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diy solar

Is Cell Compression Necessary?

If I understand your application, of course you do NOT need to go to the trouble of cell compression. You describe keeping those cells normally charged at 95% capacity, with them discharge cycling only a couple of times a year. I recall that Lishen, for one, claims 3000 charge/discharge cycles WITHOUT cell compression (and then still retaining 80% of original capacity at 3000:cycles). It sure sounds like you wouldn‘t get much benefit from cell compression.

If I were you, I would still clamp them lightly with rigid tie rods, or something simple like Kapton tape, but not fuss with springs. The cost of some simple rigid constraint is slight.

But I would be more comfortable with your plan if your power supply had SOME reserve capacity above your normal 15-25 amp load. I would want a 30-amp power supply. Why call upon the batteries for any more than momentary starting surge if that is avoidable at nominal cost? Of course, this depends on how much of the time the system is calling for the upper limits of that 15 to 25 amp load, which I can't tell.

Most of us here think in terms of applications that are much different than yours.

I think I'm going to put a sheet of some sort of thin closed cell foam between the cells and strap them together.

I did manage to find an AIMS charger which Will has featured on his YouTube channel, it's rated at 37.5 amps and has a Lifepo4 setting. They are on backorder everywhere, Amazon had one that had been returned at a great price.

I understand my application is much different than most here and in reality, AGM batteries are probably better suited to my needs. (at least simpler) The only problem with them is from my experience in the telecommunications field, even with the most fastidious care, they begin deteriorating as soon as they are placed in service and will be scrap in 5 - 10 years. The added bonus to the Lifepo4 battery is this gives me the option to repurpose some of the solar panels on my roof in a SHTF scenario with much more usable capacity. I already have a 40 amp MPPT charge controller and wiring harness made with MC4 connectors so that I can quickly unplug the SolarEdge optimizers and connect to a few of the easy to reach panels. It will be an ugly toss the wires over the side of the roof and bring them in through a garage window sort of thing, but I'll have some level of off grid power when the zombie apocalypse happens.
 
I understand my application is much different than most here and in reality, AGM batteries are probably better suited to my needs. (at least simpler) The only problem with them is from my experience in the telecommunications field, even with the most fastidious care, they begin deteriorating as soon as they are placed in service

Lithium do seem a little complicated and fussy at first, but once you get familiar with them, you will find them easy to manage. I put my first large lithium battery (21 kWh) in almost daily service almost 4 months ago. It replaced a 24 kWh AGM set. I fussed with the lithium a lot at first, but I find that they run perfectly with no I more attention than the AGM. I’m in the process now of doubling up to 42 kWh. I encourage you to make the change.

Be sure to set your system voltage carefully to prevent the batteries from continuing to charge slowly. I would plan on keeping them at 85% or 90% SOC to have a bit more headroom for that reason. With the batteries at your desired upper charge limit, adjust the system.voltage, while you measure battery current, to be sure no charging current continues to flow into the batteries. As you probably know, unlike the AGM batteries, it is damaging to keep lithium continuously trickle charging at some FLOAT voltage.

I only mentioned that your application is different than most IN THIS FORUM because I think that accounts for some of the quick response that you need to compress the cells. Your application is certainly common in the wider world.
 
i found bulk packs of 7mm neoprene mouse pads on ebay, $1 each, nearly perfect size. im going to calibrate the squish and clamp them into a wooden box, same ish idea. it is near 1100lbs of pressure though for a 2p8s pack... thats a lot of tape. hope you dont have to adjust it or replace a cell.
 
Completely unsubstantiated but man, that just seems like an awful lot of pressure.
 
it does, it does indeed. there are several good threads on it in the lifepo4 diy section, with graphs provided by one of the manufacturers. some gains were had at even a few psi, loss of cycling capacity was shown in the higher teens psi. 12psi at 90%SOC is the prime number they shoot for. 12psi times the large side area times 2...

i like the elastomeric answer since it provides even pressure across the wall and in between cells allowing minor expansion contraction in a narrow range of pressure without stressing out the terminals and the pack fanning out at the bottom like some have.

ill bet if you clamp them all together as a pack and add a bunch of tape it will probably work fine and exceed spec for cycling. i once used that tape to hold a large shade structure to rebar in the earth in the desert in windstorms. good stuff.
 
