough to burn you - but it's VERY warm. Like, you plug it in, and you'll feel the change in a few seconds. If you're standing on the ground where there's substantial pressure between the heating elements and your feet - it's too hot for my comfort in say 5 minutes. (I think my socks were a bit damp, and that made the heat seem even more dramatic.)

I've made a few changes to my setup yesterday and I'll be testing those out - namely there's a little bit of fabric between my feet and the heating elements - and that may adjust things some. But I think the general take-home for me is that this setup is going to be best when you run it for a few minutes at a time. That will warm your feet and you'll be good.

But I can tell you that it's also disappointing to turn the heat off. You just want them to stay toasty all the time.

If that's what you want, then I'd suggest LiPo batteries instead of the USB pack.

Also, I tested a buck converter with the USB pack. I've never messed with these - it's a PWM (pulse width modulation) device - so it's quite efficient. However the minimum voltage drop is about 2v, so you're left with too little voltage for good heat. Thus, using this to moderate the USB pack seems like a failed plan. [And yes, it's less efficient.] Plus it was quite hot (the buck converter itself), so finding a way to package it (and keep it small/easy) would be challenging.

I, as alluded to above - I'm using a single battery pack. Put it in a pocket. I then run wires down each leg to the boot heaters. The wires have couplers at the waist and ankles, so it's trivial to manage them - they don't get tangled, taking off your boots is no problem, etc.

This also means that turning on/off the heaters is as simple as unzipping my pocket and unplugging the USB cable from the battery.

I have some pics I'll share once I've settled on most of the details. But here's the general list of parts I use.

Heat-shrink tubing (I got 30m of 3mm): https://www.amazon.com/gp/product/B00SPNM5XY
Electrical connector/couplers (Micro JST-PH 2.0 Connectors, get pairs, not just M or F): https://www.amazon.com/gp/product/B07N2LYY7Q/
Cozy-Feet: https://www.amazon.com/gp/product/B0009HMFPC/

I have dozens of USB cables lying around, simply cut the end off one. Keep it short, because the wire inside is really small guage (28awg is what most of mine have), and you'll lose a lot of voltage if you leave it very long. The red and black cables are + and - ~5v, respectively. The green and white are data, you won't need them.

I solder a pair of those 2-pin connectors (JST-PH 2.0 from above) on. You can use a male or female here, just make sure the rest match for each set of connections, down to the boots. If you're doing more than one complete set, you probably want to do them all the same.

I used some of that heat-shrink tubing to insulate the solder connections and protect the wire to the connector heads.

I then used an ethernet patch cable to rob for wire. [If you want an excuse to buy a tool - the vice-grip auto stripping tool is one of the best tool purchases I've ever made. https://www.amazon.com/IRWIN-VISE-GR...p/B000OQ21CA/] It will strip the jacket off the cable, and the 26 awg pairs nicely. It even does the 28 awg ok, if you're careful.]

You'll want to estimate how much wire for each leg. One will be a bit longer, since it's going to cross over at the waist. I just guess by draping some cable from around the ankle to a little above the waist. (You'll get a little more length when you add the electrical connectors to each end.) And you can always leave a little loop somewhere if you have a little too much length. Better a little too long than too short. [As a farm kid growing up, hearing the saying; "They cut it three times and it was still too short..." comes to mind, still.]

Same deal with this set of wire. Heat-shrink a pair of wires. Solder connectors on each end. Heat shrink those. And a final shrink of the wire up to the connector heads. [Remember to slide the heat-shrink on before you solder the ends on. A "friend" managed to fail to do this dozens of times! ]

Desolder the wires from the cozy feets. I used leads of about 17" to get from the heating element to above the cuff of the boot. This is going to depend on the height of the cuffs and your foot size - but I'd still guess 17-20" is about right.

As before, heat shrink the wire as close to the connector on the heating element as you can. Also, untwist the wire where it runs under the foot. that will keep the bulk down and be less irritating.

A piece of black electrical tape on the solder connectors on the heating element is good. Also, keep those solder connections as smooth as possible. Less to poke up and snag things.

My current "test" boots have Intuition tour-wrap liners. So, rather than poke a hole in the liner, I simply ran the cable up the side of the liner, and out the wrap just above the toe box. Then through the lowest lacing hole and on up. My feet may be more tolerant of minor irritation, but in any case, this has worked perfectly fine for me. You can feel the wire sometimes, but it's never bothersome. I'd suggest you do the same until you're pretty sure you'll need something "better."

I put my boots/liners on simultaneously. If you put on a liner and then the boot, that may change your calculus. If so, there are videos on Youtube about putting in hot-tronics and the like to give you ideas. Do some searching and watch those first.

