cold weather variable heating for lifepo4

[Visionquest]

I want to design an insulated box with heaters for a 12v Lifepo4 battery build. I'd like to apply just enough power inside the box to keep the cells around 40 degrees without on/off cycling of the heat. E.g. - the colder it gets below 40 degrees, the more power will get applied to the heating pads. I'd like it to work down to zero F where it would apply full power. So I'm looking for help with any of the following areas...either example builds or suggested products.

1) How to size the heating pads power. If I use R10 insulation around the outside of a 14" x 8" x 9" box with four EVE280aH cells and a BMS inside, how many watts of heating power will be needed to maintain the cells at 40 F in an ambient temp of 0 F? (assume no self-warming in the cells)

2) What to use to do a variable voltage control of the heaters? At 0 F they should be at full "12v" voltage. At 40 F ambient they should be down to zero voltage. I'd like the heater control to apply as much voltage as needed to maintain cells at 40 F throughout the range of ambient temps from 0F to 40F. Does this product already exist somewhere or do I need to design it from scratch? (My background is electronics design.)

3) what is the best sensors to use and best place put them in the cell stack? in between the cells? top? sides? Terminals? What will be the most representative of internal temperature?

Thanks for any help and suggestions!

 

[sunshine_eggo]

1) calculate heat load:

Heat Loss R-Value Calculator - Calculator Academy

Enter the surface area (ft^2), the temperature differential (F), and the R-value into the Heat Loss from R-Value Calculator. The calculator will evaluate

calculator.academy

1 BTU/hr = 0.2928W

2) No idea. Given that you have a unique surface area, it's unlikely something off-the-shelf exists.

3) See #2.

With so many thermostatic control options out there, I see this as unnecessarily complex for little to no value.

 

[Daddy Tanuki]

are you 12 or 24 volt? go with something like the inkbird like everyone else does. set the temps that keep your batteries safe. if you are 48 volt then like me you will burn several supposed 48 volt relays trying to find one that works. in the end I wired multiple 25 watt 12 volt pads in series until I got the temp i wanted without relays and controllers. making it variable sounds like a massive PITA unless you just want the challenge.

 

[JRH]

I want to design an insulated box with heaters for a 12v Lifepo4 battery build. I'd like to apply just enough power inside the box to keep the cells around 40 degrees without on/off cycling of the heat. E.g. - the colder it gets below 40 degrees, the more power will get applied to the heating pads. I'd like it to work down to zero F where it would apply full power. So I'm looking for help with any of the following areas...either example builds or suggested products.

1) How to size the heating pads power. If I use R10 insulation around the outside of a 14" x 8" x 9" box with four EVE280aH cells and a BMS inside, how many watts of heating power will be needed to maintain the cells at 40 F in an ambient temp of 0 F? (assume no self-warming in the cells)

2) What to use to do a variable voltage control of the heaters? At 0 F they should be at full "12v" voltage. At 40 F ambient they should be down to zero voltage. I'd like the heater control to apply as much voltage as needed to maintain cells at 40 F throughout the range of ambient temps from 0F to 40F. Does this product already exist somewhere or do I need to design it from scratch? (My background is electronics design.)

3) what is the best sensors to use and best place put them in the cell stack? in between the cells? top? sides? Terminals? What will be the most representative of internal temperature?

Thanks for any help and suggestions!

I am doing what you’re wanting to do , for the last two winters….but with a much larger insulated battery box .( 4ft x2ft x2 ft.) …a 25 watt incandescent refrigerator light will keep mine 45 to 50 F…

probably a 10 watt light bulb would keep yours 40ish F ..ya may need a 15w or maybe a 5w ….dunno. I have tested compact florescent and LED too…they all work at some wattage , but you have to figure out what works in your situation .
just plug it up near the bottom and leave it on..…experiment…you may need to run it only at night.….??? …do some tests…
I elevate the batts off the bottom about 3/4 inch with fir strips to
get air under em …
A very small 3 -5w whisper fan will help by stirring the air around.

 

[Madcodger]

I am doing what you’re wanting to do , for the last two winters….

What state or climate zone, and how much/what type of insulation for the box, please? I'm in Maine, so trying to get an idea for what might be needed here. Get down to about -20F on rare occasions (although not this year).

 

[Daddy Tanuki]

What state or climate zone, and how much/what type of insulation for the box, please? I'm in Maine, so trying to get an idea for what might be needed here. Get down to about -20F on rare occasions (although not this year).

shit its almost a formula at this point. 12 or 24 use one of the many temp controllers on ebay or amazon. 48 wire them in series until they hit the temp point you want....

 

[Visionquest]

1) calculate heat load:

Heat Loss R-Value Calculator - Calculator Academy

Enter the surface area (ft^2), the temperature differential (F), and the R-value into the Heat Loss from R-Value Calculator. The calculator will evaluate

calculator.academy

1 BTU/hr = 0.2928W

2) No idea. Given that you have a unique surface area, it's unlikely something off-the-shelf exists.

3) See #2.

With so many thermostatic control options out there, I see this as unnecessarily complex for little to no value.

