diy solar

diy solar

LiFePO4 heating pad for cold temperatures

That is great data. Even if the set up you have is running the full 30V...... that is only 180W.

I can figure it out with the voltage & current when you measure it, but do you happen to know the resistance of the wire you use?
Just measured it at 1.4ohms for ~ 12". It is 32ga wire.
 
I kinda wish I would have seen something like this before my project. I have circular saws, jig saws, and lots of long box cutters, but I couldn't find a good way to cut smooth lines thru my 2 inch XPS. I ended up falling back to a good old hand saw. This looks like it would have avoided some of my ugly results.
Sorry, I had been meaning to post it up a long time ago but kept forgetting to.
 
I just cut that out:

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2.05 amps. At idle not cutting the voltage would be about 5.5v when cutting and the wire cooled down the voltage would be about 4.5v. Wire would glow red at ~ 2.75v (less than 3v). 8" of wire between the alligator clips (only good use for them) :)
 
I gave up on hand saws and utility knives for cutting rigid foam insulation. I could never keep a straight line and always made a mess. I started using a circular saw and that worked pretty well. It's still messy, but at least my lines are straight. I may have to give the surgical steel a try though. I have 18" sections of various gauges.
 
Dang! I loaned out my power supply today to a friend that is using it to charge LiFePO4 batteries. It'll be at least a week before I can try this.
 
Commenting for notifications. I really hope I can find time this week to hook my heaters up and test them, and subsequently contribute here.

4x 15w pads. The cells are in a plywood box with 1/4 inch poron in each end and 1/8 inch in the sides and bottom.

104F snap disk cutoff switches at the very bottom of each end cell in case the controller fails, redundant temperature sensors in case one gives up the ghost (to avoid tearing it apart to replace em) and to measure different points.

A victron battery sense about halfway up.

And the BMS sensor at about the same height.

If Battleborn has the audacity to sell a 30w heat pad for $220 then it must work well enough, so I figure 60w should suffice for me too.

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If Battleborn has the audacity to sell a 30w heat pad for $220 then it must work well enough, so I figure 60w should suffice for me too.
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.
 
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.
Yeah, I think that would be too much, or at least I hope it is. I have two 12W pads to heat my 8s 230Ah pack.

I also don't think I would put them directly on the cells. I found that even if the heating pad does get too hot, having a 0.25" think aluminum sheet between the heat and the cells really spreads out the heat to hopefully prevent any too-hot-too-quick.
 
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.
Well. Or just... turn it off. With a temperature controller.
 
Yeah, I think that would be too much, or at least I hope it is. I have two 12W pads to heat my 8s 230Ah pack.

I also don't think I would put them directly on the cells. I found that even if the heating pad does get too hot, having a 0.25" think aluminum sheet between the heat and the cells really spreads out the heat to hopefully prevent any too-hot-too-quick.
That's why my temp sensors are right next to the pads. My plan is to heat the bottom of the cell to 70F and just let it soak through. Since the thermocouple is so close to the pad it'll prevent anything getting too hot.

But testing will tell ultimately.
 
That's why my temp sensors are right next to the pads. My plan is to heat the bottom of the cell to 70F and just let it soak through. Since the thermocouple is so close to the pad it'll prevent anything getting too hot.

But testing will tell ultimately.
True, testing will tell. So your intent is basically to measure the heat of the pad, or almost. I get what you are trying, but I'm just not sure it will work.

I don't know what thermostat you plan to use, but most have a hysteresis setting. Mine is set to 10°F, so if I did like you and set the thermostat to 70°F, it will turn the heat ON when the temp gets down to 60°F, and turn the heat back off when the temp gets up to 70°F. In your set up, I think the probe next to the pad will get up to 70°F really fast, before much heating of the cell will happen. Then it will take a while for it to get down to 60°F (or whatever your hysteresis would be). So I think the duty cycle of your heater will off the vast majority of the time, and on for only a short time.

The idea of using a heat sink like an aluminum plate helps in multiple ways. It spreads the heat out uniformly, it heats up more slowly, and it cools down more slowly. So it smooths out the heating process.

I am anxious to hear how your system works, so do post back here. I may have missed it: Are you building an insulated box for this pack? If so, can you tell us something about it? Or - if I did miss it - point me back to where you described it.
 
