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

[Zwy]

Yes @KJA2001 , the 4.2V setting is just asking for trouble.

Second, the small leads that came with the power supply are limiting charging rate. I use 10AWG leads with rings terminals, not alligator clips. When top balancing, I use a BMS with cells in series until one cell hits high disconnect at 3.65V, remove busbars and install diagonal 10AWG leads for parallel charging at 3.65V. Doesn't take long, then I remove the leads and install busbars back on and capacity test the pack.

 

[timselectric]

I'm assuming that you set the voltage at 4.2v to get the amperage up. So that it will charge faster.
I hope that you are a good babysitter.

 

[wiseacre]

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 .

I found the pressure treated cells have a remarkably stable SOC and shelf life

Other cells I've slightly compressed in the typical plate and threaded rod approach

 

[JJJJ]

Mine are basically held in place. While they are in a fixture things are only finger tightened. The fixture had the benefit of securing a solid piece of polycarbonate sheet.

 

[upnorthandpersonal]

@KJA2001

Don't put that supply at 4.2V. Get did of those crocodile clamps and banana plugs and use ring terminals with proper size wire. You'll run at the max current your supply can provide well below 3.5V.

With your current set-up you will easily get bloated cells because you'll miss the 3.65V cut-off. Compression (or rather, a fixture) is not to prevent bloating by the way, but if anyone here wants some of the details, check out "4D imaging of lithium-batteries using correlative neutron and X-ray tomography with a virtual unrolling technique" and search 'compression' in that article. Link:

4D imaging of lithium-batteries using correlative neutron and X-ray tomography with a virtual unrolling technique - Nature Communications

The combination of X-ray and neutron CT enables 4D studies, i.e. to explore the evolution of 3D structures with time. Here the authors apply this approach to a Li-ion primary cell, revealing elsewhere unseen trends in the spatial distribution of performance aided by a new ‘unrolling’ methodology.

www.nature.com

 

[wiseacre]

Mine are basically held in place. While they are in a fixture things are only finger tightened. The fixture had the benefit of securing a solid piece of polycarbonate sheet.View attachment 150010

The fixture looks familiar ?
The 2 x 8 underneath makes it fairly easy to lift each pack if I must

 

[KJA2001]

I'm assuming that you set the voltage at 4.2v to get the amperage up. So that it will charge faster.
I hope that you are a good babysitter.

Yes, I started at that voltage to get the current up and then dropped it down once the cells reached 3.5V. The voltage across each cell was never higher than 3.55V. I was a very attentive babysitter!

 

[TomC4306]

Mine are only fixated so the terminals don't move. No actual compression.
Though, at full charge it is almost impossible to extract a cell. Easier to get them out when discharged

 

[DIYrich]

What about compression vs temperature? I saw on one video that it is optimal to keep the batteries under 30c (86f). Allowing gaps between the batteries would, in theory, keep the temp down.

 

[wiseacre]

What about compression vs temperature? I saw on one video that it is optimal to keep the batteries under 30c (86f). Allowing gaps between the batteries would, in theory, keep the temp down.

Only an observation
My batteries (fixed with slight compression) are in a well insulated (heated) box to protect from frigid temps
I don't see many days with temps higher than 90F
I thought it would be cooler to open the box in summer but found the opposite to be true
Temps in the sealed box remain below ambient outdoor temps
I don't have high discharge rates so my batteries don't produce much heat if at all

 

[timselectric]

If you are working the batteries hard enough to raise their temperature. You don't have enough battery capacity.
In my opinion.

 

[Skypower]

Edit: for un swollen cells!
I use two boxes with 8 cell each.
A centralized spring is used to compress at 550 lbs for 10 psi and WHEN they expand they expand approximately .025” per cell they should still be under 12 psi across the wide surface. The ends and pusher are doubled/glued for a 1-1/2” thickness needed to not bend over time. The adjustment end plate has a hole in one doubler to clear the washer to facilitate loading the spring while shortening the overall length. This is the dimensions and list of what’s in use but my next one (not finalized) will be longer and higher for a compartment for the bms and a cover the only thing thing that stays the same is the pusher. Material List for:

Eight Cell Compression Module
(lithium "Squeeze Box")
Fits most 272/320 Ah Prismatic LiFePO4 cells with allowance for spacer/insulators
sheets on wide battery surfaces (9 places).
Home Depot 2' X 4' plywood Sheets, Radiata Pine 1/2"(.480) & 3/4"(.725) thick:
1pc Bottom 3/4" X 7" X 29-1/8"*
2pcs Sides 1/2" X 9" X 29-1/8"*
4pcs Ends 3/4" X 7"* X 8-5/16" (2per end)
2pcs Thrust plate 3/4"X 6-15/16" X 8-1/8"*
*Denotes surface grain direction. Doubled thrust plate has the only vertical grain.
Ends sit on bottom and between sides. 1pc Spring, 634.6 LB @ .56" deflection
from MSC (mscdirect.com) PN 07661879.
1pc 3/8-16 four pronged Flange nut
1pc 3/8-16 X 3" Bolt, zinc plate (not stainless! it may bend)
1pc 3/8 fender washer, 2pcs 3/8-16 nuts
Grease for threads and washer surface.
Teks sharp point lath screws #8x1-1/4" to align while gluing.
Clamp up assembly squarely before gluing. Drill tight clearance holes through first side
and pre-drill small hole for threads.
Titebond 3 glue, re-coat edges after a few minutes soak in before assembly.
When done, protect with Minwax Helmsman Urethane 3 coats or you may wish to treat with fire retardant.
Sand between coats, warning very DUSTY!
One roll of Formica from Home Depot for cell spacers or purchase “epoxy insulator sheets” from 18650batterystore.com
If you treat the wood with fire retardant, do so after gluing because it may interfere with bond.

 

[sunshine_eggo]

Someone didn't set voltage with the leads disconnected.

 

[Skypower]

Only an observation
My batteries (fixed with slight compression) are in a well insulated (heated) box to protect from frigid temps
I don't see many days with temps higher than 90F
I thought it would be cooler to open the box in summer but found the opposite to be true
Temps in the sealed box remain below ambient outdoor temps
I don't have high discharge rates so my batteries don't produce much heat if at all

The cells are also a thermal reservoir, the terminals are a good conductor of heat from the core and buss bars are a good radiator. That can be a double edged sword. If the ambient temperature is warmer than the cells and there’s air flow, the cells are going to absorb heat. At night when it cools it’s the other way round. My first two batteries are under the workbench and the air is relatively stagnant. They do tend to get warmer running the HVAC in the afternoon. I have a fan on a timer that goes on at 3AM and off at 8AM so it gets a cool head start at it for the next day.

 

[justgary]

Calendar aging is my only enemy.

the majority of us will calendar-age these things to death

calendar aging will be a factor

Everybody mentions calendar aging, but nobody knows when that is. If your cells last years, I guess you can blame the calendar for their death. What if compression would have kept them working for twice as long?

The spec sheet says 660 pounds. Why does everybody go off and second guess this? The use case with solar is probably closer to an EV than just sitting your cells on a shelf. I would agree that sitting on a shelf they probably don't need compression.

 

[timselectric]

Everybody mentions calendar aging, but nobody knows when that is. If your cells last years, I guess you can blame the calendar for their death. What if compression would have kept them working for twice as long?

The spec sheet says 660 pounds. Why does everybody go off and second guess this? The use case with solar is probably closer to an EV than just sitting your cells on a shelf. I would agree that sitting on a shelf they probably don't need compression.

If you drive your solar system around. Then you should at least fixture them so that they don't move around. If your solar system powers your traction motor. Then you can gain lifespan from compression.
Most people are using these batteries for solar storage. And if their system is designed correctly. They're not pushing them anywhere near what an EV system would.
But if you want to compress your cells. It won't cause any damage. Or shorten their lifespan. Do whatever makes you feel comfortable.
For the rest of us. It's not worth the effort and time.

 

[justgary]

It's not worth the effort and time.

This is conjecture, and not supported by any studies I have seen. I'll ask again, how long is the calendar life of modern EVE cells?

 

[timselectric]

This is conjecture, and not supported by any studies I have seen. I'll ask again, how long is the calendar life of modern EVE cells?

I'll let you know when mine are dead. Or, I'll leave it to my kids to let you know.
As far as studies, maybe someone else can look for them for you.
As I said before, do what makes you feel comfortable.
Nobody is telling you what to do. We are just stating what we are doing and why.

 

[upnorthandpersonal]

how long is the calendar life of modern EVE cells?

It mostly depends on temperature. You should account for about 1% at temperatures up to 25C and up to 10% for temperatures up to 40C per year. There are several studies on this, but these numbers are from memory. I'll look up the sources if you want when I have some more time.

 

[Martijn]

Mine are only fixated so the terminals don't move. No actual compression.
Though, at full charge it is almost impossible to extract a cell. Easier to get them out when discharged

When using this in an RV/truck/whatever, this would worry me. Especially when using solid busbars between terminals.