Is Cell Compression Necessary?

[Brett V]

While I'm waiting on the slow boat, I have time to plan ahead on my installation. I've purchased 16 200 ah cells that will be configured in a 2P8S 24 volt block with a JBD 100 amp BMS. My application is a 1500 watt inverter that rarely goes above 60 amps surge when one of the refrigerator or freezer compressors starts up. The other 99% of the time it loafs along at 15 - 25 amps load. I'll be using a fixed voltage charger set at a voltage (TBD) to charge and maintain (float) around 95% cell capacity. This charger has very good voltage regulation and its maximum current delivery is 20 amps. There is a small parasitic load of around 500 ma all the time for the standby function of the inverter. This is basically a UPS for my essentials and a quiet alternative to running the generator at night if there's a power outage. Historically it only gets used a few times a year.

Sooo... since I'm going to have a relatively low charge and discharge rating in relation to the capacity, do I really need to go to the trouble of binding and compressing the cells? I'll be removing the AGM batteries from the rack and the entire block of cells will fit on the top shelf.

 

[MisterSandals]

do I really need to go to the trouble of binding and compressing the cells?

Yes

 

[DThames]

Sort of like maintaining proper air pressure in your tires. You don't have to. Better if you do. To what degree.....depends on how anal you are.

 

[Brett V]

To what degree.....depends on how anal you are.

I don't see my OCD as a disorder, just wanting to do things right...so I guess that settles that!
Damn... a sheet of 3/4' plywood is gonna cost as much as the cells LOL.

 

[DThames]

I don't see my OCD as a disorder, just wanting to do things right...so I guess that settles that!
Damn... a sheet of 3/4' plywood is gonna cost as much as the cells LOL.

I made a frame with 3/8" AL plates, threaded rod, and some die springs to provide calibrated pressure in the middle of the recommended range. As I was tightening the compression to the level that I had planned, I became a bit concerned of the total force and said, "Good Enough". So mine are less than ideal.

 

[deeuubee]

After reading pages and pages and pages... I think the general consensus is that you should if you could.
If you have an open rack, it would be simple and the 30% longer cycle time is a big benefit if you cycle every day.

Me, If I cycle 90 times a year camping, it will be a lot. So, 2500 cycles/ 90 days = 27 years
If I'm around that long, I will gladly treat myself to a new set

I am playing around with how to get the 675 pounds of pressure on the pack to compress it.
If I find a way to compress it and fit it into the case I have, I will, just to say I did.

 

[Bob B]

The specification for cell compression is for the EVE 280 ah cells .... so, while we don't know what effect it has on other cells ..... common sense would dictate that the cells be bound in some fashion to ease stress on the bus bars ... and MAYBE the compression will increase cycle life.

 

[deeuubee]

I just received 8 CATL 280 Ah cells that are already bulging. I don't know if, what compression will do.
But, not being flat is driving me crazy.

 

[DThames]

After reading pages and pages and pages... I think the general consensus is that you should if you could.
If you have an open rack, it would be simple and the 30% longer cycle time is a big benefit if you cycle every day.

Me, If I cycle 90 times a year camping, it will be a lot. So, 2500 cycles/ 90 days = 27 years
If I'm around that long, I will gladly treat myself to a new set

I am playing around with how to get the 675 pounds of pressure on the pack to compress it.
If I find a way to compress it and fit it into the case I have, I will, just to say I did.

This may or may not help, but you can see the springs on the end of the threaded rod.

 

[ArthurEld]

I just received 8 CATL 280 Ah cells that are already bulging. I don't know if, what compression will do.
But, not being flat is driving me crazy.

The bulging doesn't hurt anything but it would drive me crazy too. If they're not too bad they flatten out when you put them in a fixture.

 

[Q-Dog]

I don't compress my 206ah cells, but I do constrain them because I don't like them moving around. The first time I saw the bottoms spread out, with the cells only held together by the bus bars, I stopped the charging and made a frame to stop the spread.

