Cell Spacing

[Tulex]

After reading some threads about battery issues, I started to think about cell spacing, as too much compression can cause cell damage.
The data sheet for the Eve MB30, which is in the resource area, refers to this.

"Customer shall fully consider the influence of the cell swelling force when designing the module. The product generates expansion force during use, and the expansion force is about 60000 N when the cell capacity attenuates to 60% under the test conditions of 15 mm steel plate + 0.0 mm GAP (the space for cells to expand). Customers shall consider there liability of structural strength in the product design process, and it is suggested to reserve 2.0 mm ~ 2.5mm expansion space while grouping the cells."

That's 5/8" for a row of 8. I've seen everything from compressed cells to taped cells to packed cells. I haven't seen one of those metal box kits in person that everyone is using, or how premade batteries are made, but I can't imaging there's that kind of room. I've only really heard that you have to have a divider to prevent the cells from touching, and now many cell suppliers are shipping with a thin plastic sheet to put between them.

Obviously this is a huge concern as a more likely cause of battery failure. Over compression causing oozing, and constant flexing of the terminal. So for those who have built using box kits or who have been inside purchased batteries, how is this addressed?

 

[Lt.Dan]

The Seplos cases do come with a thin (2mm-ish) foam to put in between each cell. Not sure if it aids in helping the cells "breathe" in between cycles, but it sounds better than the plastic sheeting.

 

[Tulex]

For me there is a major conflict in all of this. Some people point to the data sheet to support compression, while the data sheet also says to leave spacing. Hard to compress with space.

 

[SparkyJJO]

For me there is a major conflict in all of this. Some people point to the data sheet to support compression, while the data sheet also says to leave spacing. Hard to compress with space.

Compressed air!

 

[Steve_S]

I am in process of doing another batch of battery packs. 24V/280AH/8S Steel Cases with JK Inverter BMS'.
The kit comes with the Fiberglass FR4 separator sheets for the bottom, back, sides & front + a large sheet to over the underside of the case top over the cells. Link to cases I am using
When installing the 280AH cells and tightening in the front plate (BMS mounts to it) the cells were NOT compressed or fixed into place - there was enough "slack" that the cells could move side to side & front to back my roughly 3mm. I then installed a fine mesh polyurethane foam on the sides & front between the FR4 sheets & casing... then when tightening everything in it was nice & snug but not heavily compressed. The polyurethane mesh I used is such that it can be very compressed (thanks to the mesh) and is more "forgiving" than a solid sheet of foam.
* AS I build these cases & test them, this is a very workable solution and the cells are quite happy overall.

Sadly, some people try to convert metric to foot/inch pounds and make a mess of it... that's been beaten to death. The cells DO Expand and Contract based on charge, temps etc... I have several packs in operation at this time as well. I have 2 that were "compressed" and taped together with Fiber Tape, 2 with no precompression and just slotted into box, 1 with cells that can move a little (slack). NB: I keep powerhouse temp at 15C/58F in winter and I have allowed Powerhouse to drop to 9C/32F for testing (part of my Thrash Tests). I am using only EVE cells.

My observations as follows.
- 2 Compressed packs: Cells stay pretty even regardless of SOC in general. These appear to be slightly slower charging when colder (they catch up about 20-30 minutes after the others). No issues are apparent when discharging and cells stay within spec.
- 2 without compression but "snug": They charge at a slightly faster pace than the above two. When 100% they are quite snug (no movement possible, but below 50% they can wiggle a tiny bit (with effort) ). They begin to deviate after 3.370 when "cool" but are fine at 20C temp & up.
- 1 Box with "loose" cells that can float about 2mm in either direction (except for busbars of course) is my "Randy Pack" as it can deviate more when cold or hot and have seen runner cells trigger below 3.0V when cool.

When these were allowed to drop to 0C/32F Charge was of course disabled. Discharging worked just fine. The "un-fixed" packs deviated more with highed cell differentials. IMO this did affect capacity as LVD was reached by runners - based on the numbers that cost about 10% of working capacity.

"Fixing" the cells prevents stress on terminals & the internal "jelly rolls" as a consequence. It is not intended to "squish" the cells but to keep them snug through the working voltage ranged & temperatures.

