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# DIY boot heater - Page 3

There was just enough space, with a bit of Dremel work to fit the buck converter I had (from Radio Shack). It was tight but it just fit with the wiring. I've honestly never pushed my packs to the limits because I haven't really needed them weather-wise (hence my reluctance to pay full pop for Thermics or Hotronics).

1 vs 2 vs 3 batteries with or without a converter all comes down to the power, current, voltage, resistance relationship that follows Ohm's law. You need to start with one or two variables and work the rest out from there. It's hard to say what is suitable on one piece of the puzzle only.

i.e in the Thermics, the resistive elements are about 10 ohms, and the middle setting of the battery pack is about 1.8V (IIRC). That gives you a a current draw of 1.8/10 = 180mA for a power output of (1.8)^2/10 = 1/3W

Assuming you are working with the same resistance of 10 Ohms, the 3.7V battery with no voltage conversion will give you a current draw of 370mA and a power output of ~1.4W and in theory with a 4200mAH battery, you should get 4200/370 = 11 hours.Given that Li-Ion have a pretty steep drop off, you get in reality maybe 80% of the theoretical (guess) so 9 hours and maybe a 10-15% drop in performance for cold so roughly 7.7hours.

Now given that 1.4W is roughly 4x the middle setting of the Thermics, you will probably want to go the buck converter route and step down the voltage to let's say 2'ish volts. Following the same logic (assuming a 10 ohm load) you get

2/10 = 200mA

(2)^2/10 = .4W

[(4200 x 0.8)/200] * .85 = 14.28 hours.

Note that this doesn't take into account the efficiency of the buck converter itself, which could be as little as 40% and as high as 80%. Say we split it down the middle and call it 60% you still get about 8.5 hours.Now this is still a pretty simplistic modeling of the system, and even if I am off by say another 20%, that should still get you to somewhere close to 6.5 - 7 hours (and I don't guarantee any of the above will actually happen in real life ;) )

I think it will still take some trial and error, but ohm's law and a few assumptions should get you to the ballpark. Start with the the three basic elements, the battery, the heater element, and some means to control the voltage output. My caveat is ensure you have the proper charger for whatever battery you choose. A non-contact (laser) thermometer may be a good idea to to check the output temperature before putting your feet on it.

Have fun,

ZG.

Ok so who is going to share a diagram how to make the controller for the handy laymen?

When I run 4 batteries straight it nearly ignited my feet ....3 batteries is a little on the weak side.    Something to cycle the heat would be ideal.

RR

http://www.sentex.ca/~mec1995/circ/pwm555.html

Put the heating element where the motor is in the example.

D3 isn't needed driving a resistive load, no inductive kick.

This is mine. output voltage is controlled by a potentiometer on the flip side of the converter. You need a small jewellers screwdriver to adjust.

Zero gravity.

I gave up on building my own after i burned out the mini transformer.  Started spending more on it than i could just buy a battery pack and 3-setting adjustable battery pack.

I bought the therm-ic basic kids where i just have to replace the 4 AAA batteries after each use.  I purchased Duracel 1.2 volt 800 mAh rechargeable batteries.  They are NiMH.  I don't think using them on even the medium setting will last all that long.  Then again, in MN my daughter was skiing this morning at -12F and they lasted for about 2 hrs on medium setting.  Maybe i should not expect too much.

Now to my concern:  I saw on the web that they don't make a 1.5 volt AAA Li Ion rechargeable battery.  What they do make is a 3.6 or 3.7  rechargeable LI Ion, but my concern is that they will burn out the battery pack (or the transformer within).  Will the transformer built into the therm-ic basic kids reduce the voltage to where it will not burn up?

If it's safe to buy a 3.6 volt Li Ion, i'll do it, but probably take your advise and buy the recommended battery charger as well.  Thoughts?

Thanks,

John

Thanks to this thread and the other one, I've ordered:

http://cozywinters.com/shop/TH-7470-7524.html

http://www.pololu.com/product/2118

And putting them into:

http://www.pololu.com/product/1159

Wired together with:

http://www.pololu.com/product/2180

http://www.pololu.com/product/2181

Plan was to use 4 NiMH batteries to power all day without any worries. Still waiting for them to come in and will test with 3 or 4. The support guy at pololu said the 2118 was a better converter than the one ZeroGravity posted and would fit in the battery box with a little trimming of some plastic posts.

I was thinking of using some rivets and webbing to make loops to slide over my boot straps. I prefer the battery packs up front as my last commercial packs were damaged by the lift bouncing on them. Not wanting to spend \$200 again I've waited until I found this site.

Do the therm-ic heaters just have +/- wire? I looked at the hotronics pads and they they use 3 that go to a little chip. Wasn't sure how to wire those up so I went with the therm-ic.

Any other tips?

Thanks for the inspiration!

Rob

I got everything in hand now and did a rough test. So far so good! The infrared thermometer says they heater is a nice 87f. The pads are 10ohm and I have the converter putting out 2.25v using 4 AA NiMH batteries (claiming a min of 1900mah).

