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Heating Pad Install Advice for LiFePo4

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Yblocker

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Jan 4, 2022
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I just finished installing a small LiFePo4 house battery and 30 amp DC DC charger under the seat of my Vanagon. The system works well for my needs.

PXL_20220119_232238420.jpg

I do some cold weather camping, and my understanding is that these batteries should not be charged in temps below 32 degrees F.
I am thinking about the linked heating pad as a solution. I'd wire in some kind of temp controlled relay for upper and lower temp cut in/out.

Question is, how to mount the pad and how close to the battery? Direct contact? Mount it on a piece of aluminum or something and get it within an inch or so in the battery box?? Any experience out there?
 
That is almost the identical pad as I used. I actually have two of them for my 8S 24V battery in our cabin in the mountains. I think it is a good one for your purpose.

You may not have room, but I put a 1/4" aluminum plate between the heat pads and the battery. The plate causes the heat to rise more slowly, and spreads the heat across the entire surface of the battery.

I've got details on what I built here: https://diysolarforum.com/threads/horseflys-cabin-solar-lifepo4-upgrade.27472/

For a thermostat on a 12V battery, several folks here use this: https://www.amazon.com/gp/product/B076Y5BXD9
 
I'd second the suggestion for an aluminum heat spreader.

To reliably read the internal temperature of the battery, you can epoxy the thermistor inside an (uncrimped) lug and add it to one of the battery terminals. Don't trust tape.. if the thermistor falls off it may read as cold while the battery is being cooked to death by the heating pad.

Here's a slightly nicer (IMO) option for a thermostat:

Amazon.com: Inkbird Dual Stage DV 12V Digital Temperature Controller Fahrenheit Thermostat Heating and Cooling for Homebrewing Brew Fermenter Fridge Incubator Greenhouse : Industrial & Scientific

Amazon.com: Inkbird Dual Stage DV 12V Digital Temperature Controller Fahrenheit Thermostat Heating and Cooling for Homebrewing Brew Fermenter Fridge Incubator Greenhouse : Industrial & Scientific
www.amazon.com

It has the sleep well at night feature of opening the relay and activating a buzzer if an open or short circuit at the thermistor is detected.
 
Thanks for the tips. I'll see what I can cobble together and will report back.
 
eXodus said:
product disappeared


RV tank heater pads - you can just google them. They come in difference sizes - usually start heating around 45F
Click to expand...
These probably work fine. The problems I see:
  • They all seem to have a built in thermostat that can't be set and isn't very precise. The first one you linked to says that it will turn on at 45°F, plus or minus 5°F, and turn off at 68°F plus or minus 5°F. 40°F may be fine for the turn on temp (I would prefer a bit higher), but I think 73°F is warmer than needed to just keep LiFePO4 cells happy.
  • I have no idea where the thermostat is, or how it will relate to the actual temperature of the cells you're trying to keep warm.
  • That first one also consumes 78W while on. That is way more than should be needed. The little cheap pads I use consume 12W at 12V.
Of note: I believe @HRTKD used a similar pad but managed to get it without a built in thermostat, so he could control it with his own thermostat. That makes it more acceptable, at least for me.
 
Horsefly said:
Of note: I believe @HRTKD used a similar pad but managed to get it without a built in thermostat, so he could control it with his own thermostat. That makes it more acceptable, at least for me.
Click to expand...

That is correct. I bought pads that were specifically designed for use on batteries, with no thermostat. I use an external thermostat that turns the pads on/off based on the temperature observed at the top of the battery. The pads are low amperage, so the heating is low and slow (controlled). A writeup of my install can be found here:

diysolarforum.com

LiFePO4 Battery Warmer Install

This is my install of a custom set of battery warmers from Ultra Heat. I had contacted them through email asking about a warming profile for LiFePO4 batteries. They responded with a design to my specifications, including warming pads that fit my batteries exactly. While I initiated the...
diysolarforum.com diysolarforum.com
 
I found this controller. Switch
I need to look through my supply of metal around here to make a little sandwich heat exchanger.
Copper or aluminum I suppose.
 
I am in the process of making an insulated/heated battery box for my 4s 302AH cells. Here is the 12V controller that I bought, and the heating pads that I am going to try. The heat pads are 7.5W 12V and I will be using 4 of them for a total of 30 Watts......current draw from the battery should be approx. 2.3 amps when the pads are on. My box is made from plywood and has 1" foam insulation on all four sides, the top, and the bottom. I also lined the inside of the box with aluminum sheeting that is approx. .030 in thickness. The heater pads will be attached directly to the aluminum sheeting inside the box. Here are the links to both the heat controller and the pads. NOTE: I have not finished this up yet, so I have no solid results on how it may or may not work.....so YMMV.....


 
Last edited:
triplethreat said:
I am in the process of making an insulated/heated battery box for my 4s 302AH cells. Here is the 12V controller that I bought, and the heating pads that I am going to try. The heat pads are 7.5W 12V and I will be using 4 of them for a total of 30 Watts......current draw from the battery should be approx. 2.3 amps when the pads are on. My box is made from plywood and has 1" foam insulation on all four sides, the top, and the bottom. I also lined the inside of the box with aluminum sheeting that is approx. .030 in thickness. The heater pads will be attached directly to the aluminum sheeting inside the box. Here are the links to both the heat controller and the pads. NOTE: I have not finished this up yet, so I have no solid results on how it may or may not work.....so YMMV.....


