Building an Lithium Battery Heater Kit

[jakegw2]

This past fall I performed the dreary task of swapping out my Battleborn lithium batteries for the original AGM batteries since I cannot guarantee that temperatures in my area will never get below -4 degrees F. This is a quite a pain as the batteries are located in an metal box underneath the van.

Ever since then I have been thinking about how to add a battery heater kit so that I could just turn it on, plug the vehicle in, and leave the lithium batteries installed all winter.

So this weekend I built and installed my own homebrew Lithium battery heater setup. This is how I did it:



Parts that I used:
Digital temperature controller ($12)
Heating pads ($60)
Rocker switch w/ indicator light ($9)
Long-cable temperature probe ($8)
AWG 14 wire, red ($14.5)
Heat-shrink Butt Connectors ($10)
Electrical Tape, Red ($7)
Electrical tape, Black ($9)
Yoga mat ($9)
2-Pin Waterproof 12V connectors ($9.5)
DC Fuse Kit ($13)


Tools:
Multimeter that can read DC current non-contact ($39)
Soldering Iron, 60W & kit ($17)


Assembly Notes:

1. First off let me state that I am not trying to keep the batteries toasty warm. My primary objective is to protect them in storage on the coldest winter days. In my area every few years we will see a nighttime temperature as low as -10 to -15 degrees F. These are the rare events I am guarding against where the internal temperature cannot be allowed to fall below -4 degrees F.
   
   A second goal is to extend the usable season to include overnights where the temperature gets down to 20 degrees F or so. Since the batteries need to be above freezing to charge my design goal is to be able to raise the internal temperature by at least 12 degrees F through heating.
   
   
2. I started by prepping the heating pads. The heating pads contain a bimetal disk temperature control switch that is normally closed but opens at 20 degrees C (68 F) and resets at 5 degrees C (41 F) (e.g. turns on again). These are notoriously unreliable and in fact when I tested them with a bag of frozen peas I found that one of the two pads I received didn't work, I could hear the switch turning off then on again repeatedly. Since I have an external controller it is best to just pull these out. To do this I pressed on the pad starting at where the wires entered it and found the small square object close by. I then cut into the pad carefully, avoiding the wires, then cut it off and replaced it with a butt splice.
   
   
   
3. Next I tested the resistance and current for the pads. Despite being rated for 78W at 13.5V (which given P=IV and V=IR we can solve for both and see that this would mean 5.8A and 2.3 Ohms) I measured 2.6 Ohms on one and 3.0 Ohms on the other (and the expected 5.2A and 4.5A respectively in current which is only 70W and 61W of heating).
   
   This is too much power for a battery in my opinion, I don't want to cook them or risk overheating one side. I also don't want to have 9.7A of draw whenever the are on. To reduce the power a bit I wired them in series and found that I get about 2.4A draw, which works out to about 16W of heat per pad. Hopefully this will be just about right.
   
4. I next installed the controller in my electrical panel. I had just added an MPPT solar controller as part of a separate project so everything was already disassembled. For power I tapped off of the solar controller battery connection. The 2.4A draw will be trivial for those AWG8 wires. I added a fuse and practiced my soldering techniques creating spliced wires to feed the controller display power and load on the positive side and a spliced wire to connect the controller display and switch light on the ground side. The rocker switch came without instructions but I eventually figured out that it needed it's own ground connection and drew power from the positive connection flowing through it.
   
   This method had the advantage that only two wires needed to be pulled to the battery tray - the positive load wire and the temperature probe. I grabbed negative for the heating pads from a negative connection near the battery tray.
   
   
   
   
5. After getting the batteries out from under the camper I applied the heating pads. The adhesive is exceptionally sticky, so rather then applying it all I cut horizontal strips in the protective backing and pulled them off so that only about 25% of the adhesive actually ends up touching the battery case. If I need to pull off the pads in the future it will not be quite so miserable of a job this way. I also added a piece of yoga mat as insulation under the battery and on the top of the battery to hold some heat. Not too much as I don't want the battery to overheat in hot conditions, but just a bit to buffer it from the cold and ensure even heating inside the battery case. I added a second thickness of yoga mat over the spot where I am inserting the temperature sensor to help ensure the measured temperature is truly coming from the battery and not the outside. The heating pads wrap around 3 sides of the battery and I left the 4th side open to promote cooling in the summer.
   
   Next I created a wiring harness using the waterproof connectors that create the serial connection between the batteries but let me disconnect them individually or disconnect both of them. My backup plan here if the serial connection does not produce enough heat is to replace this with a parallel connection that will drive the full 9.5A into the pads. Not my preferred choice, but an option.
   