Finally, I want to adhere the heating element to the foot-bed. There's a million ways to do this. Duct-tape was what I used in my initial test setup. It works. But it leaves a lot of residue behind when you remove it. I could sew some covers that would go over the foot-bed, or similar approaches.

However I have some synthetic footie socks I got cheap from Ross. At <$1 a pair, it seemed like a good sacrifice and it was very quick. I cut all but the heel off and stretched the front of the sock over the front of the footbed. It entirely covers the heating element and stops around mid-foot. I've done nothing to lock it down - and will leave it that way for a while. When I do feel like locking it down, tape or hot-glue seem like good options. This may also go some distance in diffusing the heat so the high heat output is less "harsh."

Again, once I have at least a few more days on this setup, I'll update this post with any additional observations. I have some pics I'll post then too.

If you're in Portland OR and want to share some of the heat-shrink tubing, let me know. 30 meters is probably more than I'll use in the rest of my life. It was very cheap though, and I think very worth doing.

[Edited again, with measured, rather than calculated values.]
The output of the CozyFeet heaters at 4.2v is about 3.8w per element - thus 7.6w total. That's a lot of heat.

On my 13,000 mAh USB battery pack, I estimate that would be a bit more than 6H of continuous heat - but that would be nutty hot, IMO. In my case, I'd expect to be "on" far less than half the time, probably as little as 5-10m/hour. At 15m/hr - I'd expect them to last about 24.5 hours. We'll see how accurate that turns out to be.

[mntlion]

thanks.

(this could be a good re-sale option for you) buy them from china, wait 1-2 months. Mod them with bigger battery, and sell for 1/2 of a hottronic/thermic.

[jono]

This is very well done. Thanks for taking the time to follow up with complete details!

[gregorys]

So, one other possible option that I've carpet tested.
It hadn't occurred to me [I dunno why] to use the Warmfits with a USB battery.
They are ~6 ohms, so will put out less heat at 4.2 volts.

I currently have a jerry-rigged setup, since I will probably return these because I'm mostly unimpressed with their performance - at least compared to the alternative and without doing some mod work.

However with the mod to use a USB battery, [albeit with a higher total price for an end product] I think I could recommend these.

The battery down-sides are gone, since you can swap batteries any time easily.
The heat output seems substantially better, and will probably stay a lot better vs the standard battery. [The regular battery will start at around 4.2v and as the battery discharges, it will go down to around 2.6v when the cell should protect itself from excessive discharge. At those lower voltages, it won't be putting out nearly as much heat.] The USB battery output is regulated and should put out the same voltage across the entire charge cycle.

The only remaining downsides are the wires to the foot-beds. They are pretty flimsy. Heat shrink tubing will help, but I wouldn't be surprised to see them break where they enter the foot-bed. If that happens, you'll get to MacGyver a fix. And since the crowd who might pick this solution seems less MacGyver'y - that seems unlikely. But again, probably they'd last at least one season - so even at $30 a year, provided they stay available in the future, doesn't seem that steep a price.

Total cost goes up a bit, compared to CozyFeet project. [Honestly, probably only $20 more, if you compare apples-to-apples. The total adds up, though, if you plan on multiple setups.]

Warmfits - $30.
Trivial supplies to adapt the wiring connectors - $5-10 [free if you got the stuff laying around.]
USB battery. $10-30 or so, depending.

You also get the little pouches for the stock batteries. You can probably adapt these to hold a USB battery at each boot. Or adapt the instructions I posted above to do a -single-battery-for-each foot approach.

Some soldering, probably. A little modification/splicing of USB cable(s).

Pretty easy project.
If someone finds a source of just that footbed, for cheap, that would be the bees knees.

---
A final caution here - I'll probably add this to the prior post too.
I've done some reading, on sustained temperatures and burns to the skin.
NASA appears to have a standard that any temp <113F with any normal materials is probably without any risk.

However, the CozyFeet can produce temps far in excess of that, in a warm room without any heat-sink. [Namely your feet, boots and liners.] I'd guess that would be bad for long term exposure.

The Warmfits with the USB battery can also produce higher temps, though substantially less than CozyFeet with the same battery.

If you were incapacitated, and your heaters were on, it's possible they'll burn your feet.

If you have issues with sensation, [hello diabetes] for example, any of these heating devices could be hazardous because you'd lack the required sensation to know when your feet were getting too hot - namely pain.

So, use any of my posts with caution. You'll have to determine for yourself how risk tolerant you are. [Hey, you're skiing. Burning your feet probably is less risk than a ton of other things you did today!]