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LiFePO4 heating pad for cold temperatures

upnorthandpersonal said:
Yeah, you need to use a heat spreader (aluminium block or something) with those.
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I'm using those same 7W polyimide heaters stuck the bottom of an .060 aluminum plate. In free air, I can't get the plate much above 100 degrees. With the cells on top of it, it doesn't get more than a few degrees above cell temp as the cells are conducting the heat away as is intended.

Here is a link to what I did. https://diysolarforum.com/threads/resisters-as-the-heater.32424/#post-396707
 
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Just to give feedback on the heating system I set up to warm up the two 280Ah packs in my van, thanks to the info I found here on this forum.

I have four 12W heating pads wired in parallel that make up an external DC circuit (fused), controlled by a thermostat. The pads make contact with the cells through the opening I cut in the bottom of the cases (the weight of the batteries compresses the EZ-Cool material underneath, so yes, the pads do contact the cells).

The thermostat temp probe is placed in one the battery cases, between one cell and the insulation.

The heating circuit draws about 3,75A.

The thermostat is set up so that heating starts at 5 degrees Celsius (41 F) and stop at 10 C (50 F).

On top of that, I have two BMS controlled 15 W pads in each pack that only heat when a charge is present (the BMS cuts charging under 5 C and powers the pads until the sensors reach 10 C).

I am including a picture showing how the pads are placed under the boxes (that was in the build process last year, things are more neatly organized, now).

Here is a snapshot of the system temperatures, taken this morning here in the Quebec city area, Canada, with the van in the driveway:

Outside temperature : -18 C (approx. -1 F)

Temperature inside the van (as given by my Victron MPPT sensor on the positive terminal of my Lynx distributor) : - 10 C (approx. 14 F)

Battery temperature (as reported by the BMS - the two probes are placed on the top of the cells) : 6 C (approx. 43 F)

Between outings, the van sits in the driveway. According to the EVE cells specs, the cells can rest one month at -20 C, but I would rather play it safe. So when outside temperature is supposed to go below -15 C, I warm up the van’s interior with a small portable 120V heater and activate the heating circuit.

In a test made last week to see how long it would take to warm up the batteries before going out, I let the batteries reach -15 C, then started the 120V portable heater, activated the heating circuit and applied a small charge so that the BMS controlled pads would also heat. It took five hours for the batteries to warm up to 10 C.

The heating is rather modest and slow, but I am quite happy with this set up so far.
 

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Fpoet said:
The thermostat is set up so that heating starts at 5 degrees Celsius (41 F) and stop at 10 C (50 F).

On top of that, I have two BMS controlled 15 W pads in each pack that only heat when a charge is present (the BMS cuts charging under 5 C and powers the pads until the sensors reach 10 C).
Click to expand...
Consider having the thermostat heaters come on a few degrees before the BMS cuts charging.
 
Horsefly said:
Or maybe more than a few degrees. It won't cost you much in terms of energy to do the heat, and it avoids the issue.
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Yes, both valid points, it is probably best to avoid triggering the BMS as much as possible. I will revise the settings accordingly.
 
For those trying to heat smaller cell packs, here's a heating pad test we did on a 4s 200ah pack (8.5" x 8" x 6.75") that some may find useful.

We applied a pair of 3x4" 12v 25w (6 ohm) silicone heating pads directly to the cells, on both, long sides of the pack (total 4 pads). Wired them series, parallel which dropped the total current for all 4 pads to 2.2a (26w) at 13.6v. With all 4 pads wired in parallel, they got too hot to comfortably hold, approx. 160f, and drew almost 9a of current. Two pads in series, approx. 120f (1.1a).

Used a power supply to power the pads for this test. As for future plans, haven't decided whether to power the pads via the heating port on a JBD 200a BMS or use a separate thermostat.

Temp sensor for the cells was located on the top, center area of the pack, in between the 2nd and 3rd cell.

Placed the cell pack (with the 4 heating pads active) inside a generic, plastic Group 24 marine-type battery box (with a top attached) and *no* insulation, outside around 10pm. Outside temps 8f. Cell temps 64f. 8 hours later, outside temps had dropped to (zero) 0f. Cell temps had only dropped 2 degrees to 62f. An IR gun confirmed cell temps on all four sides and top.

Wanted to confirm these 4 pads would heat the cells low and slow using minimal current, and give us the capability to safely charge the cells in sub-zero (<0f) temps. For us, this test was a pass on both counts.
 
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OTRwSolar said:
For those trying to heat smaller cell packs, here's a heating pad test we did on a 4s 200ah pack (8.5" x 8" x 6.75") that some may find useful.

We applied a pair of 3x4" 12v 25w (6 ohm) silicone heating pads directly to the cells, on both, long sides of the pack (total 4 pads). Wired them series, parallel which dropped the total current for all 4 pads to 2.2a (26w) at 13.6v. With all 4 pads wired in parallel, they got too hot to comfortably hold, approx. 160f, and drew almost 9a of current. Two pads in series, approx. 120f (1.1a).

Used a power supply to power the pads for this test. As for future plans, haven't decided whether to power the pads via the heating port on a JBD 200a BMS or use a separate thermostat.

Temp sensor for the cells was located on the top, center area of the pack, in between the 2nd and 3rd cell.

