LiFePO4 heating pad for cold temperatures

Matt-M

New Member
This is a good plan but the only downside is that you will need some mechanism to hold back the charging until the batteries get above 35F (or whatever you decide the safe to charge temp is).

That’s the job of the BMS. I am using the Overkill 8 Cell model, and you can set both temp for charging cutoff and for resumption. So, right now, the low temp disconnect is set to 32°F and the reconnect is 37°F. The heaters will come on at 35° as long as there is amperage from solar.

There will likely be some fine tuning, but that’s what I’m starting with.

FYI - for the last couple weeks the Solar Charge Controller (Victron SmartSolar) has been trying to send charge through the BMS which is below cutoff temp, and nothing seems to have suffered any damage.
I plan to set the "discharge not allowed" signal once under the safe discharge temp to disable all loads as I presume any humans or pets in the RV are now frozen and do not need power.

LOL!
 

Horsefly

Solar Enthusiast
Yes, a box to contain the heat likely helps. I didn’t do that because the pass-thru space in this RV is heated. Didn’t want to insulate from the heat. Of course the heater only runs when we are in it. Thus the heater pad option for when it is not occupied and the main RV heater is off.

waiting on parts...
I have a hard time seeing this as a logical approach. So the heater pad will only be used when there is no other heat, and you are NOT going to have any sort of insulated box around them? I hope you realize you will be using significantly more Ah from your battery heating up the surroundings instead of your battery. Isn't there a way to have the battery in an insulated box, but remove the lid to the box when you are occupying the RV?

Anyway, I don't think I would approach it this way, but clearly you have considered it.
 

HRTKD

Boondocker
I have a hard time seeing this as a logical approach. So the heater pad will only be used when there is no other heat, and you are NOT going to have any sort of insulated box around them? I hope you realize you will be using significantly more Ah from your battery heating up the surroundings instead of your battery. Isn't there a way to have the battery in an insulated box, but remove the lid to the box when you are occupying the RV?

Anyway, I don't think I would approach it this way, but clearly you have considered it.

I agree with this. I've seen one install where the owner made a box around the batteries and extended the furnace duct so that it goes into the battery box.
 

jwelter99

Solar Enthusiast
That’s the job of the BMS. I am using the Overkill 8 Cell model, and you can set both temp for charging cutoff and for resumption. So, right now, the low temp disconnect is set to 32°F and the reconnect is 37°F. The heaters will come on at 35° as long as there is amperage from solar.

There will likely be some fine tuning, but that’s what I’m starting with.

FYI - for the last couple weeks the Solar Charge Controller (Victron SmartSolar) has been trying to send charge through the BMS which is below cutoff temp, and nothing seems to have suffered any damage.


LOL!

My issue is my Rec BMS tells my Victron gear over Canbus when it can charge and when it can discharge, voltage targets, SOC, etc, etc. This is both my MultiPlus and SmartSolar VE.CAN model.

So what happens is the REC detects low temp and issues a "do not charge" which causes everything to stop charging - so at that point to run a heater is 100% off battery until temp comes up and the "ok to charge" is issues which resumes charging.

It sounds like you have the ability to still have the charge equipment on, but the battery isolated via BMS, so that charge equipment can power the heater and other loads until the BMS reconnects to allow the batteries to charge? I don't have that "in between" state.

I also have a critical contactor that is NO that the REC will close when it's inside safe ranges for temps and voltages; but open when critical to completely disconnect the battery.
 

HRTKD

Boondocker
It sounds like having a "dumb" BMS that doesn't communicate with the charging equipment is a blessing in disguise.

My BMS does not communicate with anything except its own app. That allows me to have the BMS stop charging at 32° F while the charge equipment (MPPT and converter) are set to keep charging well below that. My battery warming pads will get some help from the charge equipment instead of pulling amps only from the battery.
 

jwelter99

Solar Enthusiast
It sounds like having a "dumb" BMS that doesn't communicate with the charging equipment is a blessing in disguise.

My BMS does not communicate with anything except its own app. That allows me to have the BMS stop charging at 32° F while the charge equipment (MPPT and converter) are set to keep charging well below that. My battery warming pads will get some help from the charge equipment instead of pulling amps only from the battery.

Yes I could do the same with just disabling the Canbus integration but that would mean lots more programming and dealing with charge and discharge via the inputs to each device, or battery-protects where those inputs don't exist.

But yeah sometimes less integration and intelligence is better ;)
 

HRTKD

Boondocker
But yeah sometimes less integration and intelligence is better ;)

According to my wife, I'm the King of less intelligence. I disagree, of course, just not where she can hear me.
blahblah.gif
 

Kristof

New Member
This is a good plan but the only downside is that you will need some mechanism to hold back the charging until the batteries get above 35F (or whatever you decide the safe to charge temp is). This is the tricky logic I'm trying to work out using a Cerbo GX (instead of the Simarine you are using)

I want heaters on when I am "ready to charge" (B2B putting out power, AC power supplied to MultiPlus, or Solar production) to get battery up to temp to then allow it to charge. I want that to happen quickly to not waste "charge time" if solar, B2B, etc.

So I may decide to just put the battery heater on a timer to come on a couple hours before daybreak when temp below 35F, or perhaps allow it to be powered continually when below 35F, depending on how quick it warms up the 80kg of batteries and how well the battery box maintains temp.