Going to test them a few hours and see how they last.

Just have to find the soldering iron and I can finish up a set to test on Sunday.

Sounding good. Why did the Pololu guys recommend that particular convertor?

I think it's a little smaller and more efficient. And half the price was nice.

Still not sure what to do with the connector on the Thermic. Should and can it be cut off and stripped easily to solder? Or are there connectors available that will fit?

Since you are buying mating connectors with leads, just cut the Thermic connector off and solder the new one's leads on.  Use some heat shrink insulation to cover the solder connection.

A quick internet search of "DIY heated insoles" turned up these \$17 inserts from Home Despot: http://www.homedepot.com/p/Cozy-Feet-Heated-Shoe-Insoles-CF/202933436#

They seem to come wired, with a battery case, so all you'd need to do would be to add a voltage regulator.

Worth considering?

Jim - Have you done this? There are 4 very small wires wrapped in plastic so stripping them doesn't look easy. If there is a better mating connector I'd like to use that instead.

CharlieRN - I looked at those on Amazon (also cheaper there) and the reviews are very mixed. Maybe with a voltage regulator it would work longer than the reviews. I don't want to be changing batteries or carrying them on the slopes. You also need to get some kind of adhesive and cover for the pad and wire.

I tried to get it installed by Sunday but ran out of time. So they work well on the counter for the day but I haven't done a field test yet. I was at Mt Snow on Sunday and didn't need anything as it reached 35ºF in the afternoon. Kids were pay their age so 2 lift tickets for \$18 was a no-brainer :)

I've been following this tread and thought of my own connection. I assumed the Thermic's 4 pin connector is following the standard dimension.

http://www.ebay.com/itm/171070278989?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

http://www.ebay.com/itm/281215490269?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

Using the above two, I can create a connector that will compliment the thermic's connector. I am planning on using double sided tape, plastic card cut into rectangles and heat shrink tube to secure the connection. Or I could use epoxy or duct tape to make the joint rigid. Now.. with the other end of the wire, I am hoping to use 5.5mm male connector. Corresponding 5.5mm female plug will be connected to the battery pack.

rlloyd-- does that give you an idea of alternative way of making the connection?

I plan on using two 18650 cells per heater and an old altoid tin as a case. I haven't physically tested the fit, but I suspect it'll work okay.

Hey,  I know you guys mentioned that Therm-ic sells the empty cases but I found a website that sells both the heating inserts and the basic cases for 100\$

http://www.alpineaccessories.com/Therm-ic-Basic-Powerpack-with-TrimFit/productinfo/THBTF/

I guess it's a matter of time but since I was gonna buy the heating inserts for 50\$,   paying another 50\$ to get the cases is worth it I think.  Anyways,  I though some folks looking into this might find the link worth considering instead of building the whole system and spending hours doing so.

I checked those out and those look nice for not being DIY. That page also shows the temp settings for each setting. I think that was what I was looking for. At 2.25v mine are at 89-90ºF. I think by boosting it to 2.5v I should be at the factory 1 setting of 95º.

I tried mine out yesterday. My feet never got cold. They were a little cool maybe but never felt cold. I think going to 2.5v will do the trick and I will try them this weekend at Mt Snow. I'll post the results if anyone wants to hear them.

I figured out the total for mine - If you need to buy AA NiMH batteries, Amazon has nice ones. Using that pricing its a little under \$70. If you have batteries then it's about \$50. The cases are \$1 on ebay, voltage converter is \$5, \$1 for connectors and wires, Insoles with covers \$40.

The hardest part is trying to figure out how to attach them to the ski boot. I just tucked them into my elastic pant cuff. Didn't notice them moving around or anything. I tried some velcro around the velcro strap on my boot. I think that will work but I wasn't sure if it would slide out so I need a pouch or something.

Open to ideas on mounting?

I will say do not mount them on the back of the boot. I went on the lift with my son and they slowed it down a little, I could feel the back of the chair hitting my boot.

Stay warm!

Rob

A couple thoughts.  Two AAs NiMH in series is 2.4V.  Four in series-parallel is the same with double the life.  Might work without a controller.  The Sanyo eneloop batteries are highly rated and available on Amazon.

Quote:
Originally Posted by JimH

A couple thoughts.  Two AAs NiMH in series is 2.4V.  Four in series-parallel is the same with double the life.  Might work without a controller.  The Sanyo eneloop batteries are highly rated and available on Amazon.

I thought of that but wasn't sure how long it would last. My plan was with 4 was I may not need to recharge for a weekend of skiing. With 5 sets that's 20/40 (4 or 8 per person) batteries to charge for the next day. As long as they are turned off during breaks I think it would work. I'm using Eneloop batteries and they are nice. I have not tried the Amazon brand yet but they have a similar spec (1900mAh) and are less \$.

I forgot about the series/parallel thing. What's better? have them all in parallel or series? I think the voltage regulator is a good idea as it will also boost the power if needed for a steady voltage.

