DIY battery heating orientation - Between Cells or bottom?

pshoe · Oct 18, 2024

FYI: this is a in vehicle battery in North America with wide temp ranges

I'm preparing to run some tests on heating cells from below and from between the cells. I'm probably a month off from having all of the parts to test. This is a 4s configuration.

My cells are only 38mm thick and about 125mm tall so I'm going to test 12w silicone heaters between cells 1 and 2 then between 3 and 4. I'm also testing a 20w from the bottom. I have a scientific freezer that can do 60 below, but I'm not sure I'm going to go past maybe 20 below. It will be interesting to see how much the cells heat themselves through discharge and charge. The cells I have are rated for charging below freezing, but the curve has to be steep. Testing hopes to show that.

Anyone have any experience with testing heaters? Anyone have a preference for bottom vs between cells? I'm more or less curious and want to gather some opinions before I run the tests.

Bottom is attractive for design simplicity perspective. Between cells seems like it could be faster given the thickness of these cells. But I'm not sure fast is better. It would be if it's 20 below and you want to use a lot of power in an emergency.

Someone will ask... At the moment I'm considering custom heating circuits rather than using the BMS. Part of that choice will be to have simple ways to run the heat with a manual switch if needed. I also want the option to heat without the low temp cutoff active. It also opens things up to many more BMS choices.

S Davis · Oct 18, 2024

I would have concerns putting the silicone pads between the cells, the cells need a flat surface to expand against. I went a different route and heated my battery boxes with space all around the cells, worked great down to 7 degrees last year.

pshoe · Oct 19, 2024

If I had more room/space insulating and heating differently could be an option. I'm putting 177ah into a space of 330mm x 170mm x 160mm. Then I am replicating that maybe 50 or 100 times. These are going in ABS battery boxes and mounting in a fixed location that has existing mounts and mounts in contact with the metal chassis with no insulation. The vehicles could be parked and not heated in negative temps and then asked to perform.

The heat pads samples being manufactured are the silicone mat type. I had them designed with the leads on a tab outside of the compression or cell area. Yes, I agree they should be as flat as possible. Maybe less important if the fixture isn't compressing and is more simply in contact. We will see. You can also get Kapton style. You can get them with adhesive as well so you could stick them to the sides of cells that are free standing. That might work with a fixed location on land. Less so with something that is going to shake like hell.

This is a Kapton style heater without the leads on a tab.

S Davis · Oct 19, 2024

Well it is all about what you are comfortable with, the cells have a lot of mass and need to be secured together well to keep them from shifting when you hit a pothole at speed. If they move independently you are going to have problems with your cell connections, high resistance, heat, fire.

pshoe · Oct 19, 2024

S Davis said:
Well it is all about what you are comfortable with, the cells have a lot of mass and need to be secured together well to keep them from shifting when you hit a pothole at speed. If they move independently you are going to have problems with your cell connections, high resistance, heat, fire.

For sure. This is my second rodeo. The first design with less ah has around 50 in the field with no issues. Everything gets mechanically fixed, taped and glued. This will be my first test with heat between cells.

Thanks for chiming in.