LiFePO4 heating pad for cold temperatures

[HRTKD]

That is indeed the exact same thermostat that I had, that I no longer use.

 

[scottyfz6]

I just read the whole thread. I have 2 - 12 watt bee hive warmers under 2 - 60 amp lfp batteries. I also have one of the cheap amazon thermostats with the sensor between the batteries. I also have a 12 volt bosch relay shutting off the charging current when the heaters are on. My temps are set at turn on at 4C and off at 8C. The heaters claim a max temp of 85F so they should not cook my batteries.

My trailer if all foam SIPs and the batteries are touching the panel on 3 sides.

In a test it took 2 hours to heat the batteries from 3C to 8C. Damn thermostat is in C only.

 

[backwash]

that is pretty close to what I see on my battery heaters performance wise. I have a pair of 12V 280ah LFPs batts. each bat has a pair of 7W reptile heaters affixed to each side (powered by the inverter) and it takes about 2.5 hours to heat up. (total 28W load for heating) they turn on at 42 degF and turn off at 65 degF. I've placed my thermocouple on the positive post of the one battery. I use the relay contact available from the BMV-712 which activates a SSR for the temperature control on/off. works pretty great and I can use the victron bluetooth app to see the current battery temp and when the relay is closed / heating and open/off. In the summer I will look at reconfiguring the relay and the SSR to turn on a fan for cooling if I need it.

 

[scottyfz6]

t works pretty great and I can use the victron bluetooth app to see the current battery temp and when the relay is closed / heating and open/off. .

I got even more clever, the switch that turns the heaters on has 2 leds, one is internally connected to light up when the switch is on, the other has another terminal that I connected to the output of the thermostat. So when the switch is on its lite to let me know its on, the other led lights when the thermostat is closing the contact, so I have a visual that the heaters are on. I can also peak at the thermostat for the current temp.

 

[Jacob Scoville]

If they were extremely low power, would it be okay to stick the silicone pads directly on the batteries? I'm using two 100AH Renogy batteries and I have ordered a handful of 1 amp 12v pads (one on each side of each 100AH battery, 8 total) that I plan to run at 5v (or 9v in very extreme cold). Not very much heat but I was planning to go heavy on the insulation and I'm looking for a very slow constant heat. At 5v or 9v it's pulling a little less than .2 amps and .5amps respectively.

 

[HRTKD]

My pads are stuck on the long sides of my batteries - short sides of the 4s cells. Each pad is rated for 1 amp and I'm running them at 12v.

 

[backwash]

If they were extremely low power, would it be okay to stick the silicone pads directly on the batteries? I'm using two 100AH Renogy batteries and I have ordered a handful of 1 amp 12v pads (one on each side of each 100AH battery, 8 total) that I plan to run at 5v (or 9v in very extreme cold). Not very much heat but I was planning to go heavy on the insulation and I'm looking for a very slow constant heat. At 5v or 9v it's pulling a little less than .2 amps and .5amps respectively.
View attachment 30967

mine are stuck on the short side of the stack. 2 per battery and that seems to be fine. each cell gets an even amount of heat as the others.

 

[8milimeter]

UPDATE: After several days of testing I have come to the conclusion that the temperature spread on the heating pads does not need to be so large.
I have two battleborn heat pads surrounding (3) 100AH Battleborn batteries. Each heat pad has two elements, so four heating elements total. The power draw on all four is approximately 3.6amps. The thermostat sensor is taped to the top of one of the outside batteries. When the thermostat triggers it takes about 90 minutes to heat the batteries from 34 to 44 degrees "F". After the thermostat trips off at 44 degrees the battery temp gardually climbs another 5 degrees. I have decided to change the settings on the thermostat from 34-44 to 34-38 degrees "F". This I hope will keep the batteries above freezing and below 44 degrees.

 

[Jacob Scoville]

UPDATE: After several days of testing I have come to the conclusion that the temperature spread on the heating pads does not need to be so large.
I have two battleborn heat pads surrounding (3) 100AH Battleborn batteries. Each heat pad has two elements, so four heating elements total. The power draw on all four is approximately 3.6amps. The thermostat sensor is taped to the top of one of the outside batteries. When the thermostat triggers it takes about 90 minutes to heat the batteries from 34 to 44 degrees "F". After the thermostat trips off at 44 degrees the battery temp gardually climbs another 5 degrees. I have decided to change the settings on the thermostat from 34-44 to 34-38 degrees "F". This I hope will keep the batteries above freezing and below 44 degrees.