Hi all
This thread has come at a fortunate time for me as I'm looking for ski boot warmers for this winter and don't want to spend £140 x4 for my family.
I found this item and bought a couple to see hoe they go,

Only £.70 each!!

I hooked them up to 4x aa batteries and they did get warm but the batteries died after about 2.5 hours

How do these compare with the thermic heat elements that early posts say run hot with 4x aa for a lot longer

Any thoughts apart from the obvious cost difference would be much appreciated!
Peter

Where is this image from? where can i buy the connectors pictured? thanks

Quote:

Originally Posted by Richie-Rich

This type would even be better:

That is a pretty standard 4 pin fan connector.    You can buy them as parts for pennies, from Digikey frex.

You also can buy them for a couple of bucks as part of fan extension cables,  anywhere from Office Depot to Amazon.    The advantage of buying the fan extension cable is that you don't have to fart around connecting wires to pins - it's already done for you.

perfect.. thanks!

Hey all, new to this forum. I'm a field service technician for a rental equipment company. I spend ling lengths of time out in the cold. My feet seem to be almost constantly cold. I started using the hot hands type boot warmers last year, but with this job I sometimes have long drives between jobs and you can't just turn off the disposable heaters. I always ended up with sweaty, and then cold, feet.
I tried the cheap amazon set that was mentioned above by CharlieRN also sold at home depot. They couldn't handle the constant bending of my work boots and I got tired of making solder repairs.
So I bit the bullet and orders the thermic. Just thought I'd share my battery life experience with the thermic basic packs. I use eneloop 1900mah rechargeable AAs. Yesterday was the first use. I ran them constantly for 13rs. Mostly on the low setting with about 30mins of medium use. They were still going strong when I got home.
I think maybe the "pulse" control has something to do with the battery life. I'm not sure of how it works, but it seems to work well. It will be interesting to see how long these hold up to in daily use for the average 3 months or of the year I'll need them.

Sent from my HTC6525LVW using Tapatalk

I am going to give these a shot - https://www.amazon.com/Lifemall-GL-Insoles-Battery-Powered/dp/B01LY05XQS/ref=cm_cr_arp_d_product_top?ie=UTF8  - They are cheap and have a lot of the parts needed to make this work.  Will hack them if I have to but it seems like a good starting point.

Let us know how it goes ; when the EC had the pre Christmas arctic spell I hacked a bunch of similar Hotronics that came with assorted boots ; the consistent complaint with the hacks was that they took up too much volume in the boots. The power worked fine but they hated the fit.

I removed the insoles that were already in my boots and replaced them with those.  They were exactly the same size (Mondo 45). My feet felt good in the boots and they felt fine.  I will ski with them on Monday and let you all know how it works out.  If they don't feel right I will hack them by removing the heating element and putting them in or on the original insoles somehow.

Here is the link to what I am using again - https://www.amazon.com/gp/product/B01LY05XQS/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1 They were \$20.

I skied with this setup - https://www.amazon.com/gp/product/B01LY05XQS/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1 the last few days.

On day 1 I used the included battery packs and the leg holders.  It was a complete fail - the leg holders were too small and the battery pack eventually fell off but that turned out OK because they stayed inside my ski pants.  The bigger problem was they did not seem to get warm.

This setup also came with a USB connector and the next day I used that wire to connect one USB cell phone charger to each foot.  On my left foot, I used a battery like this one - https://www.amazon.com/gp/product/B007HUTXV8/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 - this worked well but was too hot after about 15 minutes - it was not unbearable but too hot.  On the other foot I used a larger battery https://www.amazon.com/gp/product/B009V5X1CE/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 this was burning hot.  I could only keep it on for 2 minutes before it started burning.   I was pleased with the results using the USB setup but I need to tweak this setup.  I could try a smaller battery but I would prefer some sort of regulator knob I can add to this setup.  Does anyone have suggestions on how to step down the power using some sort of variable control knob?   THANKS

^ sounds like you don't need a control knob, you need a chopper aka timed on/off control.     A simple potentiometer control would *also* get hot from those levels of current.

It also sounds like there is a timed on/off control built into the battery case (8min on 1 min off 2 min on &c) but may have been bypassed by the USB plug.           How about just applying the USB battery output to the AA battery input terminals?   .

That's a really good idea!  The only problem is the battery cases that came with the unit seem defective.  I was also thinking I could just get a much smaller USB charger - like 3000mah.

One good thing is I learned how to give someone a hot foot ;-)

Quote:
Originally Posted by John Carini

That's a really good idea!  The only problem is the battery cases that came with the unit seem defective.  I was also thinking I could just get a much smaller USB charger - like 3000mah.

One good thing is I learned how to give someone a hot foot ;-)

Just getting a small unit with a lower mah capacity won't help.  You need to reduce the current flow.  That means reducing the voltage, increasing the resistance or automatically turning the current on and off.  Ideally the last would be many times a minute (pulse width modulation).

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