Completely unsubstantiated but man, that just seems like an awful lot of pressure.
It is confirmed by EVE. Don't know if it applies to other manufacturers.

 
i found bulk packs of 7mm neoprene mouse pads on ebay, $1 each, nearly perfect size. im going to calibrate the squish and clamp them into a wooden box, same ish idea. it is near 1100lbs of pressure though for a 2p8s pack... thats a lot of tape. hope you dont have to adjust it or replace a cell.
Could you please provide a link so I can take a looksee? Thanks.
 
I'll put my 3-cents in on this one. Warning - snooze material ahead. :)

1) Horizontal and vertical torque on your terminals has already been discussed. Here in earthquake-country, it would be a big bummer to suddenly need your battery, but find that the terminal has been torqued off or damaged.

2) In case of a catastrophic charging event, you want them to vent, not expand like an accordion. This would be very bad to have loose cells or maybe just electrical-taped together punching a hole in the bottom of your fiberglass boat.

3) Snooze stuff having to do with dendrite formation:
When li-ion cells are made, the positive and negative plates should not be of the exact same dimensions. Dendrites like to take the shortest path across and seek edges typically first. By making one of the plate polarities slightly physically smaller, there is a longer path for dendrites to seek. Doesn't take much.

Some of the shoddier manufacturers would cut both plates to the same size to save money, but at the risk of providing a path for dendrites to seek.

Higher quality manufacturers make theirs with the proper "offset" from each other.

So ... if cells aren't compressed, and they start to bulge and contract a bit, there is a chance that the pouch plates will no longer be in proper alignment, and skew a little bit. Enough skew, and a very inviting path for dendrites to cross first is created.
 
Shower thought. Epoxy resin pour? In a perfect world you wouldn't need to replace a cell but assuming that's a possibility epoxy could fill all gaps and resist massive amounts of pressure... don't shoot me im new
 
As they say... All dressed up and now where to go :)

I was waiting for details about compression from the CATL battery manufacturer.
They said in an email that they should be compressed, but they were closing for the May holiday and would get back to me.
Well they did, and it's disappointing. While they should be compressed, they said there is no specific pressure.
So, I'm taking that to mean snug.
 
i found bulk packs of 7mm neoprene mouse pads on ebay, $1 each, nearly perfect size. im going to calibrate the squish and clamp them into a wooden box, same ish idea. it is near 1100lbs of pressure though for a 2p8s pack... thats a lot of tape. hope you dont have to adjust it or replace a cell.
Asking again for a link please. I can't find any mouse pads on ebay that are 7mm thick.
 
K.I.S.S

I am not finished yet as I have been experimenting. I bought everything at home depot. 1/4 inch plywood and 5/16 threaded rods. If I had to do it again I would use 1/2 inch plywood and 1/4 inch threaded rods. I did install PVC tubing over the threaded rods. Works for me.
 

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K.I.S.S

I am not finished yet as I have been experimenting. I bought everything at home depot. 1/4 inch plywood and 5/16 threaded rods. If I had to do it again I would use 1/2 inch plywood and 1/4 inch threaded rods. I did install PVC tubing over the threaded rods. Works for me.
Looks familiar :)

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Well, I think that compression IS necessary, and I used 3/16 steel plate (per this photo): "small" 120Ah battery using 8" x 8" steel compression plates.
Come on ... let’s be thoughtful about this.

While I agree with you that compression is probably worthwhile for lithium batteries that are used normally/regularly, and while I have my cells under carefully considered and chosen spring compression, the original poster asked this question because he is contemplating replacing an AGM battery, with lithium, in an application that he anticipates will result in the batteries only being discharged a couple of times a year. In that circumstance, there is no payback for the effort and cost of cell compression, the goal of which is to extract a few thousand more charge/discharge cycles out of the cells.

Circumstances vary. No one size fits all.
 
That's what SHE said :D

Man I like that fancy crate build with the springs though, very nice.
 
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