 

[Gould]

I just received 8 CATL 280 Ah cells that are already bulging. I don't know if, what compression will do.
But, not being flat is driving me crazy.

Ha, just received mine as well and have the same bulging. Unfortunately my OCD is borderline hindering - I prefer to call it CDO so the letters are in order!

I haven’t cycled them yet but hoping a discharge will flatten them. Still on the fence about what compression to use but seeing them is this state, I’ll be doing something for sure.

 

[ArthurEld]

Ha, just received mine as well and have the same bulging. Unfortunately my OCD is borderline hindering - I prefer to call it CDO so the letters are in order!

I haven’t cycled them yet but hoping a discharge will flatten them. Still on the fence about what compression to use but seeing them is this state, I’ll be doing something for sure.

If you put the cells in a threaded rod fixture and set the torque when the cells are pretty full, they will all become flat after a cycle or two.
Then re torque again when they are full to squeeze them into shape a little better if needed.
The cells make the best use of the available space which gets rid of the lumps and bulges.
The recommended torque is 18 inch pounds max. Since the cells expand, full charge is where the torque is set.

 

[curiouscarbon]

dear @Brett V awesome looking current setup

please consider pros and cons of putting 5mm of a squishy and electrically insulating material such as closed cell neoprene between the cells to support them as they flex out and in over charge range

even at low rates i seem to recall seeing photos of the cell walls flexing enough to make you have a bad day if the plywood doesn’t take that force and instead busbar does

cheers; lots of people also like compression and light padding

 

[Boondock Saint]

So this thread has 675 pounds of pressure, rods with red springs, rods with 18 inch pounds max, and a full 5mm of spongy cell metrial bwtween each cell. So which is it? It seems like some of these things can't co-exist together and be true at the same time.

 

[Brett V]

dear @Brett V awesome looking current setup

Thanks! My current setup was a good multi stage learning experience, using almost entirely "surplus" parts from work, I probably don't have more than $200 invested in it.

I did some measuring and will still have room to get the new battery on the top shelf with the added thickness of 1/4" foam between the cells. I'm looking at this product, Dense charcoal foam maybe it's overkill. We used a similar product to ship HP lab standard test gear all over the world and it's pretty tough stuff.

 

[Gould]

So this thread has 675 pounds of pressure, rods with red springs, rods with 18 inch pounds max, and a full 5mm of spongy cell metrial bwtween each cell. So which is it? It seems like some of these things can't co-exist together and be true at the same time.

Here is the challenge. We know that compression helps to prolong the life of the cells. We also know what that compression range should be throughout the entire state of charge. What is highly debatable, at least from all of the reading that I’ve done, is how to do that effectively in a DIY environment. I think overkill and diminishing return are in abundance.

My approach; my cells came bloated so I know visually what I’m trying to prevent. I’ll apply my compression method of choice erring on the side of caution and cycle them a few times. If I don’t like what I see I’ll adjust. I am in no way, shape or form an expert here but I would think that the real concern here is the stress that the bulging places on the terminals. I’ll be using 4/0 as opposed to bus bars.

 

[JoeHam]

Absolutely NO.

Not NECESSARY. Let’s be honest here.

It’s just an EFFORT to extend a likely 3-5 year lifespan towards 7-10 years.

See Fortune cell holders and others including the innards of many commercial batteries which specifically make compression impossible.

Could it extend the battery life to a point where better battery technology is available and cheaper which renders these batteries inconsequential ?

A definite perhaps. ?

 

[ArthurEld]

So this thread has 675 pounds of pressure, rods with red springs, rods with 18 inch pounds max, and a full 5mm of spongy cell metrial bwtween each cell. So which is itI've never tried the spongy material. 18 inch lbs of torque is what it takes to create 675 pounds of pressure.

I think someone calculated that 18 in lbs of torque = 675 lbs of pressure. Seems like the spongy material wouldn't change the equation.
I use less than 18 in lbs

 

[RobOfYork]

I know I am confused. Compress cells or not. Foam between cells or not. Hmmm...