LiFePO4 heating pad for cold temperatures

Horsefly said:

I assume that was directed at me. I'd say one extra thermostat that costs about $10 is not much complication.

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Yes. But I agree that it would be necessary for automatic control.

What type of thermostat are you using? I'd prefer the system on/off one to be non-powered so it's not consuming and energy when not needed.

Are you using something like in my previous post.

Downside is the cheap mechanical ones aren't very precise.

Short_Shot said:

This thing should be a zero power solution to turning it on or off based on ambient temperature but not top of cell temperature. Commonly available.

View attachment 68430

Alternatively and cheaper still is this, or similar.

View attachment 68431

If you really want the remote cell temp you can use a capillary therm but the tube is really sensitive to damage.

View attachment 68434

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Or these, which several people here on the forum have used (you would want the 12V version, but I used this 24V version): https://www.amazon.com/gp/product/B076YD8H19

Horsefly said:

Or these, which several people here on the forum have used (you would want the 12V version, but I used this 24V version): https://www.amazon.com/gp/product/B076YD8H19

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Yeah I have similar ones.

I just don't want it to pull any power at all when it's not needed. Even if it's milliamps it's just additional stuff that is powered on, and more time spent on means more chances to burn itself out.

The mechanical bit will shut off the digital bit entirely.

Short_Shot said:

I'd prefer the system on/off one to be non-powered so it's not consuming and energy when not needed.

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It's been a while, but I did try to measure the current of the thermostat I used, and I think it was pretty small, like maybe 50mA.

In my situation my system is in the mountains with no humans within 50 miles for about 5.5 months per year. However, the solar charge controller will be on and providing power to the heating circuit and the batteries. I also cut off the heating circuit if the battery gets below 26V, and the JBD BMS cuts off charging if the temp gets below 4°C. Anyway, I don't care much about the small current that the thermostat consumes. Won't matter.

Horsefly said:

It's been a while, but I did try to measure the current of the thermostat I used, and I think it was pretty small, like maybe 50mA.

In my situation my system is in the mountains with no humans within 50 miles for about 5.5 months per year. However, the solar charge controller will be on and providing power to the heating circuit and the batteries. I also cut off the heating circuit if the battery gets below 26V, and the JBD BMS cuts off charging if the temp gets below 4°C. Anyway, I don't care much about the small current that the thermostat consumes. Won't matter.

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My concern isn't the power drain but the fact that those cheap controllers aren't known for having long lasting components. Last thing I need is to try to use the system and the board is shot. Or worse, NEED it to work and it doesn't.

I've got old capillary thermostats that have been in service since the 1960s at work lol

Constantly being powered on and operating should be fine for most of what's there on those boards with the exception of the display module, however even $100 automation direct controllers I've used over the years have failed on me in just a couple of years of being powered on 24/7.

Horsefly said:

Or these, which several people here on the forum have used (you would want the 12V version, but I used this 24V version): https://www.amazon.com/gp/product/B076YD8H19

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That's the thermostat I'm using. Two of them, one on each battery, but in 12v. They work well. I prefer the digital over analog. Knowing you have it set at 35° F rather than 30° F is important. My BMS and BMV-712 agree on the temperature that the other thermostat is seeing, so I know it's accurate.

HRTKD said:

That's the thermostat I'm using. Two of them, one on each battery, but in 12v. They work well. I prefer the digital over analog. Knowing you have it set at 35° F rather than 30° F is important. My BMS and BMV-712 agree on the temperature that the other thermostat is seeing, so I know it's accurate.

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Accuracy is important for the actual low temp condition.

Much less so for simply enabling or disabling the heater circuit.

Have you checked the actual temp seen at the heater itself with your setup?

noenegdod said:

This is just my gut but I think 60w might be too much for a battery that size. If it turns out to be too hot it is an easy fix to just wire series pairs and cut the wattage down.

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I have similar 15w heaters and plan to run two in series. This cuts the wattage from 60w (4 pads) to 15w of heat. I tested on the bench and the pad was warm to the touch but never hot.

SolarShed said:

just wondering, once you have your heaters installed, whether your own heater or built-in from a manufacture (BB, Renogy, etc.), how well does it actually heat the cell? For example are there cold spots in the cell that could be damaged because its not as warm as other spots?

