LiFePO4 heating pad for cold temperatures

[Oldtoad]

Still in Maine, bit of a dusting going on here, just wanted to share my build,
Could not have done it without all the great help here.
Plexiglass, more insulation and lid to be added or it would just look like a battery box.

 

[HGPilot]

Since we don't need a lot of heat produced inside an insulated box, how about using using a few 30 watts resistors to produce heat inside the enclosure? I can lookup an adjustable thermostat and we could size R according to your battery voltage so it barely keeps it above freezing?

 

[Horsefly]

Since we don't need a lot of heat produced inside an insulated box, how about using using a few 30 watts resistors to produce heat inside the enclosure? I can lookup an adjustable thermostat and we could size R according to your battery voltage so it barely keeps it above freezing?

You can find lots of threads here of people using different types of heating pads, mostly separated from the cells by some metal sheet, but I think some have actually taped the pads directly to the cells (I would not recommend that).

Resistors do dissipate heat, but it's hard to see how it would work very well. Seems like it would mostly heat the air.

I used two of these pads for my 24V 8S battery: https://www.amazon.com/gp/product/B0794V5J5H I actually keep my cells in the 50°F - 60°F range. If the outside air temperature is between 20°F and 30°F it uses around 4Ah per day to keep the battery warm.

 

[Saul]

I have a new system in Alaska. -15 -20 Fahrenheit last week (-40 on a rare occasion some years). Cabin has no heat when unoccupied. LIFEPO4 12V system. I built an insulated battery box and put a 60W forced air fan PTC heater in it. This heater is designed to be in enclosed distribution boxes. Inside the cabin today is -3.5 deg. The heater comes on for 4 min every half hour (about 4.5 amps). I use a thermal snapdisc (40-60) inline with the positive input wire as a thermostat. Temperature is stable at 50 deg +- 5 deg F.

I currently have 3 heaters hooked up and have a manual selector switch to activate each one individually.

1. PTC heater
2. Car seat warmers placed on the underside of an aluminum plate and batteries sit on top of plate.
3. Two small plain PTC heater elements with no fan secured to a small metal plate just laying there on top of the aluminum floor of the box.

Snapdisc are still used for control.

It boils down to Low, Med and High. given current flow and time on, They all come out to about the same amps drawn. It just how fast do you want it done.

Read up on the PTC heaters.

and

https://dbkusa.com/blogs/whatisptc/ptc-heater-types-explained

 

[Horsefly]

I have a new system in Alaska. -15 -20 Fahrenheit last week (-40 on a rare occasion some years). Cabin has no heat when unoccupied. LIFEPO4 12V system. I built an insulated battery box and put a 60W forced air fan PTC heater in it. This heater is designed to be in enclosed distribution boxes. Inside the cabin today is -3.5 deg. The heater comes on for 4 min every half hour (about 4.5 amps). I use a thermal snapdisc (40-60) inline with the positive input wire as a thermostat. Temperature is stable at 50 deg +- 5 deg F.

I currently have 3 heaters hooked up and have a manual selector switch to activate each one individually.

1. PTC heater
2. Car seat warmers placed on the underside of an aluminum plate and batteries sit on top of plate.
3. Two small plain PTC heater elements with no fan secured to a small metal plate just laying there on top of the aluminum floor of the box.

Snapdisc are still used for control.

It boils down to Low, Med and High. given current flow and time on, They all come out to about the same amps drawn. It just how fast do you want it done.

Read up on the PTC heaters.

and

https://dbkusa.com/blogs/whatisptc/ptc-heater-types-explained

Is heater #1 the 4.5A load? What current do the other two pull?

I'm having a little trouble visualizing this. I get that #2 is under the aluminum plate. #3 is "on top of the aluminum floor of the box". Is this under the #2 heaters?

I would think that moving air around is not the most efficient way to keep the cells warm. I would think the heater under the batteries (with an aluminum plate to spread the heat) is your best bang for the buck. It seems like a PTC heater mounted under the aluminum plate would be best, if you can get PTC heaters without their own spreaders.

How much insulation do you have?

 

[Oldtoad]

How big are the batteries amp hour wise?
4.5amps for many days in a row adds up.
You must get days-weeks when the solar panels are covered in snow or no sunlight. Ditto on the insulation question.

 

[Saul]

Is heater #1 the 4.5A load? What current do the other two pull?