   
   
   
   
   
6. I hooked everything up and ran it for two days. It is on the warmer side now, but it was about 41 degrees this morning (and climbing from a deeper low). According to the temperature probe which is located on the top of the battery at the opposite side from the heating pad the battery was at 53 degrees F. This is right about a 12 degree temperature rise and gets me a bit nervous that I don't have quite enough of a safety margin for what I want to do. The next option would be to add additional insulation. What does this group think?

 

[hbn7hj]

Not relevant to you but Battleborn and other vendors now sell a battery with the heating element built in.

 

[@Michael]

To reduce the power a bit I wired them in series and found that I get about 2.4A draw, which works out to about 16W of heat per pad. Hopefully this will be just about right.

FWIW - The Battleborn heaters are 15W, and they seem to think that's all you need. My experience in -20F is that 15W will keep the battery above freezing if the battery is insulated a bit. Your 16W will probably keep the battery above -4F in your climate.

The Battelborn supplied thermostat is flakey, so I also use an electronic temperature controller to cycle the pads.

Battleborn advises against high wattage heaters, claiming that the cases get warped.

Are your batteries outside the coach?

 

[jakegw2]

Yes, my batteries are outside the coach in a metal box mounted on the underside. I think this is the same place Winnebago puts the batteries for all their Promaster-based campers.

I know that Battleborn offers both the heated version of their batteries now (which is of course useless for me - I'm not buying new ones to replace my year-old models) as well as a heating kit. The cost of their external heating kit seemed excessively high to me and provided no way to monitor or control the heating process. That's part of why I posted these instructions here, it is cheaper and not much harder to buy everything to make the heating kit yourself, including the tools, then buying the pre-made version.

 

[jakegw2]

Oops, I nearly forgot that to connect to the switch I used ring and fork connectors. I also Added spade connectors on the wires for the temperature probe so that I could disconnect and reconnect it easily from the controller.

Looking back at the post I noticed that I didn't specify why mounting the controller close to the solar controller and pulling power from it is so convenient. There are two reasons - first it has a direct connection to the battery which will provide all the power you need for this project, and second, the wiring path to the battery will be easy to follow as you just need to run the heater positive wire and temp probe along the same path as the battery wire. They are much smaller then the battery wire so the floor penetration will almost certainly have enough room to squeeze them through without any new drilling.


Ring, Fork, and Spade Connectors ($13)

Also, if you don't have one, this tool is critical:
Wire Stripper Multi Tool ($12)

Finally, if the heat does not end up being enough then I will probably add a PWM controller next to the thermostat on the panel. Wiring would be simple, just power the controller using the same power I use for the temperature controller then run the positive wire from the temp to the PWM and then out to the battery. I would replace the Y connector I made that connects the batteries in series with one wired to connect them in parallel, then use the PWM controller to adjust the heat output.

A Pulse Width Modulation controller takes a steady DC input power and outputs a pulsed power, with the time between pulses ranging from zero (full on) to something relatively long (very low). This way the heat output can be controlled with nearly 100% of the power used going to the heater instead of being wasted.

Pulse Width Modulating Controller ($8.00)

 

[@Michael]

To your original question RE: insulation, here's what I found:

I have one Battle Born battery wrapped in the OEM 15 watt battery heater. The heater is essentially an Ultraheat tank heater sized to wrap three sides of the battery, with two 7 watt heating areas. Space is very tight, so the thickest insulation I can fit is 1/2" of XPS on three sides, 1/4" of EPS + a reflective felt pad on the other three sides. I didn't spend too much time fitting the XPS and EPS, so it has gaps.

With the relatively poor insulation and 15 watts of heat, I measured the following:

Ambient temp in the camper: -15F
Temp inside electrical cabinet: -8F
Least-insulated corner of the battery: +22F
Top of battery, under felt pad: +38F

Where the insulation is poor I see +30F temperature difference. Where the insulation is not-so-poor I see at least +45F above ambient.

My conclusion is that with better insulation (I.E. 1" of well-fitting XPS) the 15 watts of heat is more than adequate to keep the battery above freezing under most circumstances and above the low limit of -4F under nearly any condition.

At a minimum, I'd consider insulating the bottom so that the heat doesn't conduct out through the battery tray, and perhaps insulate the sides to keep the heat from the pads from radiating out into the box.

In you case, it looks like getting access to the batteries sufficient to remove the insulation in summer would be a lot of work. Because you have good temperature probes, you can monitor summer heat and determine if the insulation is causing a problem.