What’s the lowest ambient temperature been so far and how long does it take to cool back down to 34?

 

[8milimeter]

24 is the lowest. So far it only comes on in the early morning hours. The daytime temps are above freezing so it does not run during the day.
It takes about 6 hours to cool back down. I am sure it will be much faster in extreme cold. I will know more after the weekend. Friday night it will be 18, Sat 31, Sat night 21.

 

[8milimeter]

The new settings are working out great, 34-38 on the thermostat produces 34 to 41 actual on the batteries.
The thermostat shuts off at 38, but the temp rises some after it trips.
I adjusted the P2 setting to -1 so the thermostat reading would match the temp on my Victron monitor.
I really like this thermostat.

 

[HRTKD]

The new settings are working out great, 34-38 on the thermostat produces 34 to 41 actual on the batteries.
The thermostat shuts off at 38, but the temp rises some after it trips.
I adjusted the P2 setting to -1 so the thermostat reading would match the temp on my Victron monitor.
I really like this thermostat.

Great to hear that it's working!

 

[albacore]

Has anyone here tried self regulating heat trace for battery heating?

There are not many low voltage DC options but Freezstop FLV and Heatline Kompensator both look like they have potential.

What I like about these is that their energy output declines with increasing temperature so there is less energy loss from unnecessary heating. Furthermore, the self-regulation doesn't rely on temperature sensing at a single point, but instead on the temperature of the wire itself.

A temperature controller is still required to cut off the output above a certain point.

I think it's a little pricey, but 5 ft might be enough for a battery box. Overall this seems safer and less prone to hot spots than simple resistance heating, so combined with the efficiency gain it might be worth it.

I have in mind to snake a length of this stuff on the opposite side of the battery box from the temp sensors, so by the time the temp sensor reaches a high temp cutoff the cells are already there.

I'm also wondering if the self regulating aspect might make it safe to apply directly to the cells, though I doubt I will try that.

Has anyone here looked into these (or similar) products?

 

[scottyfz6]

No self regulating heat trace gets way to hot, even when its "self" regulating. Like temps over 100 degrees F. They say it wont get that hot, but they really mean when it attached to something like a pipe.

Master electrician here, I have installed miles of the stuff.

 

[Jacob Scoville]

If they were extremely low power, would it be okay to stick the silicone pads directly on the batteries? I'm using two 100AH Renogy batteries and I have ordered a handful of 1 amp 12v pads (one on each side of each 100AH battery, 8 total) that I plan to run at 5v (or 9v in very extreme cold). Not very much heat but I was planning to go heavy on the insulation and I'm looking for a very slow constant heat. At 5v or 9v it's pulling a little less than .2 amps and .5amps respectively.
View attachment 30967

So I think the lack of an actually flat surface on the sides of my batteries is slowing down the heat transfer. I was going to fill the gaps with rtv silicone sealant but then quickly realized that is thermally insulative, not conductive and that may have the opposite effect I am looking for. Any ideas for an alternative sealant/glue? Ordered a few aluminum sheets to spread the heat out a little more but I feel like it will still have an issue penetrating the battery.

 

[HRTKD]

With the heating pads on the sides of my batteries, the heat seems to be transferring well. I have my temperature probe for the thermostat located in the middle and center of the 4s pack. The temperature probe for the BMV-712 is on the post of the end cell. The temperature reported by both probes is within .5° F of each other. To me, that means the heat is spreading evenly.

 

[BiduleOhm]

Any ideas for an alternative sealant/glue?

Thermally conductive epoxy

 

[Jacob Scoville]

Thermally conductive epoxy

I found a lot over the last week, even some modified silicone adhesive like regular rtv that was really good, but like $70-100+ for a small tube. Cheapest seemed to be $45 on amazon for some MG chemicals stuff. I was just hoping there would be something that was somewhere between thermal insulating like regular RTV and thermally conductive like regular thermal epoxy/paste/etc and not 10 times the cost.
I got some inexpensive double sided thermal pads for sticking the heat pad to my aluminum plates that are coming in tomorrow, and I’ll just go from there. Kicking myself for not just selling my batteries at half price and picking up some internally heated Battleborn batteries.

 

[BiduleOhm]

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[Jacob Scoville]

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It’s still $15 for a super super tiny tube like many other computer paste options. The sides of my renogy batteries are terribly not flat and filling in all those pockets would take a lot.