I'm having a little trouble visualizing this. I get that #2 is under the aluminum plate. #3 is "on top of the aluminum floor of the box". Is this under the #2 heaters?

I would think that moving air around is not the most efficient way to keep the cells warm. I would think the heater under the batteries (with an aluminum plate to spread the heat) is your best bang for the buck. It seems like a PTC heater mounted under the aluminum plate would be best, if you can get PTC heaters without their own spreaders.

How much insulation do you have?

The two PTC heaters (connected in parallel) that lay between the batteries on top of the aluminum plate spike to about 4 amps on start up for several seconds and then are on for 35-40 min at 1 amp.

The heating pad (different than the seat warmer mentioned in the previous post) under the aluminum plate is pretty steady at about 2.5 amps for 25 min. (30 W heater)

Heating the air is exactly what I want. Every part of the battery is subject to the heated air. Constant surrounding temperature. I have no desire to heat just the bottom of the battery. All three of these methods are heating the air. Even the pad underneath. The batteries sit on top of a small neoprene pad (1/4"). A consistent 45 to 50 deg will keep the entire battery well out of the danger zone.


Next week I hope to move to Two 200AH LIFEPO4 batteries instead of Two 100AH.

The box insulation is 2" R-10 XPS foam. All sides. The box has plenty of room for the air to move around. The batteries are not crammed in there.
Interior dimensions are roughly 22"L X 14"W X 12"H. The Aluminum plate sits off the foam with a 1" air gap to aloww room for the heating pad and wires.

The car seat warmer mentioned in the previous post is pretty neat in the sense that it has its own High and Low switch. (70W & 25W).
Its all going to come out roughly the same. It take's X amount of current to heat X amount of degrees. Placement of the snapdisc thermostat is also a key factor.

 

[Horsefly]

The two PTC heaters (connected in parallel) that lay between the batteries on top of the aluminum plate spike to about 4 amps on start up for several seconds and then are on for 35-40 min at 1 amp.

The heating pad (different than the seat warmer mentioned in the previous post) under the aluminum plate is pretty steady at about 2.5 amps for 25 min. (30 W heater)

Heating the air is exactly what I want. Every part of the battery is subject to the heated air. Constant surrounding temperature. I have no desire to heat just the bottom of the battery. All three of these methods are heating the air. Even the pad underneath. The batteries sit on top of a small neoprene pad (1/4"). A consistent 45 to 50 deg will keep the entire battery well out of the danger zone.


Next week I hope to move to Two 200AH LIFEPO4 batteries instead of Two 100AH.

The box insulation is 2" R-10 XPS foam. All sides. The box has plenty of room for the air to move around. The batteries are not crammed in there.
Interior dimensions are roughly 22"L X 14"W X 12"H. The Aluminum plate sits off the foam with a 1" air gap to aloww room for the heating pad and wires.

The car seat warmer mentioned in the previous post is pretty neat in the sense that it has its own High and Low switch. (70W & 25W).
Its all going to come out roughly the same. It take's X amount of current to heat X amount of degrees. Placement of the snapdisc thermostat is also a key factor.

Sounds like you have a solution you are happy with.

Just be aware of the physics that are at play here. The LiFePO4 cells are by far the majority of the thermal mass in your battery box. Probably over 90%. Heat them, and the rest will stay warm. Heating the air around something is just about the worst way to warm that something, especially if it is the main thermal mass.

2" XPS is good. That is what I used in my heated / insulated box. I have 8 230Ah Eve cells, and I have 2 x 12V,12W silicone heating pads. The pads are under an aluminum plate, which is under the cells. I'm currently testing it with an outside temp that hasn't got above 25°F for 24 hours. It has used 4Ah to keep the pack between 50°F and 60°F. The next 36 hours will be a tougher test, as the temps are to stay mostly below 20°F, and between 5°F and -5°F tomorrow night.

 

[S Davis]

Sounds like you have a solution you are happy with.

Just be aware of the physics that are at play here. The LiFePO4 cells are by far the majority of the thermal mass in your battery box. Probably over 90%. Heat them, and the rest will stay warm. Heating the air around something is just about the worst way to warm that something, especially if it is the main thermal mass.

2" XPS is good. That is what I used in my heated / insulated box. I have 8 230Ah Eve cells, and I have 2 x 12V,12W silicone heating pads. The pads are under an aluminum plate, which is under the cells. I'm currently testing it with an outside temp that hasn't got above 25°F for 24 hours. It has used 4Ah to keep the pack between 50°F and 60°F. The next 36 hours will be a tougher test, as the temps are to stay mostly below 20°F, and between 5°F and -5°F tomorrow night.

