Should I insulate my LiFePO4 battery?

[Matty-Lee]

I'm planning on building a box out of plywood for my 12v LiFePO4 battery before I house it under the passenger seat of my van.

I live in the UK and it doesn't get really really cold or really really hot (-10 to +39 are probably the extremes) although it may get pretty warm in the van cab area with all the glass there. The charger has a low temp disconnect.

I've seen people insulate batteries and I've seen people say that ventilation is required.
The battery came with that white foam which I thought might be good to put inside the box around the battery.

The question is should I use it in my box or just keep it ventilated? Maybe use it bottom and sides and ventilate the top?

Please don't cry over the picture as I'm just testing it works.

 

[hour]

I have my lightly insulated box in the bed of my truck with a canopy on it, so it gets pretty hot in there even right now during spring time. Like 115*F / 46*C. But we still have extreme dips at night down to around freezing. What I'm noticing (with temp sensors inside and outside of the box) is that it stays in a comfortable range all day. Right before sunrise if the bed of the truck inside (probe right beside battery box) is 32*F/0*C it's like 50-55*F in the box. And towards the end of the day after the sun is beginning to finish its assault on my truck, bed 115*F, the box is about 80*F/26.6*C.

So... if I had the battery completely exposed or used forced ventilation in the box, it'd be 115*F instead of 80. The only downside to this, something that I noticed last year during the peak of the summer, is that the BMS (also sealed in my box) really gets warm. I'm guessing this is because of the 12v fridge cycling a ton and the cells frequently reaching the balancing threshold, causing the BMS to work harder. This will take the inside of the box to over 100. I wouldn't say the insulation is really working against me because I know it's still hotter in the bed of the truck.

I'd completely separate the BMS from the cells enclosure but then it's more exposed to dust and humidity, and its low temp cutoff stuff won't work (the probe will, but the onboard bms temp sensor won't be correct). TL;DR yeah I have no regrets on insulating my battery as it seems to aid in keeping it warm overnight and cooler during the day, but the BMS inside the box doesn't do me any favors in certain conditions-but mandatory in my case.

(sealed isn't the best wording - plenty of things will fly out of my enclosure if the cells pop, unintentional pressure relief ports)

 

[Northernchateau]

Compress and insulate the batteries and be sure and use a BMS with a temperature sensor so it shuts down in cold.

 

[HRTKD]

Compression first. Then insulate only during cold weather.

 

[Matty-Lee]

Thanks everyone.

Compression has been mentioned a couple of times. What does that mean?

 

[hour]

Thanks everyone.

Compression has been mentioned a couple of times. What does that mean?

Compressing the individual cells together. So imagine two squares of plywood slightly larger than your cells, one on each end, and threaded rod joining these two end plates. There are a bunch of examples of that on here done in this exact manner (would be worth looking them up in the diy lifepo4 builds subforum.) You could use other materials but if the box is going to be made out of plywood...

It limits the amount of swelling that can occur on your cells. And it restricts movement of individual cells - all four in your case would move together as one unit. If you have cells moving about independently, or swelling, it can strain your connections. Some bus bars can accommodate a little bit of this (the arched ones, or bus bars with oval holes instead of circles, or braided bus bars) but it's easy to prevent with compression. If you check out the DIY build subforum you'll probably stumble across a thread or 10 of people who have destroyed cells by using too long (or overtightened) bolts / grub screws / whatever. Treat your cell connections like the achilles heel of your project.

Compression also reduces/eliminates the potential of dust/sand/whatever getting in between the cells and wearing away at the blue film covering yours. You're going to have vibrations in a mobile environment and instead of making a pearl, grit could result in a hazardous situation. I think it's pretty standard now to place something thin between your cells as added insurance. Very thin dense plastic sheet, forget the abbreviation, but should also come up if you peruse the diy subforum.

 

[hour]

Compression first. Then insulate only during cold weather.

Sounds like a good way to ensure his battery reaches the temperature of the inside of his car. Insulation and limited air flow has certainly benefitted my project year round.

 

[Matty-Lee]

Compressing the individual cells together. So imagine two squares of plywood slightly larger than your cells, one on each end, and threaded rod joining these two end plates. There are a bunch of examples of that on here done in this exact manner (would be worth looking them up in the diy lifepo4 builds subforum.) You could use other materials but if the box is going to be made out of plywood...

It limits the amount of swelling that can occur on your cells. And it restricts movement of individual cells - all four in your case would move together as one unit. If you have cells moving about independently, or swelling, it can strain your connections. Some bus bars can accommodate a little bit of this (the arched ones, or bus bars with oval holes instead of circles, or braided bus bars) but it's easy to prevent with compression. If you check out the DIY build subforum you'll probably stumble across a thread or 10 of people who have destroyed cells by using too long (or overtightened) bolts / grub screws / whatever. Treat your cell connections like the achilles heel of your project.

