Aluminum heat spreader in direct cell contact?

[S Davis]

So here is the EVE 304ah spec sheet, notice the cycle count with temperature

25c 4000 cycles

45c 2000 cycles

So if your heat pad hits 113 (45c) that cuts your cycle life down considerably.

 

[wpns]

I’m confused, is this battery heating to 40F to prevent charging while frozen or heating to 40C to <improve performance?>?

 

[Partimewages]

If your battery box is insulated correctly 25 to 40 watts is all you need

 

[400bird]

No matter what you do with direct contact you are going to create warmer spots in the cells. Heat is bad, look up the cell specs and see what higher temperatures does to the life cycle of the cells. If you are comfortable with it that’s all that matters, I do heat transfer for a living and was not.

Sure, hot cells are bad.

But if the cells are 3c and you turn on the heater, does the contact point reach 45c or 13c?
I'd hope that the thermal transfer is good enough that the hot spot doesn't get more than 5-10c hotter than the cells.

I agree with you that hot cells are bad. I don't agree that there'd be that large of a delta between the hot spot/heater mat and the rest of the cell.

For example, the heater on my cells can reach 80c
The heater turns on at 5c and off at 10c (cell temps measured at the terminals, heat contact on the bottom of the cells)
In my testing, nothing in the system shows any noticeable temperature difference.

 

[S Davis]

It’s all about what you are comfortable with, I look at the spec sheets on stuff and base my builds on that. Most cell makers recommend compression, and in fact don’t rate the cells using them without. A lot of people ignore that.

 

[Visionquest]

I won't be heating the cells past 45-50 degrees F. The wattage of the heat pads will be around 20 watts total spread across the sides and bottom. I don't want to create hot spots which is why my original questions focus on use of an aluminum plate in contact with the heat pads. I think its clear now that I don't want to put the aluminum plate directly against the cells due to potential for shorting cases together. Placing the heat mats directly on the cells could create a hotter area where they directly touch. May not be an issue but will need some testing. The final thought I'm having is to put heat mats on an aluminum plate but electrically isolate the plate from the cells using a thermally conductive dielectric material. This would apply the heat evenly to a large surface of the cells but remain safe from shorting cell cases.

Anyone doing this?

 

[offgrid-curious]

I'm also planning on using an aluminum plate as a heat spreader for my DIY battery box, also using EVE 304 cells. Under what conditions would you expect the shell cases to short? The cell spec sheet says:



I understand this to mean that each cell is wrapped with 0.11mm of polyethylene terephthalate and the top (and I also assume by looking at the bottom) is 0.3mm of polycarbonate. Is that not adequate insulation to prevent shorting?

 

[400bird]

The 0.1 mm blue layer is not robust enough, not even close, especially in a mobile application. The cells I've handled don't have the black layer on the bottom.