 

[jakegw2]

Hi Michael, these measurements are very helpful. It sounds like you have a very similar heating solution and have some good real-world observations. I think I will beef-up my insulation a bit more before fall. Probably add more yoga mat pieces to the open sides and a pad of rock-wool insulation in plastic sheeting to the top of each battery to reduce heat loss from above.

 

[Hein-ClassB]

Great write up. Thinsulate AU4002-5 is great for insulating batteries. It is 1" thick but compressible for areas that have less space.


All the best,
Hein
DIYvan

Here is a video:

https://youtu.be/r4X3pXi5PKU

 

[jakegw2]

Hein,

That was a very helpful video, and your finished product looks great. I have a bit of a concern about using Thinsulate for the exposed battery tray under the camper however as it does get wet from road spray sometimes and certainly collects dust. Whatever I use for insulation needs to be something that won't absorb and hold water I think.

Thinsulate appears to be more expensive then I expected as well - a premium solution for sure.

 

[Frankshay]

Lithium stores best at freezing temps but need to be a bit warmer for charging (what it seems you're doing). If you're just storing though I would leave them cold.

 

[avanti]

Lithium stores best at freezing temps but need to be a bit warmer for charging (what it seems you're doing). If you're just storing though I would leave them cold.

Except that they also typically are spec'd with a minimum temperature (usually somewhere below 0F) at which they allegedly can be permanently damaged.

 

[Snowy-ClassB]

Your diagram seems to show the battery heaters taking power from your batteries. Is that so? I would caution on this - discharging a LiFePO4 battery below -4F is damaging.

Advanced RV has posted an excellent primer on RV Batteries.
https://advanced-rv.com/wp-content/uploads/2018/05/Advanced-RV-Batteries-V4-White-Paper.pdf

For LiFePO4 batteries, you want to prevent charging below 32F and discharging below -4F. There is also a spec stating that you should not store a LiFePO4 battery below -4F, but the storage spec is somewhat controversial. PleasureWay doesn't worry about storage temps in cold Manitoba - they just disconnect the battery for cold-weather storage. I have heard anecdotal reports of people letting their batteries get to -20F, and not observing any damage, although i wonder if they have reduced their capacity but just don't know it.

I live in Alaska, so i will be installing some sort of heater. I think the ideal heater would be a 120v battery blanket wired into the 120v side. This way for storage, you can plug the rig in, disconnect the battery to prevent charging/discharging, but get power to the battery blanket. If you just want to put a little heat on the batteries because you are using the rig during the winter, and don't have shore power, you can still use the battery blankets by turning on the inverter.

 

[markopolo-ClassB]

If the RV has a Converter type charger like typical Progressive Dynamics units then the batteries can be disconnected from charging and the RV will still have 12VDC when plugged into grid power so 12V pads can be used. That type of Converter/charger functions as a Power Supply and doesn't require a battery being present.

I'm curious to know what SOC are the batteries being left at for lay-up periods during winter or other lay-up periods by forum members and what periodic capacity testing they have done to track battery health. 50% SOC seems to be recommended and I have seen a test result showing a small loss of capacity when the batteries were stored after being charged to 100% SOC compared to when they were stored at 50% SOC.

Using a Power Supply type converter/charger as described above will let you leave the batteries disconnected and at approximately 50% SOC and still have 12V heat.

 

[booster]

If the RV has a Converter type charger like typical Progressive Dynamics units then the batteries can be disconnected from charging and the RV will still have 12VDC when plugged into grid power so 12V pads can be used. That type of Converter/charger functions as a Power Supply and doesn't require a battery being present.

I'm curious to know what SOC are the batteries being left at for lay-up periods during winter or other lay-up periods by forum members and what periodic capacity testing they have done to track battery health. 50% SOC seems to be recommended and I have seen a test result showing a small loss of capacity when the batteries were stored after being charged to 100% SOC compared to when they were stored at 50% SOC.

Using a Power Supply type converter/charger as described above will let you leave the batteries disconnected and at approximately 50% SOC and still have 12V heat.

Probably doesn't apply to lithium, but have tested you if the PD chargers will power 12v without a battery if they have a Charge Wizard on them. Lithium wouldn't normally have a Wizard, but others may want to take out other styles of batteries also and power the van.