If you heat the air space around the batteries they never get cold soaked. I heat the air around mine and they stayed at 45-50 degrees in single digit temps in a plywood enclosure with little to no insulation. Plus I can cool them in the summer when pulling high C rates if needed. It is a lot easier to keep a battery warm by using its thermal mass to your benefit, I looked at heating the batteries directly but the heat pads were too hot for me to be comfortable even spreading the heat with aluminum sheet.

 

[upnorthandpersonal]

I looked at heating the batteries directly but the heat pads were too hot for me to be comfortable even spreading the heat with aluminum sheet.

Even those small 7W Polyimide pads, like these? (if you tried them). I've had very good results with those.

 

[S Davis]

Even those small 7W Polyimide pads, like these? (if you tried them). I've had very good results with those.

I am using a 25 watt silicone pad on each battery , I never checked the temp except by touch. I felt like it would put too much heat directly on the batteries.

 

[upnorthandpersonal]

Yeah, 25W would be way too much.

 

[curiouscarbon]

air and contact thermal regulation are both valid approaches

good luck to everyone

 

[curiouscarbon]

re: contact heating

the most efficient place to heat and cool most batteries are at the Terminals Themselves.

this is because the terminal is directly connected to all the inner layers, but the walls are only touching the outermost layers. thermal conductivity is higher at the terminals. some people even monitor temperature at the terminal itself!

of course it is very undesirable to engineer contact cooling and heating on the terminals themselves.

there are already busbar or other connections!

however, still just wanted to share

1 source:

cooling and heating same thing different direction

good luck to all on their thermoregulation adventures!

 

[BiduleOhm]

You can screw metal cased power resistors on the busbars, very cheap and easy if you want to heat the cells via the terminals

You can even find power resistors in TO packages like those or alternatively those if you want a non-conducting package. They are a lot smaller than the classic power resistor listed above if you have really tight space constraints.

 

[time2roll]

I looked at heating the batteries directly but the heat pads were too hot for me to be comfortable even spreading the heat with aluminum sheet.

I used some silicone pads rated 12v and 15 watts. Put pairs in series for 1/4 the wattage. Sorta felt warm in my hand. Stuck one to each cell without worry.

 

[S Davis]

I used some silicone pads rated 12v and 15 watts. Put pairs in series for 1/4 the wattage. Sorta felt warm in my hand. Stuck one to each cell without worry.

As long as you can limit the heat to a low enough level I don’t see a problem doing it that way, I needed a way to cool the batteries in the summer.

They are in the back of a crew cab pickup so I suspended the batteries on vibration dampers so I have air space all the way around. I will have a circulation fan and a exhaust fan for cooling if needed.

 

[Horsefly]

I used some silicone pads rated 12v and 15 watts. Put pairs in series for 1/4 the wattage. Sorta felt warm in my hand. Stuck one to each cell without worry.

Those are the silicone heating pads, which at least limit to a max heat of 60°C. I found that the polyimide pads that @upnorthandpersonal pointed to (I bought those exact ones) heat up too hot too fast. The biggest risk to watch out for is heating one area of the cell (where the pad is attached) to a relatively high temperature before the other side has even gotten warm. That could damage the cell.

By your putting the silicone ones in series you should be limiting the heat pretty well. I would feel better if you were using a heat spreader like an aluminum plate, but maybe it isn't needed. I assume you only have them on one side of the cells, and not the other?

 

[upnorthandpersonal]

I found that the polyimide pads that @upnorthandpersonal pointed to (I bought those exact ones) heat up too hot too fast.

Yeah, you need to use a heat spreader (aluminium block or something) with those.

 

[S Davis]

Those are the silicone heating pads, which at least limit to a max heat of 60°C. I found that the polyimide pads that @upnorthandpersonal pointed to (I bought those exact ones) heat up too hot too fast. The biggest risk to watch out for is heating one area of the cell (where the pad is attached) to a relatively high temperature before the other side has even gotten warm. That could damage the cell.

By your putting the silicone ones in series you should be limiting the heat pretty well. I would feel better if you were using a heat spreader like an aluminum plate, but maybe it isn't needed. I assume you only have them on one side of the cells, and not the other?

I am using the silicone @140 degrees, I still think it is too hot for direct contact.