Do I really need to put cells under compression or just be able to withstand expansion?

[cmburns]

I'm planning on making 4 16s batteries from 280AH cells. My simple plan is to build a snug fitting box for 8 cells.(cut to size Ikea cutting boards between them). I'm ging to just brad nail and glue the boxes together. I'm going to round off the external corners of the box, then put 3-4 sets of pallet strapping around each box. There won't be a lot of tension, but it should withstand a lot of tension from expansion.

My further plan is to charge each battery one at a time, then discharge. Then I will disassemble and match the batteries between sets based on capacity.

 

[sunshine_eggo]

Note what the manufacturers do when charging:

Inside the Basen test & dispatch center

An interesting video tour showing racks of batteries presumably being charged to an even level ready for dispatch. Nothing about the equipment shown seems to indicate it can do anything more sophisticated like measure cell IR or capacity.

diysolarforum.com

If the datasheet indicates compression, that's what you need for the quoted cycle life.

 

[S Davis]

I would be worried about too much pressure on the cells with a ridged box at high SOC, too much pressure will damage the cells.

 

[timselectric]

My cells are sitting freely on a shelf.
They they are LFP and will outlast me.
Compression is important only for the first few cycles. To force the bubbles to the top.
Assuming that they weren't already cycled (for testing) , before they came to you.
If so, then they were either compressed or not. But, there's nothing that you can do about it now.

 

[S Davis]

My cells are sitting freely on a shelf.
They they are LFP and will outlast me.
Compression is important only for the first few cycles. To force the bubbles to the top.
Assuming that they weren't already cycled (for testing) , before they came to you.
If so, then they were either compressed or not. But, there's nothing that you can do about it now.

Then I guess you know more than the manufacturers, they specifically state in the spec sheets to compress the cells to get the rated life cycles. Most of them don’t even give a rated cycle life without compression

 

[timselectric]

Then I guess you know more than the manufacturers, they specifically state in the spec sheets to compress the cells to get the rated life cycles. Most of them don’t even give a rated cycle life without compression

If you are going to put them in an EV. (Which is what they are made for) Then, by all means. Go ahead and compress them. I am using them for solar storage. Calendar aging is my only enemy.

 

[LakeHouse]

The manufacturers also say to put these cells in an EV and charge and discharge them at 1C, but none of us are doing that either. Our use case is vastly different from what the cell manufacturers intended.
In my opinion the majority of us will calendar-age these things to death long before we run out of cycle life. My cells are also not compressed, I just don’t think it’s worth the effort. But like I said, just my opinion, take it or leave it.

 

[42OhmsPA]

My take on compression below. I figure by the time I have to worry about anything there will be a new breakthrough chemistry or calendar aging will be a factor. If I were running 100s of amps through them and/or they were in a mobile environment I would have approached things differently.
I mainly "secured" them because I'm never clumsy and would never bump a cell with something...

 

[S Davis]

The manufacturers also say to put these cells in an EV and charge and discharge them at 1C, but none of us are doing that either. Our use case is vastly different from what the cell manufacturers intended.
In my opinion the majority of us will calendar-age these things to death long before we run out of cycle life. My cells are also not compressed, I just don’t think it’s worth the effort. But like I said, just my opinion, take it or leave it.The manufacturers also say to put these cells in an EV and charge and discharge them at 1C, but none of us are doing that either. Our use case is vastly different from what the cell manufacturers intended.

Unless they fail from delamination.

 

[Browse]

Off Grid Garage had a recent video on this subject:

 

[Dynoman]

I'm planning on making 4 16s batteries from 280AH cells. My simple plan is to build a snug fitting box for 8 cells.(cut to size Ikea cutting boards between them). I'm ging to just brad nail and glue the boxes together. I'm going to round off the external corners of the box, then put 3-4 sets of pallet strapping around each box. There won't be a lot of tension, but it should withstand a lot of tension from expansion.