Click to expand...
Sounds like it should be fine. Here's some things to consider:
  1. I can't tell from the Amazon listing, but the thermostat needs a delta or hysteresis temperature. If you target temp is 50°F you may want the heat to turn ON at 45°F and then OFF at 50°F. The hysteresis temperature (5° in this case) lets the heat turn ON and OFF over a longer period of time.
  2. You posted two links to the thermostat, so there was no link to the heating pads. I tested lots of pads, and determined that some heat up too fast and can potentially get too hot. So you might cook one side of the cells while waiting for the other side of the cells are still too cool. One of the most important things about heating cells with heating pads is to do so slowly, so that the thermal mass of the cells can absorb and disperse the heat throughout. I ended up settling on heating pads that had a maximum temp of 60°C, instead of the 100°C or higher that most pads seemed to have.
  3. Hopefully the "aluminum sheeting" is thick enough to disperse the heat from the pads. Same thing about the low and slow heating.
I've posted a few times the graphs of my system, where the temps went up and down and when the heating pads were turned on. It all works really well as long as you apply the same thought as smoking meat: The key is low and slow.
 
I used silicone heating pads that adhere directly to the cells (AliExpress etc...). They had built in thermistors so you could track the temperature of the pads themselves in order to keep the outsides of the cells from getting crispy. However, I ended up wiring 4 pads in series which resulted in only 10 Watts of heating per pad. At this level the pads stay at a reasonable temperature and don't require separate monitoring. I definitely agree with Horsefly on low and slow, it eliminates or minimizes another possible source of battery damage.
 
Horsefly said:
Sounds like it should be fine. Here's some things to consider:
  1. I can't tell from the Amazon listing, but the thermostat needs a delta or hysteresis temperature. If you target temp is 50°F you may want the heat to turn ON at 45°F and then OFF at 50°F. The hysteresis temperature (5° in this case) lets the heat turn ON and OFF over a longer period of time.
  2. You posted two links to the thermostat, so there was no link to the heating pads. I tested lots of pads, and determined that some heat up too fast and can potentially get too hot. So you might cook one side of the cells while waiting for the other side of the cells are still too cool. One of the most important things about heating cells with heating pads is to do so slowly, so that the thermal mass of the cells can absorb and disperse the heat throughout. I ended up settling on heating pads that had a maximum temp of 60°C, instead of the 100°C or higher that most pads seemed to have.
  3. Hopefully the "aluminum sheeting" is thick enough to disperse the heat from the pads. Same thing about the low and slow heating.
I've posted a few times the graphs of my system, where the temps went up and down and when the heating pads were turned on. It all works really well as long as you apply the same thought as smoking meat: The key is low and slow.
Click to expand...
Let me try this again.......The picture will show just three....and I bought the three pack, but I also bought a single one, for a total of 4 that will be in the box. Two on each long side of the box, and placed towards the bottom of the box, as heat will rise. Sorry about the post of two of the controllers. The instructions that come with the controller should allow me a 20 degree differential between when it turns the controller on and the pads start heating, and when it turns them off when that differential setting is satisfied. I'm certainly not a pro on digital temperature controllers, but that is what it sounds like to me, the way it will work.

PS.....I edited the original post so that both the controller and the pads are shown....thanks for catching that.......brain farts get more common as the birthdays pile on. :(

 
Last edited:
Bzzzt said:
I used silicone heating pads that adhere directly to the cells (AliExpress etc...). They had built in thermistors so you could track the temperature of the pads themselves in order to keep the outsides of the cells from getting crispy. However, I ended up wiring 4 pads in series which resulted in only 10 Watts of heating per pad. At this level the pads stay at a reasonable temperature and don't require separate monitoring. I definitely agree with Horsefly on low and slow, it eliminates or minimizes another possible source of battery damage.
Click to expand...
If all four of my pads are on and heating.....it will be a total of 30 watts, or a little over 100 BTUs. Do you think that should be enough in an insulated box? I know that's hard to say.....it depends on how well it's insulated, how cold the ambient temperature is, etc.
 
triplethreat said:
If all four of my pads are on and heating.....it will be a total of 30 watts, or a little over 100 BTUs. Do you think that should be enough in an insulated box? I know that's hard to say.....it depends on how well it's insulated, how cold the ambient temperature is, etc.
Click to expand...
That is plenty to maintain a temperature. My four pads at 10W each warm the batteries 30F in a couple of hours. This is for 8 120A cells in a 2" foam enclosure.
 
I went to the storage lot to work on my RV trailer today. The 87% state of charge took me by surprise. I forgot to turn on the solar charge controller two weeks ago. Oops.
blush2.gif
The system has been running the heating pads since then with no charge. I was impressed. The lows each day have been in the low 20's and upper teens.

I have 560 Ah of battery capacity.
 
HRTKD said:
I went to the storage lot to work on my RV trailer today. The 87% state of charge took me by surprise. I forgot to turn on the solar charge controller two weeks ago. Oops.
blush2.gif
The system has been running the heating pads since then with no charge. I was impressed. The lows each day have been in the low 20's and upper teens.

I have 560 Ah of battery capacity.
Click to expand...
It is really surprising how little energy is required from the battery to keep it warm if you have it well insulated, even if it is pretty cold. Glad your results are looking like mine.
 
Horsefly said:
It is really surprising how little energy is required from the battery to keep it warm if you have it well insulated, even if it is pretty cold. Glad your results are looking like mine.
Click to expand...

I didn't think my insulation job was that great. I would call it "casual". I didn't seal any of the edges. I cranked up the on-board generator today and after some encouragement, got the converter to kick in. The shunt measured ~75 amps of charge from the converter and the MPPT together. Split between the two batteries, that was about 35 amps each. I checked on the temperature every few minutes and from 87% to about 98% state of charge the battery temp came up about 3 degrees F to around 48 degrees. That surprised me a bit, so maybe my insulation is better than I thought.

I turned off the converter as it was stuck on bulk longer that I expected. I was running the generator to power a drill press and chop saw, not really to charge the batteries.
 
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