I see some of you have IR scanners, just wondering if you can scan the cell and see how uniform the heat is but I guess it would be hard to see deep into the cell with a surface technique...its almost like there should be a temp sensor inside the cell but I guess that is not a good idea

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My conclusions so far is that these cells/batteries won't freeze (or develop cold-spots within them at all) quickly due to their mass.

My trailer is unheated, partially shaded (~50% of the day) and parked outside nearby. The JBD/Overkill has a low temp charging cut-off set for 0 degrees centigrade/32 degrees fahrenheit. It has been set to maintain the battery at ~ 3.45V per cell/13.8V total. Even though the temps are dipping below freezing at night the battery itself hasn't gotten below 6 centigrade. This seems mostly likely due to the batteries density overall and the fact that dense objects retain and lose heat more slowly and evenly. This indicates to me that absent a heavy load or an inherent issue with the batteries integrity itself, there is very little chance of it developing spots within the battery or cells that are much different than the cell/battery overall.

Horsefly said:

It's been a while, but I did try to measure the current of the thermostat I used, and I think it was pretty small, like maybe 50mA.

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Just for grins I went and measured it. With the relay off, the thermostat pulls 13mA. With the relay on, it pulls 42mA. I would assume both of those would be two times higher for the 12V version.

I know @Short_Shot you said you didn't care about the power drain, but I thought some folks here would.

SolarShed said:

just wondering, once you have your heaters installed, whether your own heater or built-in from a manufacture (BB, Renogy, etc.), how well does it actually heat the cell? For example are there cold spots in the cell that could be damaged because its not as warm as other spots?

I see some of you have IR scanners, just wondering if you can scan the cell and see how uniform the heat is but I guess it would be hard to see deep into the cell with a surface technique...its almost like there should be a temp sensor inside the cell but I guess that is not a good idea

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There's quite a bit of thermal mass in one of these cells. It will take a while to warm it up from the outside, but I'm certain the material will get to a uniform temperature soon after. So no, there won't be any cold spots.

time2roll said:

I have similar 15w heaters and plan to run two in series. This cuts the wattage from 60w (4 pads) to 15w of heat. I tested on the bench and the pad was warm to the touch but never hot.

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That was my experience as well.

Good news. I cobbled it all together real fast and plugged it in and the empty fuse location on my fuse block lit up so that's promising.

Update the second. Turned it on. It's 23c outside right now. Set it to kick on, then set it to 25c.

It reached 25c, coasted to 26, then in less than 2 minutes was back down to 25 where I stopped keeping track because that's pretty much exactly what I expected from it.

Obviously with longer run times you will see a note overshoot but shouldn't be anything in the realm of damage.

I will now disable charge and discharge from the battery (running on external power at the moment) and heat it up a bit farther to see what kind of temperature gradient I get at the bms and battery sense respectively.

For a temperature controller I think an “inside the battery box” is a better way to go than an external controller, and it’s simpler. I used the following controller to turn the heater pads on and off…if I recall correctly they are about $5 a unit.

HiLetgo W1209, 12V DC Digital Temperature Controller Board Micro Digital Thermostat -50-110°C Electronic Temperature Temp Control Module Switch with 10A One-Channel Relay and Waterproof​

Huh. At 30c it only overshot by 0.2c lol

Must have been saturated by the time it reached cutoff.

MadMacks said:

View attachment 68459For a temperature controller I think an “inside the battery box” is a better way to go than an external controller, and it’s simpler. I used the following controller to turn the heater pads on and off…if I recall correctly they are about $5 a unit.

HiLetgo W1209, 12V DC Digital Temperature Controller Board Micro Digital Thermostat -50-110°C Electronic Temperature Temp Control Module Switch with 10A One-Channel Relay and Waterproof​

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That's pretty much exactly what I'm using too. Just a different vendor that includes an acrylic case for an extra 2 bucks.


So back to the point, I'm really going to have to find a way to log the battery Temp in an appropriate location this winter. Heat it up to operating temp of 75F then turn off the heat and see how long before the bms cuts off.

My initial plan was to simply run the heats off the tow vehicle and hope the average consumption is low enough to get by for 24 hours, including running my diesel heater.

Now I'm thinking I might be able to tap some of the heat from said heater and pipe it to that side of the tongue box. It'll be much less efficient on fuel but use way less power.

I could probably blow the hot air right into the lid of the box. Bonus points if I can figure out a way to regulate it.

Anyone by chance know if those cheap controllers retain settings after power off?

