Where to place temperature sensor on enclosed LiFePO4 battery (for heater activation)?

[BilTheGalacticHero]

I'm setting up a LiFePO4 auxiliary battery for my Jeep to use when camping. I purchased a generic 100Ah Chinese battery off Amazon. This is a fully encased battery (plastic case). This is the battery...



Since I often camp in below freezing temperatures I'm making a heater to warm the battery up enough to charge as needed. I have some small heating pads and a thermostat controller. My plan is to place the pads under the battery, They'll be attached to a thin aluminum plate. Unless of course there's a better place to put the heating pads.

These are the pads...



I was going to use two in series to cut down on the heat level.

The battery is located inside the Jeep but two winters in a row it's gone down into the teens and I've had to run the engine and heater on the Jeep for quite a while before the portable battery I was using would allow charging.

What I really need to know is where to put the temperature sensor for the thermostat? It has to go somewhere on the outside of the case but the locations I see recommended are all over the place (on the heat pad, which doesn't seem to make sense, on the positive terminal, etc.).

Thanks!

 

[Crowz]

I just taped the sensor to the top of the battery case on mine. Seemed to work ok. The pads are located close to the bottom of the case on 4 sides.

 

[Partimewages]

There is nothing wrong with @Crowz suggested placement. I would add insulation around the outside of the battery area would be almost as helpful in the weather you speak of also. This would cut down on the amount of energy used to keep the battery warm.

 

[SeaGal]

I would add insulation around the outside of the battery area

2" of PIR works wonders, if you have the space

 

[Crowz]

One thing that I found that helped mine retain heat was using the foam pad that came with the batteries. All of the lifepo4 batteries I have bought ranging from 12v to 48v had a foam pad on the bottom of the battery to protect it during shipping. The pad comes up the sides an inch or two and this seems to make an excellent insulator. I have mine sitting on a concrete floor resting on the foam from the shipping boxes. Works great for me so far.

 

[SeaGal]

One thing that I found that helped mine retain heat was using the foam pad that came with the batteries.

My foam pads came all the way up the prismatics - looked impressive. I didn't use them for insulation though as a) I wanted compression between cells and b) I didn't have any specification as to their fire category.

 

[BilTheGalacticHero]

Thanks for the feedback. I am planning on insulating the battery but I don't have a huge amount of space around it. I imagine a radiant barrier like Reflectix wouldn't be useful in this care correct?

So, just secure the temperature sensor to the top of the battery on the plastic? Simple enough.

Any thoughts on heating pad placement? A lot of people seemed to think that placing them directly on the battery case was a bad idea. Is sides or bottom better?

Thanks again!

 

[Crowz]

Well the worries on pad placement has more to do with what kind of pads you went with. The silicone ones I bought run 140F in series and 210f or more in parallel. I chose the 140F route. Takes longer to warm up of course but won't melt anything

 

[BilTheGalacticHero]

Well the worries on pad placement has more to do with what kind of pads you went with. The silicone ones I bought run 140F in series and 210f or more in parallel. I chose the 140F route. Takes longer to warm up of course but won't melt anything

I think the ones I ordered are similar. You have them attached directly to the battery case?

 

[hardtop]

2" of PIR works wonders, if you have the space

What is PIR?

 

[Crowz]

I think the ones I ordered are similar. You have them attached directly to the battery case?

Yep.

This is what I'm using pad wise :

https://www.amazon.com/gp/product/B075WVPP5Y/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

 

[BilTheGalacticHero]

Yep.

This is what I'm using pad wise :

https://www.amazon.com/gp/product/B075WVPP5Y/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

That's the exact same ones I ordered. Great! Thanks!

 

[Crowz]

The pads work great. I even have a few of them on water pipes for my house. Its 25F degrees outside as I'm typing this and the controller and those pads are keeping the pipes between 50F to 55F automatically. The pipes its heating are not under the house but outside with wind blowing on them.

 

[SeaGal]

What is PIR?

Polyisocyanurate:

https://en.wikipedia.org/wiki/Polyisocyanurate

AKA Insulation board

 

[Coydon]

Polyisocyanurate:

https://en.wikipedia.org/wiki/Polyisocyanurate

AKA Insulation board

If you are concerned about the flammability of insulative materials, the article you link to mentions that PIR has been found responsible for the unusually incendiary nature of an apartment block fire in London. It also mentions that PIR will give off Hydrogen Cyanide when burnt and that toxic gases like that are the primary cause of death in such fires.

 

[BilTheGalacticHero]

Yep.

This is what I'm using pad wise :

https://www.amazon.com/gp/product/B075WVPP5Y/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

Just confirming that you're running four pads, each pair in series? That seems like a lot of heat.

 

[Crowz]

Just confirming that you're running four pads, each pair in series? That seems like a lot of heat.

4 pads

pad1 + pad2 in series = 140F per pad

pad3 + pad4 in series = 140F per pad

________pad1
pad2 Battery pad3
________pad4

The controller turns the pads on at 45F and off at 50F.

The __ is to make the spacing right the site trims the spaces off

 

[suimei]

Hello, I made a pot-shaped battery case for a lithium battery heater based on your post. Place the heater close to the bottom of the pot. Use heat-resistant insulation sheets on the floor to prevent heat from escaping. The temperature measurement point is the center inside the box, the center of the protrusion on the long side, and the corner farthest from the heater. I live in Japan, so temperatures are displayed in Celsius. This post uses machine translation.
The temperature inside the car was measured by installing a sensor between the second and third seats. We set the target temperature to 6℃ and observed the situation overnight. The outer heater side is set at 6°C, with a 1°C offset at the bottom inside the pot. At about 4:30, equilibrium was reached and a plateau of 5 degrees occurred. At this point, I switched to PID control and the temperature stabilized at 4.2-4.5°C on both the sides and ends. The outside temperature was 3.5 degrees Celsius at 4 o'clock and 2.4 degrees Celsius at 7:30, dropping slowly. It's the perfect experimental environment. This setting allows you to keep the inside of the case above 0°C even if the ambient temperature is around -10°C. The next theme is to improve the insulation of the case to reduce the corresponding temperature and power consumption.

 

[v_green57]

Being an old-skool user of FLA batteries, I mount temp sensors of any kind on the long side of a battery, half way from top to bottom and end to end. I use aluminum flue tape, as being metal, it will "absorb" the temp of the few square inches of the battery case it is stuck to and pass that along to the sensor.

I do the same with LFP batteries.

Reasoning:

Based on the teardowns I have viewed, the "package" style batteries have their internal BMS mounted on top of the cells, just under the top of the case.

The BMS, being a set of fairly high-power electronics, certainly generates some heat of its own. If your temp sensor is mounted on the top of the case, directly over the BMS, it *might* read somewhat higher than the the actual cell voltage due to the BMS.

Now, this might not matter at all, the difference could be negligible. I will have to point the IR camera at a package battery sometime while it's under charge and see. But why not make the assumption, I say. Won't hurt anything.

 

[featherlite]

This post uses machine translation

The machine translation is excellent.

I made a pot-shaped battery case

Nice work and thanks for the experimental data.