 

[jakegw2]

Snowy,

You are correct, my heaters take power from the battery (or converter if I am plugged in). I think I have all the scenarios covered to make this work however:

First, my main plan is to never let the batteries get below -4F by turning the heater on in advance of any significant cold-snap. The batteries have capacity to run the heater at 100% duty cycle for about 76 hours, and much longer if I set it to keep them just above the minimum temp threshold so the duty cycle is more like 50%. This should never be a significant problem as I store it right next to an exterior outlet and can plug in if I anticipate a long stretch of cold weather. Also, even without being plugged in my solar system can generate 20-30AH a day if there is any winter sun available (more like 5-10 if it is very cloudy). With a drain of only 2.6A/hr any available solar will greatly extend the heater run-time without shore power. I could also run the generator for an hour or so to recharge and buy myself a few more days of heating if needed.

Second, if for some reason I let the batteries get down below freezing the internal BMS will prevent charging to protect them. Applying shore power will allow the heaters to work without charging the batteries until they are warm enough to accept a charge.

 

[booster]

Snowy,

You are correct, my heaters take power from the battery (or converter if I am plugged in). I think I have all the scenarios covered to make this work however:

First, my main plan is to never let the batteries get below -4F by turning the heater on in advance of any significant cold-snap. The batteries have capacity to run the heater at 100% duty cycle for about 76 hours, and much longer if I set it to keep them just above the minimum temp threshold so the duty cycle is more like 50%. This should never be a significant problem as I store it right next to an exterior outlet and can plug in if I anticipate a long stretch of cold weather. Also, even without being plugged in my solar system can generate 20-30AH a day if there is any winter sun available (more like 5-10 if it is very cloudy). With a drain of only 2.6A/hr any available solar will greatly extend the heater run-time without shore power. I could also run the generator for an hour or so to recharge and buy myself a few more days of heating if needed.

Second, if for some reason I let the batteries get down below freezing the internal BMS will prevent charging to protect them. Applying shore power will allow the heaters to work without charging the batteries until they are warm enough to accept a charge.

What solar charger and shore charger do you have? Re Markopolo's post you may not be able to run either without a battery in the system, and/or risk voltage spike if they are on and the BMS shuts off the batteries. This is one of the larger issues with designing a full cutoff system and why many setups won't run the shore or solar charger unless there is a battery in the circuit. They also make sure the chargers would disconnect first before batteries do, to prevent harming vehicle electronics or themselves.

 

[markopolo-ClassB]

Probably doesn't apply to lithium, but have tested you if the PD chargers will power 12v without a battery if they have a Charge Wizard on them. Lithium wouldn't normally have a Wizard, but others may want to take out other styles of batteries also and power the van.

Both the 9100 series Charge Wizard and 9200 series Remote Pendant permit only time limited changes so I think the units still function as power supplies when no battery is connected. During lay-up periods, both would get the batteries to 100% or very near 100% because of either (1) Normal mode or (2) the periodic automatic Boost mode if the batteries are connected.

Forum member Winston's prismatic cells at rested 13.19V were at 50% SOC. My cylindrical cells 50% SOC voltage was 13.164V when tested (24hr rested). Davydd's lithium bank has been stored at a high state of charge (90% to 99%) but I don't think it has ever been capacity tested. There would be some very useful info there.

 

[booster]

Both the 9100 series Charge Wizard and 9200 series Remote Pendant permit only time limited changes so I think the units still function as power supplies when no battery is connected.

I thought the Charge Wizard might check for voltage like many, even timer model, multistage chargers do. The guess would be it defaults to ignoring the Wizard and using it's native 13.6v regulated voltage? If so that could be very useful for lithium as long as it would drop to that voltage fast enough if the BMS tripped out. Would not have any timing issue on startup without battery, though, so would be good for running heaters to get the lithium up to charge temps. Would we also assume that the 14.7v single voltage "lithium" charger would be a power supply at that voltage not "boosted"? I would think so, as it would make more sense, and most power supplies can be set internally, or externally, to whatever voltage you want within their range.

 

[markopolo-ClassB]

Yes to defaulting to 13.6V for the 9100/9200 standard units.

Just a note: 13.6V will get the batteries to near 100% SOC. See: https://www.classbforum.com/forums/f23/ecotrek-battery-knowledge-base-11419-2.html#post120850

 

[markopolo-ClassB]

A challenge if heating drop-in type batteries is ensuring that the center (actual dimensional center & last to warm) of the battery gets to operating temperature before the BMS permits charging after a cold soak period. The temperature sensor on Battle Born batteries appears to be near the outer shell of the battery (first to warm). See:

There's also the substantially limited current charging below 10C / 50F if following JEITA guidelines to figure out.