I can try it myself later but am curious now while standing here watching it get warm.

Decided to put my switch in and turns out, at least for a few minutes, it did retain settings.

Short_Shot said:

Anyone by chance know if those cheap controllers retain settings after power off?

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You mean those thermostats like in your picture? Yes, they keep their settings after power off.

upnorthandpersonal said:

You mean those thermostats like in your picture? Yes, they keep their settings after power off.

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Thanks lol.

I just tried it but of course that doesn't answer it for longer periods. I'll assume that means I'm good for any practical purposes.

Short_Shot said:

I just tried it but of course that doesn't answer it for longer periods. I'll assume that means I'm good for any practical purposes.

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It will. I've got one on my desk right now that was powered off for over a week and kept its settings:

Short_Shot said:

So back to the point, I'm really going to have to find a way to log the battery Temp in an appropriate location this winter. Heat it up to operating temp of 75F then turn off the heat and see how long before the bms cuts off.

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Since my battery box and heating system will mostly be unattended while it is doing its job, I'll be testing it through this winter here at my home in Denver. I bought three loggers.

Two temperature loggers. I'll put one at the bottom of the box, near the aluminum heat sink, and one resting on top of the cells.

Here's the first one. I've not used this first one before, but it seems like it should do.
https://www.amazon.com/gp/product/B01MY9A0G1

Here's the second one. I've used this type before to log the temperature at our cabin through the winter.
https://www.amazon.com/gp/product/B082WV43HH

Then I have a voltage logger that will simply log the times the heating pads are on and off. I've also used one of these before.
https://www.amazon.com/gp/product/B003DS0BS2

Hopefully I'll be able to get all three datasets sync'd up time-wise, and then I'll put it into a spreadsheet and a scatterplot, It should let me see how the top and bottom of the cells differ as the heating pads are on, and how often they are on.

I've been using Elitech data loggers for some time. I recommend spending a bit more on the Elitech loggers with PDF function. They create a PDF of dates, times and temps without the use of their clumsy software.

So. It's about 7.5C tonight and I'm out here with these heater pads cranking away at full power.

60w going into the bottom of the cells and the temperature at the very bottom next to the pads is rising extremely slowly.

This suggests the heat is being transferred into/ through the cells very well.

Pads read 24C and the midpoint sensor for the bms shows 15.5C.

Top of cell shows 12C ish looking through a hole in the lid to the black plastic top of the cell.




Dropped the setpoint of the heater and it only overran 0.1C and started dropping within a second or two so the response time is very rapid and I won't have to worry about excessively long on-off cycles.

TowerCamp said:

I've been using Elitech data loggers for some time. I recommend spending a bit more on the Elitech loggers with PDF function. They create a PDF of dates, times and temps without the use of their clumsy software.

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I noticed the PDF, but I want to combine the data from three different sensors so I can put them in one time plot. So I think I have to use the software. I tried it and it doesn't seem that bad.

Since I can, and will use this for work I've ordered this bad boy and some type k probes.



I don't need calibration, high accuracy, or precision for what I do (for work, +/- 5F is fine) but I have needed logging for some time and the cost is low enough that I can buy it rather than having my employer pay and thus own it.

I'll use this at the pads, halfway up the cells on the opposite side of the battery, one on top of the battery, and one for ambient temp I'm thinking.

I'm thinking I can detect the pads on/off via nearby sensor, though there will be some expected latency.

My goal is to ensure bottom doesn't exceed, say, 25C while the top doesn't dip below 5C. As we discussed earlier about two thermostats I ordered an inexpensive mechanical capillary thermostat which I'll use to cut off the power to the bottom digital controller based on the top of the cell reaching, say 10C or so.

The only reason I'm cutting power to the controller instead of just interrupting the pad power is, as I mentioned before, to hopefully reduce the total on-time of the digital one since those displays and cheap components don't last forever.

I'll use the shunt to get an idea of total power used per hour at X ambient temperature and if I'm really motivated I'll repeat at a few temp ranges as it gets cooler this year.


It's also possible I'm totally wasting my time with ensuring the bottom doesn't get too hot as the cells may conduct heat quickly enough that the bottom doesn't get hot enough to worry about before the top gets up to temp.

However I also don't want a false sense of security in the the bottom might be warm but the top still too cold and then I'm damaging half the cell, though I feel that's unlikely.