Aluminum heat spreader in direct cell contact?

[Visionquest]

I have done rather extensive reading on the subject of heating pads for LifePo4 builds. I'm not finding quite enough specific information to complete my 12v design, hence this new thread. This build will be 12v 302aH using EVE cells. I will be using 25watt silicon heater mats that many others here have used. I will run either two or three in series to limit the watts and top temperature. My remaining questions come down to the following:

1) If I mount them to an aluminum spreader plate, can that plate then be in direct contact with the sides or bottom of the cells? If yes, how is it ok for the aluminum to potentially connect the cases together electrically? (heat mats<=>aluminum<=>cells)

2) Instead, if I attach the heat pads directly to the sides or bottom of the cells, how can I insure that they stay in good contact....press them with an aluminum plate? Other material? (aluminum<=>heat mats<=>cells)

3) The heat pads I ordered do not come with adhesive. Is there a heat conductive adhesive that is effective at holding them in place?

Thanks for your help and experience,
Bob

 

[400bird]

1) If I mount them to an aluminum spreader plate, can that plate then be in direct contact with the sides or bottom of the cells? If yes, how is it ok for the aluminum to potentially connect the cases together electrically? (heat mats<=>aluminum<=>cells)

You need to isolate the cell cases from each other. So you can't use one large aluminum plate on all 4 cells. You could use 4 small plates, one on each cell.

2) Instead, if I attach the heat pads directly to the sides or bottom of the cells, how can I insure that they stay in good contact....press them with an aluminum plate? Other material? (aluminum<=>heat mats<=>cells)

If your mats are sized correctly to fully insulate the aluminum plate from the cells, this would be my preferred method. If the heater mat is smaller than the cells, I'd add some insulation around the heater mat to keep the aluminum plate from contacting the cells.

3) The heat pads I ordered do not come with adhesive. Is there a heat conductive adhesive that is effective at holding them in place?

I'm not sure I'd glue the heater to the cells, if you need to disassemble the pack, will the adhesive pull off the blue shrink wrap on the cells?

 

[S Davis]

I heat the battery box with no direct contact with a 25watt silicone pad and small computer fan. Works great, we got down to 4 degrees this last cold spell.

 

[400bird]

Nice looking set up
How was working with the flexibar?

Do you have more pics of the heater set up?

 

[wpns]

How about a layer of stiff foam with the heat pads on top, then set the cells on the heater pads? Gravity should keep them in place

 

[S Davis]

Flexibar worked great, you have to be careful when drilling to avoid burrs between the layers. For the heat I used a 25watt silicone element between thin aluminum plate and made a channel for the fan to blow across and it.
A 12volt digital thermostat for the heat pad and a computer fan thermostat that will power four fans with individually adjusted on/off temps. The batteries are mounted on 1” vibration dampers about 1-1/2” above the bottom of the box.

 

[S Davis]

This is a mobile system in my work truck.

 

[Visionquest]

How about a layer of stiff foam with the heat pads on top, then set the cells on the heater pads? Gravity should keep them in place

What kind of still foam will be ok long term with direct contact of the heat mats?

 

[Visionquest]

This is a mobile system in my work truck.

I like your setup though I plan to do something different. Thanks for sharing, unfortunately it doesn't help answer my questions.

 

[OffGridInTheCity]

You need to isolate the cell cases from each other. So you can't use one large aluminum plate on all 4 cells. You could use 4 small plates, one on each cell.

Just to clarify - the metal 'case' is electrically active. It's either the + or - of these cells (I don't which) but if you put the cells in series using the terminals and then place a metal conductor across the cases (electrically connecting them) you'll cause a big short. Just wanted to call this out explicitly to help you with you're design.

 

[400bird]

Just to clarify - the metal 'case' is electrically active. It's either the + or - of these cells (I don't which) but if you put the cells series using the terminals and then place a metal conductor across the cases (electrically connecting them) you'll cause a big short. Just wanted to call this out explicitly to help you with you're design.

Well said, I should have included that information. I'm about 70% sure the case is connected to the positive terminal.

 

[wpns]

What kind of still foam will be ok long term with direct contact of the heat mats?

Gee, https://www.mcmaster.com/products/foam-sheets lets you pick any temperature you want. How hot are the heat pads going to get again?

 

[Checkthisout]

Is the inverter going to be on all the time?

 

[robby]

I heat the battery box with no direct contact with a 25watt silicone pad and small computer fan. Works great, we got down to 4 degrees this last cold spell.

View attachment 201088View attachment 201090

Nice setup.
Only thing I would add is a strip of foam tape around the Lid edge to keep even more heat in during the Winter.

 

[S Davis]

Nice setup.
Only thing I would add is a strip of foam tape around the Lid edge to keep even more heat in during the Winter.

It’s under my bed that has a 5” foam mattress, a little extra heat is fine.

 

[Daddy Tanuki]

I heat the battery box with no direct contact with a 25watt silicone pad and small computer fan. Works great, we got down to 4 degrees this last cold spell.

View attachment 201088View attachment 201090

nice, neatly done setup.

 

[Visionquest]

Gee, https://www.mcmaster.com/products/foam-sheets lets you pick any temperature you want. How hot are the heat pads going to get again?

I expect no more than 100 degrees F on the pads though designing for somewhat higher will be a good idea. Maybe 150.

I just ordered poron sheets from mcmaster for another application so I'll take a look through their catalog. Good suggestion.

 

[S Davis]

I was not comfortable putting that much of a hot spot on the cells, cells degrade faster with heat and it can’t be good for them.

 

[400bird]

The heater might be capable of reaching 100f, but with good thermal contact, it should just dump watts of heat into the cell without being a significantly higher temperature. There must be some delta between the two materials for heat to transfer, but it shouldn't create 100f hot spots right where it contacts the cells.

 

[S Davis]

The heater might be capable of reaching 100f, but with good thermal contact, it should just dump watts of heat into the cell without being a significantly higher temperature. There must be some delta between the two materials for heat to transfer, but it shouldn't create 100f hot spots right where it contacts the cells.

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.