LiFePO4 heating pad for cold temperatures

I did not find proper information regarding what energy is needed to heat up a battery in order to get it to a working temperature. So I did some experiments with my 48V 280Ah battery. With 1kWh I was able to heat it up from 2C to 22C. This is with proper heat insulation (6cm EPS) and in a 24-hour time window.

Let's consider the insulation perfect. The total cell weight is ~90kg. This means that you need ~1W of energy per kilogram of LiFePo4 cells to heat them by 1 degree Celsius.

Now let's consider the information on the internet about the LiFePo4 cells' specific heat energy. The numbers are at ~1000J/kg. Or roughly 0.3W per kilogram of cells to heat them up by 1 degree Celsius. Looks likely the insulation I have is far away from perfect.

This should give a rough estimate of the required heating time with a specific heating pad. I guess this would be useful when somebody is trying to decide what heating element is needed for his case.
I tried to address this mathematically in a thread a couple of years ago: https://diysolarforum.com/threads/on-keeping-lfp-warm.17629/
 
Hi, my apologies if this has been suggested already. After going through about 18 of the 36 pages of replies here, I decided to just go ahead and describe the dead-simple, cheap and effective battery warming system that I used last winter.

I have an off-grid cabin here in Bethel, Vermont. The cabin temperature frequently reaches subfreezing temperatures for several days/weeks in a row.

Electrical system consists of four 360W solar panels, two Growatt SPF 3000TL LVM-ES inverter/charger/MPPT controllers in parallel (120v). Batteries are one Jakiper 48v 100ah lifepo4 running in parallel with four series-connected 12v 200ah AGM batteries.

I built an easily removable enclosure, similar to the engine covers used with inboard speed boats - 2x4 frame, 1/2" OSB and interior lined with 1 1/2" rigid foam.

In the box I have a $20 120v electric baseboard heater thermostat like this one connected to the inverter that controls a 120v socket. Heating pad plugged into that and lying on top of the lithium battery. Alternatively, one or two (for redundancy) low wattage (~15w) light bulbs. Not too close to the thermostat. Have thermostat set to turn on when temperature in box drops below 40°-50°F.

An afternoon's work, $100 or so in parts and materials, simple, reliable. Little technical expertise needed. In the spring, take the box off and stick it in a corner.
I have a converted bus with 2-200 ah lifepo4 batteries stowed in the unheated underbed storage. Last year we lived in the RV for most of the winter while staying at several ski areas with many sub-zero days. It required we run our diesel heater more than necessary to try to get enough heat to keep the batteries above freezing. This year I am going to move the batteries closer to the living section wall, insulate with additional 2" foam insulation, add small fan to pull air from the heated space. I am going to use your idea of the baseboard heater thermostat. I am going to try using a 19.7x19.7, 12v, 35w pet heating pad with a possible programmable timer.
 
I have a converted bus with 2-200 ah lifepo4 batteries stowed in the unheated underbed storage. Last year we lived in the RV for most of the winter while staying at several ski areas with many sub-zero days. It required we run our diesel heater more than necessary to try to get enough heat to keep the batteries above freezing. This year I am going to move the batteries closer to the living section wall, insulate with additional 2" foam insulation, add small fan to pull air from the heated space. I am going to use your idea of the baseboard heater thermostat. I am going to try using a 19.7x19.7, 12v, 35w pet heating pad with a possible programmable timer.
The 12V self regulating mobile tank heaters work fairly well. I used them in my unheated garage in NY during winter, and they kept the battery box above freezing the whole time.
I moved south, so I have two if you want them. They are 8x25 inches. I covered them in high heat tape to prevent any electrical shorting to my LiFePo4 cells. (I never trust the blue plastic).
 

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The 12V self regulating mobile tank heaters work fairly well. I used them in my unheated garage in NY during winter, and they kept the battery box above freezing the whole time.
I moved south, so I have two if you want them. They are 8x25 inches. I covered them in high heat tape to prevent any electrical shorting to my LiFePo4 cells. (I never trust the blue plastic).
How did you insulate your battery box? I'm about to put together a system for my father & stepmother that'll run the garage door openers, but have been trying to figure out what type and how much insulation to use building the box.
 