Compression also reduces/eliminates the potential of dust/sand/whatever getting in between the cells and wearing away at the blue film covering yours. You're going to have vibrations in a mobile environment and instead of making a pearl, grit could result in a hazardous situation. I think it's pretty standard now to place something thin between your cells as added insurance. Very thin dense plastic sheet, forget the abbreviation, but should also come up if you peruse the diy subforum.

Thanks for the explanation and warning.
Currently my cells are stuck together with 3M VHB tape and the bus bars do have some play built into them.
I'll look into compression. Strange I've not seen it mentioned on YouTube.

 

[hour]

Thanks for the explanation and warning.
Currently my cells are stuck together with 3M VHB tape and the bus bars do have some play built into them.
I'll look into compression. Strange I've not seen it mentioned on YouTube.

Some of the first videos from Will I watched utilized giant pipe clamps (worm clamps? something like that) to keep the cells bound tightly together.

Here are some links from here:

https://diysolarforum.com/threads/best-way-to-compress-cells-in-packs.11299/

https://diysolarforum.com/threads/another-cell-compression-thread-this-time-about-foam.16537/page-2

(actually, "cell compression diysolarforum" on google yields quite a few results)

And here are some videos:

If I recall correctly, the guy in the first video (lithiumsolar) had issues with his approach as the bands were impossible to evenly torque or something, and that caused the cells to not sit flush against one another. Threaded rod is too cheap and easy to consider using bands in my opinion.

I also attached my cells with VHB with nothing else in between, so you're not alone. I'd say I'd do it differently if I could go back in time, but I've built another several since learning of separating plastic sheets... still using VHB... not sure what my thought process was there

If you're good with wood working you could probably build the box to perfectly fit the cells. This guy went this route.. I don't know how tight they are but it certainly limits how much swelling can occur. He is stationary though, so he doesn't have to think about the cells bouncing up and down on bumpy roads.

 

[Matty-Lee]

Some of the first videos from Will I watched utilized giant pipe clamps (worm clamps? something like that) to keep the cells bound tightly together...

Excellent, thanks.

 

[Northernchateau]

Thanks everyone.

Compression has been mentioned a couple of times. What does that mean?

Compression helps reduce internal delamination and increases battery cycle life.

 

[hour]

Some real world summer results with a lightly insulated ammo can for what it's worth. Getting direct sunlight, not a cloud in the sky, UV index 11+. Been hot as hell since the second I woke up this morning.

Outside is 96*F
Sealed truck bed (camper shell's slider windows closed) is 118*F
Ammo can battery in truck bed is 96*F at bottom of the cells, and 102*F at top of cells - BMS is up there cranking out heat

Nothing I can do to get it lower, but if I used forced air ventilation, or left the lid off, or didn't insulate - I'd expect the full 118*F in the ammo can.

So I would not recommend removing insulation for the summer, or ever. The catch (I guess) is that come 3am when the bed is 65*F it'll likely be over 85 in the ammo can. I suppose one has to choose avoiding the worst of the heat vs keeping still unideal temps in the box for a longer period of time. In my mind avoiding the extremes is the better call.

I put my travel router in a project enclosure with forced fan ventilation, PWM controlled, so the fan speed adjusts automatically as needed. That little twat is 126*F and is powered by a lipo pouch battery (constantly charging over USB). There's also a 12v->5v step down in the project box. Thinking I'm going to remove the battery and power directly from USB after seeing the data today.

 

[MOCOMofo]

Some of the first videos from Will I watched utilized giant pipe clamps (worm clamps? something like that) to keep the cells bound tightly together.

Here are some links from here:

https://diysolarforum.com/threads/best-way-to-compress-cells-in-packs.11299/

https://diysolarforum.com/threads/another-cell-compression-thread-this-time-about-foam.16537/page-2

(actually, "cell compression diysolarforum" on google yields quite a few results)

And here are some videos:

If I recall correctly, the guy in the first video (lithiumsolar) had issues with his approach as the bands were impossible to evenly torque or something, and that caused the cells to not sit flush against one another. Threaded rod is too cheap and easy to consider using bands in my opinion.

I also attached my cells with VHB with nothing else in between, so you're not alone. I'd say I'd do it differently if I could go back in time, but I've built another several since learning of separating plastic sheets... still using VHB... not sure what my thought process was there

If you're good with wood working you could probably build the box to perfectly fit the cells. This guy went this route.. I don't know how tight they are but it certainly limits how much swelling can occur. He is stationary though, so he doesn't have to think about the cells bouncing up and down on bumpy roads.

You're the real hero for linking all the vids!!!