LFP in cold weather

[Sipma02]

Yes, I've read several other threads about LFP's not being charged below around 32º-35º, but I have further inquiry In our Sprinter build, that would be too big of an issue, because we'll have a diesel heater for the cold nights. Batteries are mounted under the front seat. But for when we aren't in the van, and as a backup, I plan on using some small heating pads to warm the batteries up.

My question is: How warm do the batteries have to be? How do I measure that, is it as simple as having a thermometer on the pack itself? If the heating pad is on the bottom of the battery, but the air around the battery is 20º-30ºF, is that alright?

 

[Luthj]

To be safe the battery should be over 40° all over. A probe mounted on the side opposite of the heating pad is best.

The seat pedestal will easily conduct significant amounts of heat away. In the winter I found the pedestal will be 30 or 40° cooler than the interior of the van. So you will need insulation both on the pedestal walls and on the bottom.

 

[GMB]

Battery Monitor with temp sensor: https://www.amazon.com/Victron-BMV-712-Battery-Monitor-Temperature/dp/B07F3C7ZV7/ref=sr_1_3

Stand alone temp sense if you have a Victron SCC: https://www.amazon.com/Victron-Battery-Temperature-Solar-Controllers/dp/B07FK6H3YF/ref=sr_1_4

 

[Sipma02]

To be safe the battery should be over 40° all over. A probe mounted on the side opposite of the heating pad is best.

The seat pedestal will easily conduct significant amounts of heat away. In the winter I found the pedestal will be 30 or 40° cooler than the interior of the van. So you will need insulation both on the pedestal walls and on the bottom.

Yeah, that's what I was afraid of. What do you think about a small, 5-7 CFM computer fan pulling air into the pedestal? Assuming the temp of the van interior is 60-70ºF, it could pull a decent amount of air into the under seat/pedestal area. One problem is the if the air is 70º in the van, near the floor may be 10-20º cooler. Still, pulling 50º+ degree air into the pedestal could help.

 

[Sipma02]

Also, an edit to my original post: (in bold)

Yes, I've read several other threads about LFP's not being charged below around 32º-35º, but I have further inquiry In our Sprinter build, that would't be too big of an issue, because we'll have a diesel heater for the cold nights. Batteries are mounted under the front seat. But for when we aren't in the van, and as a backup, I plan on using some small heating pads to warm the batteries up.

My question is: How warm do the batteries have to be? How do I measure that, is it as simple as having a thermometer on the pack itself? If the heating pad is on the bottom of the battery, but the air around the battery is 20º-30ºF, is that alright?

 

[Luthj]

Yeah, that's what I was afraid of. What do you think about a small, 5-7 CFM computer fan pulling air into the pedestal? Assuming the temp of the van interior is 60-70ºF, it could pull a decent amount of air into the under seat/pedestal area. One problem is the if the air is 70º in the van, near the floor may be 10-20º cooler. Still, pulling 50º+ degree air into the pedestal could help.

No reason it can't work. Just take into account the noise factor. Really the best option to to include a thermostat and heating pad. Even if its a backup, its good to have. There may be situations where you want to keep the van just above freezing while your away as well.

Insulting inside the pedestal isn't that tough. If you like, remove the pedstal, and install some 1/8" rubber as a thermal break between it and the floor. Then insulate the floor with some 1/2" foam. That should do the trick, assuming you keep your interior above 50F.

 

[Sipma02]

No reason it can't work. Just take into account the noise factor. Really the best option to to include a thermostat and heating pad. Even if its a backup, its good to have. There may be situations where you want to keep the van just above freezing while your away as well.

Insulting inside the pedestal isn't that tough. If you like, remove the pedstal, and install some 1/8" rubber as a thermal break between it and the floor. Then insulate the floor with some 1/2" foam. That should do the trick, assuming you keep your interior above 50F.

Cool, thanks! I agree. Sounds like a plan!

 

[snoobler]

Insulation of the battery itself will also improve temperature readings. Consider wrapping the battery itself.

 

[Luthj]

I see no disadvantage to insulating the battery directly, except if you plan on high current usage in hot weather. Even with insulation the battery will reflect the average interior temperature.

 

[Sipma02]

Another clarification: The cells should be fine in cold (down to 0ºF or so) as long as they are not being charged, correct? But they can be discharged when they are that cold, or just sit there? (I have a charge disconnect switch)

 

[zorlig]

Another clarification: The cells should be fine in cold (down to 0ºF or so) as long as they are not being charged, correct? But they can be discharged when they are that cold, or just sit there? (I have a charge disconnect switch)

Yes you can discharge them at cold temperatures. Charging should be limited below freezing, but what the exact C rate is where the is an issue isn't something I've seen before, and I think depends on temperature.

I think someone asked battleborn about this, here is another thread on the topic. The gist is that it's probably not an issue. I suspect for a typical solar system for example it won't be possible to damage them, C rate too low.

Charging LiFePO4 at below 32F // BAD Battleborn advice

So I saw this quote the other day, “According the engineer at Battleborn that Will interviewed you can charge consumer LiFePO4 batteries at 25F ambient”. I am going to say that the Engineer at Battleborn was incorrect ... Unless you know for 100% certain that your INTERNAL ambient temperature...

diysolarforum.com

 

[Luthj]

Discharge is fine.