Placed the cell pack (with the 4 heating pads active) inside a generic, plastic Group 24 marine-type battery box (with a top attached) and *no* insulation, outside around 10pm. Outside temps 8f. Cell temps 64f. 8 hours later, outside temps had dropped to (zero) 0f. Cell temps had only dropped 2 degrees to 62f. An IR gun confirmed cell temps on all four sides and top.

Wanted to confirm these 4 pads would heat the cells low and slow using minimal current, and give us the capability to safely charge the cells in sub-zero (<0f) temps. For us, this test was a pass on both counts.
Click to expand...
Do you know how many Ah were used by the heating system for your overnight test? Is your long term plan to add insulation?
 
Horsefly said:
Do you know how many Ah were used by the heating system for your overnight test? Is your long term plan to add insulation?
Click to expand...
I believe 2.2a for 8 hours would equate to 17.6ah.

No plans to add insulation. This pack is used inside a small, sealed battery compartment that’s located just inside the interior of our truck camper. Access is via a small exterior door which does create some heat loss. Fortunately, this compartment only tracks a few degrees cooler than the interior which is heated by a propane heater. The only time we'd need the heating pads is if our truck camper's propane heater quit working.
 
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Amazon has small heating pads listed as motorcycle handlebar heaters and they are about 4 inches square sold as a pair they get very warm. I used a pair hooked up to a cheap thermostat and set for 35 f on and 38f off. Worked last winter for my overland rig.
 
fairly simple as many have done this, aluminum plate to help spread the heat. 25 watt heating pads. wire them in series to drop the out put and allows you to spread out the heat across more of the plate. a simple heat measuring control unit that runs on 12 volts to control the pads.

now here is the fun fact: wire enough of them in series and insulate it well and you do not even need the controller. about half way through last winter my 48-12 buck controller burned up and I hooked up the pads all in series and let them rip. no issues temps stayed stable without a controller until i disconnected them in the early spring as they were no longer needed. so wire them in with a switch and turn them on in the fall when the temps drop below 8C and turn them off when it regularly sits above 8C. this winter i am looking at a different controller but in honestly do not need it. on in the winter, off in the spring. BTW for 48 cells (now 64) I used sixteen 25 watt pads wired in series 8 on the bottom, and 4 on each side. originally wired in series parallel and then rewired for all in series after converter died. only difference was in total power consumed. it went up a bit when on 24-7 and all in series. insullation was the 4" blue foam around the batteries and taped shut at seams to prevent any air leakage.
 
FilterGuy said:
PIR is fire resistant. It will chare and carbonize, but it will not sustain a flame. The more traditional Rigid polyurethane (PUR) is far more problematic in a fire situation.
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First time hearing of PIR too. Tagging @diyernh since they also mentioned it in another thread.
Saw this on wikipedia:

A 2011 study of fire toxicity of insulating materials at the University of Central Lancashire's Centre for Fire and Hazard Science studied PIR and other commonly used materials under more realistic and wide-ranging conditions representative of a wider range of fire hazard, observing that most fire deaths resulted from toxic product inhalation. The study evaluated the degree to which toxic products were released, looking at toxicity, time-release profiles, and lethality of doses released, in a range of flaming, non-flaming, and poorly ventilated fires, and concluded that PIR generally released a considerably higher level of toxic products than the other insulating materials studied (PIR > PUR > EPS > PHF; glass and stone wools also studied).[10] In particular, hydrogen cyanide is recognised as a significant contributor to the fire toxicity of PIR (and PUR) foams.[11]
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Wow, amazing read while waiting for code to compile and waste a Friday away. We are running a DC 30 A charger in our van to charge a Renogy 50AH lithium battery in Colorado and now that the temps are getting colder overnight, I found this thread informative and have ordered some parts for making a battery case with heater. One thing I need to do is either make or buy some angled 2GA battery lugs. Do folks see any issue with bending straight one to make them work versus buying angled ones? I have a stash of straight one and would prefer not adding to the collection :)
 
HeathenNomad said:
One thing I need to do is either make or buy some angled 2GA battery lugs. Do folks see any issue with bending straight one to make them work versus buying angled ones? I have a stash of straight one and would prefer not adding to the collection
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The danger is that they don't end up with a flat surface to mate with the cell pads (or whatever they are being bolted to) If you put the part that needs to be flat in the jaws of a vice you can keep it flat while bending the part that hangs out. (If the vice has serrated jaws, put a piece of flat metal between the jaw and lug so the serrations don't imprint on the lug)
 
FilterGuy said:
The danger is that they don't end up with a flat surface to mate with the cell pads (or whatever they are being bolted to) If you put the part that needs to be flat in the jaws of a vice you can keep it flat while bending the part that hangs out. (If the vice has serrated jaws, put a piece of flat metal between the jaw and lug so the serrations don't imprint on the lug)
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Makes sense and by cell pads, I am guessing you mean the battery posts? Right now my battery is upright and I use flat lugs, but with the battery on its side in a to be built box, I need angled ones.
 
Some metals don't bend well. I tried bending 1/8" T6061 into a right angle and it broke. Hopefully copper works better for you.
 
HRTKD said:
Some metals don't bend well. I tried bending 1/8" T6061 into a right angle and it broke. Hopefully copper works better for you.
Click to expand...
There are bending radius specs to follow when bending higher strength aluminum allows like 6061-T6.

Aluminum Minimum Bend Radii | Cumberland Diversified Metals

CDM buys and sells aluminum coil, strip, sheet and plate but also trades tube, bar and structural products.
www.cumberlandmetals.com www.cumberlandmetals.com
 
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