Some will want to argue the points. TBH I don't care to engage in that nonsense again, this is my experience/observations & conclusions extrapolated from my own tests & real world observations over a few years.

Hope it helps, Good Luck.

 

[Tulex]

I am in process of doing another batch of battery packs. 24V/280AH/8S Steel Cases with JK Inverter BMS'.
The kit comes with the Fiberglass FR4 separator sheets for the bottom, back, sides & front + a large sheet to over the underside of the case top over the cells. Link to cases I am using
When installing the 280AH cells and tightening in the front plate (BMS mounts to it) the cells were NOT compressed or fixed into place - there was enough "slack" that the cells could move side to side & front to back my roughly 3mm. I then installed a fine mesh polyurethane foam on the sides & front between the FR4 sheets & casing... then when tightening everything in it was nice & snug but not heavily compressed. The polyurethane mesh I used is such that it can be very compressed (thanks to the mesh) and is more "forgiving" than a solid sheet of foam.
* AS I build these cases & test them, this is a very workable solution and the cells are quite happy overall.

Sadly, some people try to convert metric to foot/inch pounds and make a mess of it... that's been beaten to death. The cells DO Expand and Contract based on charge, temps etc... I have several packs in operation at this time as well. I have 2 that were "compressed" and taped together with Fiber Tape, 2 with no precompression and just slotted into box, 1 with cells that can move a little (slack). NB: I keep powerhouse temp at 15C/58F in winter and I have allowed Powerhouse to drop to 9C/32F for testing (part of my Thrash Tests). I am using only EVE cells.

My observations as follows.
- 2 Compressed packs: Cells stay pretty even regardless of SOC in general. These appear to be slightly slower charging when colder (they catch up about 20-30 minutes after the others). No issues are apparent when discharging and cells stay within spec.
- 2 without compression but "snug": They charge at a slightly faster pace than the above two. When 100% they are quite snug (no movement possible, but below 50% they can wiggle a tiny bit (with effort) ). They begin to deviate after 3.370 when "cool" but are fine at 20C temp & up.
- 1 Box with "loose" cells that can float about 2mm in either direction (except for busbars of course) is my "Randy Pack" as it can deviate more when cold or hot and have seen runner cells trigger below 3.0V when cool.

When these were allowed to drop to 0C/32F Charge was of course disabled. Discharging worked just fine. The "un-fixed" packs deviated more with highed cell differentials. IMO this did affect capacity as LVD was reached by runners - based on the numbers that cost about 10% of working capacity.

"Fixing" the cells prevents stress on terminals & the internal "jelly rolls" as a consequence. It is not intended to "squish" the cells but to keep them snug through the working voltage ranged & temperatures.

Some will want to argue the points. TBH I don't care to engage in that nonsense again, this is my experience/observations & conclusions extrapolated from my own tests & real world observations over a few years.

Hope it helps, Good Luck.

Lots of good info, thanks. I don't really want to get into the compressed or not thing either, only in that it's conflicting in the data sheet. Personally, I believe the compression is taken out of context from cell testing, was never meant to be used in practice, but I haven't read enough to stand behind that belief. I do believe it's clear what they say about having a gap.
I built my first three batteries by lining the whole box with a light duty blended EPDM foam and also used that between cells. Something like 11 psi to compress 25%, then just snugged the pusher plate, no actual compression.

 

[Steve_S]

I think you also hit on a GOTCHA that many trip over... is when to "fix / bind" the cells. The fine print (several makers) indicates to bind the cells at 50% SOC / 3.200VPC collectively. Not when 100% or 25% SOC and if you measure the cells with a digital caliper when at 25%, 50% & 100% it becomes obvious why.

EVE has stepped up with their newer docs that have been proof read by folks with good working english. Some of it is still not as clear as it should be IMO as it leaves some room for "debate".

Now LFP is still evolving faster & faster and we are now starting to see cells with higher C-Rates for charge/discharge and more temperature tolerances with higher capacities due to improved electrolytes & more refined materials. CATL's new Honeycombed internal design is getting them some incredible numbers but we won't see that in ESS & Cell Vendors for quite some time. Boy, some folks will be freakin in Q4 when a lot of new cells & new capacities hit the streets...