Thanks for that link. Very helpful starting point for figuring out heating levels as I actually design and build the box. I also agree that my approach is likely too complex / unnecessary but part of this process for me is having fun designing and building.

 

[Visionquest]

are you 12 or 24 volt? go with something like the inkbird like everyone else does. set the temps that keep your batteries safe. if you are 48 volt then like me you will burn several supposed 48 volt relays trying to find one that works. in the end I wired multiple 25 watt 12 volt pads in series until I got the temp i wanted without relays and controllers. making it variable sounds like a massive PITA unless you just want the challenge.

Just 12v and I will look at the inkbird type solution though the challenge is all a part of it for me since I am a retired engineer. Smiles. Thanks for responding!

 

[Checkthisout]

Absolutely no point to the variable control.

I have lots of ideas that would work but rigging everything up would be a waste of time with no benefit.

 

[Visionquest]

shit its almost a formula at this point. 12 or 24 use one of the many temp controllers on ebay or amazon. 48 wire them in series until they hit the temp point you want.... its not friggin rocket science. put them on a 1/4" or 6mm aluminun sheet and stop asking questions that have been answered a million times.. read... its already written down repeatedly in all of its various permutations.

I have read and researched all the normal solutions. So, yes, I do my work ahead of just asking the same questions. What makes my investigation unique is looking for a variable heat control for which I have not seen in other posts. I may give up and go with the "tried and true" if I run into a brick wall. Otherwise I will post my solutions after my build. Still hoping someone has run across a variable controller.....

 

[kommando]

Still hoping someone has run across a variable controller.....

Thermostatic control switch, set a target temp to turn on and let it turn on and off to regulate a bulb.

 

[Adam De Lay]

Here's what I did to keep my 12v cells warm during the winter. Worked great and used minimal power.

 

[JRH]

Here's what I did to keep my 12v cells warm during the winter. Worked great and used minimal power.

Haaaaaaaaaaaa…that 250 watt bulb could cook rotisserie chicken that small of a space…
I love it..
The duty cycle of 250/watt light would be about 1 min per hour….

J.

 

[JRH]

What state or climate zone, and how much/what type of insulation for the box, please? I'm in Maine, so trying to get an idea for what might be needed here. Get down to about -20F on rare occasions (although not this year).

I’m up on a mtn in the NC/ TN Appalachians. It’s below zero here often, and -10 to -15 F and below now and then. Lota snow …lotta wind ….but there are many variables to be considered for each person conditions .

I’m using 2 inch insulation and plywood on all sides of my box …it has removable lid for summer use .they are all 100 amp 12 v batts ..

A little heat and little fan and you can do it easy …
With this said , one can also complicate it to many levels…

I just want simple stuff that works and requires very little remembering or attention or effort….and won’t burn the place down if you forget to do somthing.

I don’t want to trust a thermostat that I don’t need to achieve my goal , especially a cheap China one…

A simple light , and a simple computer type fan and bingo it works perfect ..AC or DC …
Whatever works for you…

Insulation is the key.

The only tweak would be if its Gona be super cold switch bulbs out for the time ya need more heat…then switch back later…

Good luck.
J

 

[AmpD]

Computers have a lot of temp sensors and variable speed/temp controls for fans and cooling equipment. Most of it is low voltage dc.

 

[Adam De Lay]

Haaaaaaaaaaaa…that 250 watt bulb could cook rotisserie chicken that small of a space…
I love it..
The duty cycle of 250/watt light would be about 1 min per hour….

J.

Well, we use them for chicken...just not prior to them being eaten...
That bulb was the first thing that came to mind because we have a handful of those lamps for our brooding boxes.

 

[NC_hydro]

I’m up on a mtn in the NC/ TN Appalachians. It’s below zero here often, and -10 to -15 F and below now and then. Lota snow …lotta wind ….but there are many variables to be considered for each person conditions .

Near Roan Mountain?

 

[JRH]

Well, we use them for chicken...just not prior to them being eaten...
That bulb was the first thing that came to mind because we have a handful of those lamps for our brooding boxes.

Yea , I got one for my chicks they seem to like it….… . they also work well on humans… in the bathroom when you step out of the shower when it’s chilly …
J…..

 

[JRH]

Near Roan Mountain?

Nope …..pretty up there I will say… used to buy 100 lbs of trout from ole Will Cabel for 10 years in elk park…..was up on top of Beech for 15 years but moved down to a more civilized and less crowded altitude ….im over on the north watauga side of the the high country…only about 4000 ft elev.

I’m not as young and robust as I once was…but I’m very remote and can’t see a soul near me
…I love it…..
J.

 

[Madcodger]

I’m up on a mtn in the NC/ TN Appalachians. It’s below zero here often, and -10 to -15 F and below now and then. Lota snow …lotta wind ….but there are many variables to be considered for each person conditions .

I’m using 2 inch insulation and plywood on all sides of my box …it has removable lid for summer use .they are all 100 amp 12 v batts ..

Thanks! Very helpful and I appreciate your reply.