 

[willo]

I will second the victron gear - I have the smartsense, a bmv712 and smartsolar charger - all work very well together. I'm finally adding a 24/3000 multiplus to the mix - it too has temperature sensing. Now that I have more time with the gear, I have to say that victron works well. I added a 12v 500a solenoid to mine and the SCC controls that for discharge cutoff. You can get all the control you need from just a victron smart controller and a battery smartsense. I suggest running extra systems to verify status, but that's me. I've found the BMV712 to be great for quick status checks of my SOC and load status. The panel meter contains the bluetooth interface so I get better reception from it over the charge controller when I'm monitoring things from inside the house.

You can use the batteries below freezing but don't charge below freezing - thus you can add an electric heat pad to get them back to temp. My rig currently features a foam insulated box for the battery pack. I suggest upping the value a bit for safety - I use 5c instead of 0c to allow for variation through the pack and temp sensor issues for example.

It's very easy to tie a DS1820b sensor to a raspberry pi or arduino for additional temperature monitoring....

 

[snoobler]

I will second the victron gear - I have the smartsense, a bmv712 and smartsolar charger - all work very well together. I'm finally adding a 24/3000 multiplus to the mix - it too has temperature sensing. Now that I have more time with the gear, I have to say that victron works well. I added a 12v 500a solenoid to mine and the SCC controls that for discharge cutoff. You can get all the control you need from just a victron smart controller and a battery smartsense. I suggest running extra systems to verify status, but that's me. I've found the BMV712 to be great for quick status checks of my SOC and load status. The panel meter contains the bluetooth interface so I get better reception from it over the charge controller when I'm monitoring things from inside the house.

You can use the batteries below freezing but don't charge below freezing - thus you can add an electric heat pad to get them back to temp. My rig currently features a foam insulated box for the battery pack. I suggest upping the value a bit for safety - I use 5c instead of 0c to allow for variation through the pack and temp sensor issues for example.

It's very easy to tie a DS1820b sensor to a raspberry pi or arduino for additional temperature monitoring....

You'll be able to ditch the solenoid and program the Multiplus for the low voltage cut-off.

 

[Sipma02]

I built a false floor in a box (ammo can lined with thin roll insulation-not reflectix) and stuck a silicone heat pad to the bottom of the floor which is made of thick aluminum sheet. A silicone baking mat sits on top of the aluminum plate and the cells on top of that.

A 3d printer heater control operates the heat pad and never exceeds 50w draw, I guess kind of like dimming a light bulb. It can get scary hot if you don't do this. I made the mistake of plugging in the control pin to the wrong spot on my microcontroller and smelled... hot hot hot... 30 seconds later. Glad I noticed. The floor is never allowed to exceed 90*F measured at the pad itself. This is fairly well distributed through the thick aluminum and radiates in to the rest of the box+is absorbed by the cells bottom to top. The target temperature is the average of a probe on top of the cells and one in the middle of the cells. I played around with PID control and trying to supply the perfect wattage to the pad at the right time to reach and maintain a temperature, i.e. cruising on 5w or something once it's up to temperature. I abandoned this for reasons forgotten and opted to just flip it on and off @ a static wattage.

This setup worked all of last winter with the battery in the bed of my truck and days (on end) where the high temp was barely above 0*F. It also worked when a foot of snow sat on top of my 200w of panels for days at a time. Those days I would have been better off just disabling heat since solar collection wasn't going to happen anyway. I don't try to keep my battery at its happiest temperature, I just want it above 45*F or so to satisfy my MPPT & BMS low temp cutoffs.

I also screwed around with trying to heat at say 4am so by sunrise-ish I'd be warmed up enough to accept charge. This worked but I didn't really want to push my luck. I wasn't consuming enough power to even think about, even with aforementioned low/no charge days.

I tried motorcycle grips and they burned and wrinkled almost immediately with a 12.00v power supply. I also tried a 12v heater with PC fan, looks like a computer heatsink but blows heat. May have been labeled as an incubator heater. Did not trust that at all and hard to get meaningful readings with probes at the cells/between the cells blowing hot air at them. Everything in the path of the fan became a billion degrees while the cells remained a cold block. The silicone heating pad was by far the most solid feeling product I evaluated for this use.

TL;DR heat from below with thick aluminum to disperse heat. Silicone heat pads are reliable and work fine while sipping power. Design your box to allow for something more than cardboard box-thickness insulation like I had to use. Can't wait to replicate this heating system in a much larger future off-grid project.

For anyone still interested, I found this thread to be very helpful, specifically, this post. You can see the OG thread here: https://diysolarforum.com/threads/winter-heat-for-battery-bank.10814/#post-119831

TLDR: Put a heating pad under the batteries, controlled by 3D printer heater controller (which operates on a repeat duty cycle, on-off-on) to be around 90ºF. Heating pad is attached to a metal plate which dissipates/spreads out the heat.

This is what I plan to do. Thanks for the write-up, @hour !