Well. Or just... turn it off. With a temperature controller.
Please, Im not trying to pick your set up apart, just have a conversation. As you have said, the temp sensors are right next to the pads which will mitigate any overheating compared to what would happen if the sensor was on the opposite end of the cell and the pads were cooking the other end until the heat made it through. That wasnt what I was trying to imply you were going to do.

Generally speaking it is nicer for everything involved if the power of the heater is low enough so that when required it turns on and stays on until its job is done and then turns off. On for a few seconds and off again constantly is not quite as nice for all the components involved. Believe me, I know I very well could be wrong, as I said it is just a gut feeling.
 
Please, Im not trying to pick your set up apart, just have a conversation. As you have said, the temp sensors are right next to the pads which will mitigate any overheating compared to what would happen if the sensor was on the opposite end of the cell and the pads were cooking the other end until the heat made it through. That wasnt what I was trying to imply you were going to do.

Generally speaking it is nicer for everything involved if the power of the heater is low enough so that when required it turns on and stays on until its job is done and then turns off. On for a few seconds and off again constantly is not quite as nice for all the components involved. Believe me, I know I very well could be wrong, as I said it is just a gut feeling.
No negativity intended.

I deal with temperature controls on a daily basis so I'm pretty well sold on knowing the temperature at the heater rather than farther away.

When you use a thermocouple much farther away you have much less control over the actual temperature of the system at the pads.

Much less heating power can alleviate this but if that power becomes insufficient on a cooler than expected day you risk coming up short.
 
True, testing will tell. So your intent is basically to measure the heat of the pad, or almost. I get what you are trying, but I'm just not sure it will work.
You're not far off to be honest.

With cold material the fluctuations happen fairly quickly when measuring at the heater itself. I use both methods on my machines at work. Some heaters have the thermocouple built into itself and others are embedded into the heated mass.

The ones that aren't directly on the heater very often overheat during the lag behind turning on and getting the t/c up to temp. This way works faster but risks excess temps.

The ones built into the heater itself fluctuate quickly but never overheat what I'm heating. Keep in mind the thing you're heating will be drawing heat away from the heater too.

There's a balance to be reached between the two extremes of course.

I currently have a super cheap $5 controller to test it with. Setting a 10 degree hysteresis would be excessively wide for a system with slow response as you've described. The one I use is adjustable to less.

That said, PID controllers are cheap enough to upgrade if it doesn't work out then the bottom of the cell will just stay put at set point.


Oh and it's a plywood box with a thin neoprene liner and 1/4 inch of poron at each end, plus plastic cutting board type mat. Not a lot of insulation but should be enough. Certainly a lot more effective R value than the thin heater pads around an uninsulated battery case.
 
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With cold material the fluctuations happen fairly quickly when measuring at the heater itself. I use both methods on my machines at work. Some heaters have the thermocouple built into itself and others are embedded into the heated mass.

The ones that aren't directly on the heater very often overheat during the lag behind turning on and getting the t/c up to temp. This way works faster but risks excess temps.

The ones built into the heater itself fluctuate quickly but never overheat what I'm heating.

There's a balance to be reached between the two extremes of course.
I don't know if you saw what I did. I have two thermostats, wired in series. The probe for one is on top of the aluminum plate below the cells. The heating pads are mounted below the plate. This thermostat - call it T1 - is set to 90°F with a hysteresis of 10°F. The probe for the second thermostat (T2) is on the top of the cells, and the thermostat is set to 50°F (60°F for my testing) with a hysteresis of 10°F.

So in operation, it should work like this.

If the temperature of the cells (as measured on the top) gets down to 40°F, T2 turns on. If the temperature of the plate is 80°F or lower, T1 is ON. Since both thermostats are on the heating pads are turned on.

If the aluminum plate gets up to 90°F before the cells are warm, T1 will turn off until the plate cools down to 80°F again. So the heating pads are turned off, even though the cells haven't gotten to 50°F.

T1 may cycle on and off several times, making sure the heating is happening low and slow.

Eventually the temperature of the cells reaches 50°F, and T2 turns off. The entire heating system will then remain off until the cells drop down to 40°F.

Although this required the expense of a second thermostat, I feel really confident that it will allow me to heat the cells but not cook the side next to the heating pads.
 
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