How did you insulate your battery box? I'm about to put together a system for my father & stepmother that'll run the garage door openers, but have been trying to figure out what type and how much insulation to use building the box.
I made the battery box out of a truck toolbox. It fit 32 x 280ah cells perfectly. I glued fairly thin foam padding inside on the sides and bottom before putting the cells in.

I then added half inch cutting board material over the terminals to hold the BMS's, and make sure the box cover chains never hit a terminal. I added rubber padding between each cell to avoid shorts (dont trust the blue cell covering!) Lastly, I laid Rock wool insulation, which is non-conductive, fire proof and water proof. I put the two heating pads along the front and back of the box along the cells.
 

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The 12V self regulating mobile tank heaters work fairly well. I used them in my unheated garage in NY during winter, and they kept the battery box above freezing the whole time.
I moved south, so I have two if you want them. They are 8x25 inches. I covered them in high heat tape to prevent any electrical shorting to my LiFePo4 cells. (I never trust the blue plastic).
Bro how did those run you? i would not mind playing with a set of those for shits and giggles. let me know how much and a palpal addy and if we can agree on costs, i will take them off your hands to play with.
 
Bro how did those run you? i would not mind playing with a set of those for shits and giggles. let me know how much and a palpal addy and if we can agree on costs, i will take them off your hands to play with.
What is your zip code? I will check on shipping costs.
 
What is your zip code? I will check on shipping costs.
96349-0113 for the military mailing address, 238-0026 for the local Japanese address whichever is easier for you, if same then give me price for both so I can compare with and without import taxes. (no import taxes for base, but limited on size etc.
 
96349-0113 for the military mailing address, 238-0026 for the local Japanese address whichever is easier for you, if same then give me price for both so I can compare with and without import taxes. (no import taxes for base, but limited on size etc.
No cost for the heating pads. Just shipping. I will package them up in a box. PM me with your full address. What service? Air Force?
 
No cost for the heating pads. Just shipping. I will package them up in a box. PM me with your full address. What service? Air Force?
You can get two of them for around $66 on Amazon. Not sure if shipping cost would be lower.

Facon 7-1/4"x25" 12V RV Holding Tank Heater Pad, CW-ST725, use with Up to 50 Gallons Fresh Water/Grey Water/Black Water Tank, 2Packs Heating Pad with Automatic Thermostat Control https://a.co/d/eYH6LZ6
 
How did you insulate your battery box? I'm about to put together a system for my father & stepmother that'll run the garage door openers, but have been trying to figure out what type and how much insulation to use building the box.
Rock wool / mineral wool at most any hardware store (assuming you are USA then lowes / home depot)
They're made from left over slag from steel production / mineral stuff respectively.
both are basically stone and do not catch fire.. until like 4000 degrees
All foam will melt at not that high temps some catch fire, so I don't use around any batteries.

If you want you can get concrete board internally of the battery box and surround it with rock wool.. which is what I do. If I had a fire it likely wouldn't leave my box at all, even if all the batteries went up in flame
 
I live in tropical areas so this doesn’t affect me. For a while I had two small independent systems because of a big load a few times a day on a large inefficient inverter and another using a Victron 12/800 to power essentials 24/7. Each had their own battery and panel because of voltage difference.

For this situation would it work to have a small AGM configured to power only a 15w heating pad keeping the LFPs warm?
 
How did you insulate your battery box? I'm about to put together a system for my father & stepmother that'll run the garage door openers, but have been trying to figure out what type and how much insulation to use building the box.
Like many here, I used 1" EPS foam board. I believe it goes by "Foamular" at Home Depot.
 
for some strange reason this thread has popped up i y feed four times indicating it was a new answer... each time it was @100 Proof talking about his insulation (good to go Bro, no complaints on you.) is the forum screwing up... are the ods messing up? whats up folks. i read this on the day it was originally posted...
 
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