I plan to set the "discharge not allowed" signal once under the safe discharge temp to disable all loads as I presume any humans or pets in the RV are now frozen and do not need power.
Not sure my plan is extant.

The BMS on the battery pack will control the "can charge/Cannot Charge" functions. The temp control will turn the heaters on at 40F and off at 50F as required. Whatever type of heating system you choose should determine how fast the cells heat up and the amount of power used to heat them. The equation is simple on the heating side - if you draw say 30W to heat for an hour you will lose X amount of AH from your battery which will be made up the next day from the solar draw. My Victron 40 amp MPPT solar charge controller has a Bluetooth network with the temp on the battery as well as the BMS, ensuring nothing happens unless they are all happy. Makes the heating equation easy.

Night.
 

orangeminnie

Solar Enthusiast
Relion is maybe the only one to do it right, for an internal heater that is internally controlled. Below 0dgC, all incoming charging current is diverted to an internal heater. When the battery is sufficiently warmed, the charging can begin. No battery power is ever used to run the heaters. Perfect! I think Battle Born missed the boat with their system.
 

Matt-M

New Member
I have a hard time seeing this as a logical approach. So the heater pad will only be used when there is no other heat, and you are NOT going to have any sort of insulated box around them? I hope you realize you will be using significantly more Ah from your battery heating up the surroundings instead of your battery. Isn't there a way to have the battery in an insulated box, but remove the lid to the box when you are occupying the RV?

Anyway, I don't think I would approach it this way, but clearly you have considered it.
I have thought about it, but maybe I will reconsider. The heater vent that runs into the pass-thru area is in the middle of my electronics panel - had to cut a hole to let the warm air pass though:

IMG_0193.jpeg

Adding ductwork from that vent would be difficult - it's just a plastic grille.

My assembled battery packs look like this:

Screen Shot 2021-02-24 at 2.15.19 AM.jpg

And there will be three of them in front of and below that panel. Here, the first one is in place:

IMG_0194.jpeg

My plan was to just cover the terminal area with a Lexan shield. Here is a render from AutoCAD:

shield.png

So, if it turns out that an enclosure is needed for the heaters to work properly, then I would scrap the Lexan shield idea, and instead built a Lexan enclosure around three sides and the top - the rear is too close to the back wall and the power panel to put anything on that side. The side walls would touch the back wall though, so it would basically be enclosed on all sides. I could add a couple of smaller diameter (about 2") inlets on the top, and a couple vents near the bottom to release warm air to the rest of the compartment. The ducting to those inlets would just be attached on the surface of the current vent - it would get a fair amount of warm air, but not the entire flow. Here is a render of what that might look like:

box.png

That enclosure can be added later, so for now, I will proceed with the heating pads and test how it works.
 
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Horsefly

Solar Enthusiast
I see concern about the risk of metal touching the wrapped cells. I was pretty certain that someone here (or somewhere?) verified that the voltage measured between the aluminum case and either positive or negative terminal is a surface / static charge, and presents no risk if shorted. I think I saw where someone actually shorted between the case and either terminal, and had no current. Can someone confirm?

If that's true, I think we can stop worrying about using aluminum (or other metal) in fixtures touching the wrapped aluminum cases.
 

Horsefly

Solar Enthusiast
Ok, I just tried it myself on one of the little 25Ah Navitas cells:

Between the case and the negative terminal measured about 2.5V. Between the case and the positive terminal was about 0.5V. I shorted the case to the positive terminal, and there was no current. I then shorted between the case and the positive terminal, and again - no current.

So we all can stop worrying about using metal against the cells.
 

DerpsyDoodler

Solar Enthusiast
Ok, I just tried it myself on one of the little 25Ah Navitas cells:

Between the case and the negative terminal measured about 2.5V. Between the case and the positive terminal was about 0.5V. I shorted the case to the positive terminal, and there was no current. I then shorted between the case and the positive terminal, and again - no current.

So we all can stop worrying about using metal against the cells.

Some spec sheets specifically mention not allowing the case to short while under load. I do not know if that is short to a battery terminal or otherwise. Regardless, I will be making sure my cells are electrically insulated from each other and from any metal touching them. Perhaps it’s over-cautious, but I’d rather be cautious than sorry.
 

Horsefly

Solar Enthusiast
Some spec sheets specifically mention not allowing the case to short while under load. I do not know if that is short to a battery terminal or otherwise. Regardless, I will be making sure my cells are electrically insulated from each other and from any metal touching them. Perhaps it’s over-cautious, but I’d rather be cautious than sorry.
I think that's fine @DerpsyDoodler, and it is generally good to be cautious. I mainly wanted to put the fact out there that the case is no real potential between the case and the terminals, at least for many / most aluminum-cased prismatic cells. I have a hard time believing that there isn't on some cells and there is on others. I think the prudent thing for people to do is to verify on their own pack (like I did on the Navitas) before assembling a fixture with metal contacting the wrap.
 

Matt-M

New Member
@Matt-M, that's a nice board you have there. That's a lot of devices in a compact space.
A little tighter than it should have been! All the components were laid out just fine - the problems came when I tried to make 1/0 cable make a tight bend. It all worked out, but I would have made it a bit larger if I had to do it again.

Here's the fun part... I built this in my garage and put a camera above to capture a time-lapse. Will eventually be put into a full video on my new YouTube channel, but here is a preview of the time-lapse. You can see the difficult wires on the top left near the end - they supply the big inverters. You can also see some mistakes and re-do's on the upper right.

 
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