My further plan is to charge each battery one at a time, then discharge. Then I will disassemble and match the batteries between sets based on capacity.

My fixture is using yellow pine wood 1 inch x 10 inch cut to length and four 1/4 inch course threaded rods with 1/4 inch nuts & washers on the 8s Lifepo4 battery banks.

This is Info I used and torqued a little less at 5 Inch Pounds with a torque wrench at about 3.2 to 3.3 volts charge in each cell. The cells are essentially just held in place.

The spec from EVE was 300 KG force which rounds off to 660lbs. Battery face is approx 6.85"x 7.874" = 53.94 sq inches
660lbs/53.94sqin=12.23 lbs per sq inch
Divide 660 by 4 bolts that's 165 lbs Axial (clamping) force per bolt.
Using 4 course 1/4 in threaded rods that should equate to roughly 8 INCH pounds torque per bolt. Realistically, that's a snug twist of the wrist on a regular nut driver for the average build mechanic.

Ran across this EVE LF280K spec sheet with diagrams dated April 2022. The Testing Cell Clamp diagrams are shown. It is interesting that it shows 10mm metal plates 6 M6 bolts on a single cell.

 

[Anomof]

The need for supports on either side of a single cell is as necessary if not more than stacked cells in compression, because as two compressed cells expand against each other the pressure from the opposing sides rises, during charging above 3.4V.

 

[Anomof]

Applying locktight to the bolts or double bolting is necessary as bolts tend to loosen with repeated cycles of increased pressure. To avoid the added requirement of retorking the bolts periodically to maintain the proper tension.

 

[gian83]

if the cell are precompressed from the manufacturer you dont need any compression.
Like most people said you only need compression to squeez those gas bubble out from new cells when you first charge them if indded the manufacturer didnt do it .

 

[PreppenWolf]

My system has been active since Nov. My total cycle count on my 105kWh bank according to my BMS tracking? 24.

So 50 cycles per year at my current rate. Let's double it and say 100. Still never going to cycle the batteries to death.

 

[Grimmster]

Depends on the cell (energy density) and case material/strength and whether you want to absolute best cycle life. Also, how do you know the manufacturer actually did the proper compression during break in?

It does seem like in most battery teardown videos there are very few actually compressing the cells. Of course they are trying to produce the cheapest possible product.

LiFePO4 Cell Compression | Current Connected

Cell compression is squeezing/fixturing the outer aluminum case of (LiFePO4) cells to prevent delamination of the internal components.

www.currentconnected.com

 

[Cheap 4-life]

My cells are sitting freely on a shelf.
They they are LFP and will outlast me.
Compression is important only for the first few cycles. To force the bubbles to the top.
Assuming that they weren't already cycled (for testing) , before they came to you.
If so, then they were either compressed or not. But, there's nothing that you can do about it now.

How would anyone ever know for sure if they were cycled before delivery.. They wouldn’t so compression is a must even if all it does is force out the bubbles in the first few cycles.. also if mistakes happen like over charging or over discharging I’d assume proper (springs or poron foam) compression could possibly prevent unnecessary expansion bloating or delamination or whatever..

 

[timselectric]

How would anyone ever know for sure if they were cycled before delivery.. They wouldn’t so compression is a must even if all it does is force out the bubbles in the first few cycles.. also if mistakes happen like over charging or over discharging I’d assume proper (springs or poron foam) compression could possibly prevent unnecessary expansion bloating or delamination or whatever..

If it makes you feel better to compress. It shouldn't do any harm. As long as you don't over do it.

 

[KJA2001]

I compressed mine gently. I had to make an enclosure anyway, compressing with some angle iron and two threaded bolts added very little effort. I needed to use thick wooden end plates to spread the load applied.

 

[42OhmsPA]

I compressed mine gently. I had to make an enclosure anyway, compressing with some angle iron and two threaded bolts added very little effort. I needed to use thick wooden end plates to spread the load applied.View attachment 150002

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