LiFePO4 heating pad for cold temperatures

[Trabuco]

Am using these in my build:

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$10, 12 watts each, 100mmx120mm.

4 per side of these glued to an aluminum plate will just about perfectly fit my 4 x 280AH cells.

With combinations of parallel and serial connections, I will have the flexibility to drive 96 watts total in parallel, or signficantly less for a more gentle heat without the need for any additional electronics.

Plus I can change after building as needed by only swapping a few connectors,

 

[Horsefly]

Last summer I bought these: https://www.amazon.com/gp/product/B081T57SP4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 They are no longer available, but were $14 for the set of four, if anyone cares.

I needed 24V pads, and there were not many choices. These are rated 30W, and when I hook one up to 25.8V from my bench power supply it does indeed pull about 1.34A. That part is good. I don't have the intended aluminum plate yet, so to test it out I taped one to the back of the steel side panel of an old desktop PC. I powered it up and within 3 minutes the other side of the steel panel was at 180°F, too hot to touch. I then saw some smoke coming out from under the panel so I killed the power. I don't have any good kapton tape, so I had used some duct tape for my experiment, and the tape was not happy.

Of course the caveat on this experiment is that I didn't have batteries or other high heat capacity mass sitting on the top of the panel. Still, I think this thing was getting so hot so fast that I would worry it would damage the bottom of the cells before the body of the cell got up to temperature. It seems the goal should be to heat slowly, to allow the heat to radiate through the whole pack.

I think I've seen people elsewhere on the forum saying they are going to use 120W (10A at 12V), and I can't imagine that not being a problem.

I have a couple of alternatives:

1. I tried hooking two of the pads in series. The Amp draw drops down to 0.659A, just under 17W. After about 7-8 minutes the top of the panel had gotten up to just under 100°F. My guess is that this heats slow enough that the heat can radiate through the cells from the bottom without over-heating the bottom of the cells. This may work.
2. I could buy some of the similar 12V heating pads, running a couple in series. I'm now a bit confused about what the right wattage should be. I'd rather not buy a ton of them just to use a process of elimination until I find the right size. The things I have read in this thread - in light of my experiment - have me confused.

 

[HRTKD]

Low and slow. My pads are rated for 1 amp each. It takes between 30 and 45 minutes to bring my batteries up to temperature, depending on where the start temperature is. My typical range is 35° F to 45° F. Pads on adhered to the long sides of the battery and the temperature sensor is located on the top/middle of the battery.

 

[Horsefly]

Low and slow. My pads are rated for 1 amp each. It takes between 30 and 45 minutes to bring my batteries up to temperature, depending on where the start temperature is. My typical range is 35° F to 45° F. Pads on adhered to the long sides of the battery and the temperature sensor is located on the top/middle of the battery.

Thanks @HRTKD - I had seen yours (nice work by the way), but I couldn't find any wattage for your pads. Now I guess you have answered that: ~12W each. I had assumed that I would put my heat pads under the pack, separated from the pack by an aluminum plate to dissipate the heat across the base of the cells. I also was going to use some silicone pads (kitchen trivets) underneath the heat pads. What made you decide to put the pads on the sides?

 

[HRTKD]

What made you decide to put the pads on the sides?

I didn't want the pad under the battery. That's a lot of weight. They can be ordered that way, but the incoming wires would be on the bottom of the battery. That could alter the stability of the battery, as slight as that would be. With the pads on the side, neither of those points is an issue. With pads on both sides of the battery I get a lot of surface area covered.

My pads are specifically designed to be used on batteries. Therefore there is no need for plate to spread the heat.

 

[Horsefly]

Update: I just re-looked at the specs of the heat pads I bought, and it says the maximum heating temp is either 170°C or 200°C (two values quoted in two places). YIKES! That's way too hot. I guess I need to buy some with a lower max. The beehive pads that @Trabuco pointed to may fit the bill, with a max temperature of "only" 60°C.

 

[Horsefly]

I didn't want the pad under the battery. That's a lot of weight. They can be ordered that way, but the incoming wires would be on the bottom of the battery. That could alter the stability of the battery, as slight as that would be. With the pads on the side, neither of those points is an issue. With pads on both sides of the battery I get a lot of surface area covered.

My pads are specifically designed to be used on batteries. Therefore there is no need for plate to spread the heat.

Oh yeah, yours is mobile, so I guess I understand the stability issue. Mine is stationary (basement of our cabin), so I guess our issues are different.

Since heat rises, I'm going to stick with putting the elements under the pack. (until I change my mind )

 

[HRTKD]

Update: I just re-looked at the specs of the heat pads I bought, and it says the maximum heating temp is either 170°C or 200°C (two values quoted in two places). YIKES! That's way too hot. I guess I need to buy some with a lower max. The beehive pads that @Trabuco pointed to may fit the bill, with a max temperature of "only" 60°C.

Still too hot in my opinion. But as long as you're not putting the pad directly against the battery and there is plenty of circulation around the battery to allow the heat to get around, it might be OK. I may be biased on this, but I think you guys are killing yourselves with all these whacky workarounds.

 

[Horsefly]

"Whacky workarounds"? I resemble that!

Not disagreeing, but what exactly are you calling a whacky workaround?

 

[HRTKD]

"Whacky workarounds"? I resemble that!

Not disagreeing, but what exactly are you calling a whacky workaround?

Attaching pads to plates and hoping the heat will spread around the entire battery without cooking the bottom of the battery first. A fan to circulate air around the battery would help, but that's just making whacky, whackier.

My pads do not have internal thermostats. They're designed to be slow heaters, allowing the heat to spread throughout the battery, while avoiding excessive temperatures.

 

[Horsefly]

I certainly don't think a fan is necessary, or even a good idea.

I think most of what is discussed on this board is some degree of whacky, so I'm Ok with that.

 

[HRTKD]

I never got to thermodynamics in college before I changed majors, so I can't figure out heat transfer equations. I paid the man to do that for me. Yes, it cost me more than most of the solutions that are being proposed/tested/implemented. But I have no worries that my system is safe for my batteries.

 

[BiduleOhm]

Since heat rises, I'm going to stick with putting the elements under the pack. (until I change my mind )

Heat doesn't rise. Hot air rise only when surrounded with colder air and only if there's gravity. And it's not even because it's hot (that's just a side effect) but because it's less dense than colder air.

Your battery will be heated by conduction only, so putting the pad(s) on the side is a far better mechanical solution without compromising the thermal side of things

 

[Fpoet]

Still too hot in my opinion. But as long as you're not putting the pad directly against the battery and there is plenty of circulation around the battery to allow the heat to get around, it might be OK. I may be biased on this, but I think you guys are killing yourselves with all these whacky workarounds.

What's the max temp of your pads?

 

[Horsefly]

Heat doesn't rise. Hot air rise only when surrounded with colder air and only if there's gravity. And it's not even because it's hot (that's just a side effect) but because it's less dense than colder air.

Your battery will be heated by conduction only, so putting the pad(s) on the side is a far better mechanical solution without compromising the thermal side of things

Good point. I knew that, at one point in my career. Makes me wonder what else I used to know that I don't anymore.

 

[fafrd]

Update: I just re-looked at the specs of the heat pads I bought, and it says the maximum heating temp is either 170°C or 200°C (two values quoted in two places). YIKES! That's way too hot. I guess I need to buy some with a lower max. The beehive pads that @Trabuco pointed to may fit the bill, with a max temperature of "only" 60°C.

Watts matter, but so do W/area (energy density)

My 40W carboy heater spreads those Watts over a surface of 275 inches^2, or less than 0.15W/inch^2.

In never gets too warm to touch operating in ambient...

 

[Horsefly]

Watts matter, but so do W/area (energy density)

My 40W carboy heater spreads those Watts over a surface of 275 inches^2, or less than 0.15W/inch^2.

In never gets too warm to touch operating in ambient...

Ahhhh a carboy heater. That brings back some memories with my buddies homebrew. By it's nature, that would be a low and slow heat. I would assume there is no such thing as a 12VDC or 24VDC carboy heater though.

 

[fafrd]

Ahhhh a carboy heater. That brings back some memories with my buddies homebrew. By it's nature, that would be a low and slow heat. I would assume there is no such thing as a 12VDC or 24VDC carboy heater though.

I suspect you can put DC through them but will lose power as a result.

I just measured and they have a resistance of 350 Ohms, so ~68.6mA @ 24V or 1.65W

They are actually designed to be cut and halved, so two halves in parallel would give you 137.2mA through each half or 3.3W through each half for a total of 6.6W. 2 Carboy heaters set up that way would provide over 13W which may start to be an option depending on what level of W you are designing to offset based on your insulation and worst-case (cold) temps.

 

[Trabuco]

Still too hot in my opinion. But as long as you're not putting the pad directly against the battery and there is plenty of circulation around the battery to allow the heat to get around, it might be OK. I may be biased on this, but I think you guys are killing yourselves with all these whacky workarounds.

I'm trying to keep the build simple and flexible.

They're 12 watts. And will be attached to an aluminum plate on a battery to further distribute the heat. And as I stated, can be wired in series with no additional controls to reduce the wattage even further if I think it's getting too warm.

The 60C rating is likely max open air temp without being attached to a massive heat sink like a 50 pound battery.

 

[HRTKD]

The 60C rating is likely max open air temp without being attached to a massive heat sink like a 50 pound battery.

Yes, but are you absolutely, 100% certain of that? Are you really willing to test that theory on an expensive battery?