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

[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.

 

[Pappion]

If I were to do compression, I would use springs or belleville washers. To set a known compression and allow for temperature expansion and contraction.
I did not bother in the rush to get it done. Also did not want to use flammable wood. I am not running high currents for Inverter, so it's not likely I would benefit from compression.
BTW: My solid bussbar between terminals have a bend (hump) and slotted holes. They seem better for temperature changes.

 

[JJJJ]

For those who install their systems in a moving vehicle, then movement would be a consideration. A good flexible bus bar might be a consideration.

A friend has field stripped Tesla batteries. These have the potential to be exposed to some fairly extreme conditions. They have coolant systems built in to control temperature. They are also interconnected by fine wire. The packs I have seen were glued together, so while not compression, this could mimic a fixture to prevent movement.

Granted this is a different chemistry. The take homes for me are controlling temperatures where my batteries are stored, keeping them safe in a location so they are not bumped and moved around, utilizing lower C rates for charge and discharge and setting the operational voltage levels to conservative settings.

I don't worry about the calendar age. If I were installing these in a commercial application where they were required to run for XX years without maintenance then that would be a different conversation. In that case I would be installing a commercially prebuilt battery for that application.

 

[TomC4306]

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

I'm NOT using them in an RV/truck/whatever(?)
They're sitting in my basement powering my house

 

[DIYrich]

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?

Based upon below, I would guess that batteries kept under 85°F degrade about 3% per year, maybe 2% if kept under 75°F.

https://www.frontiersin.org/articles/10.3389/fenrg.2023.1108269/full#:~:text=For LFP, degradation is less,low as compared to NCA.

Temperature, SOC and time are key factors related to calendar aging in LIBs via three main mechanisms: loss of lithium inventory, loss of active material in the electrodes, and a rise in cell internal impedance.

The primary cause of loss of lithium inventory is the consumption of Li-ions by side reactions, including the creation of solid electrolyte interface (SEI) on the surface of graphite negative electrodes, the electrolyte breakdown processes and binder decomposition.

Loss of active material is often caused by electrolyte decomposition, and electrode particle cracking during storage.

Internal impedance in LIB increases owing to SEI growth, electrolyte degradation, and binder decomposition during storage.

Most researchers have concluded that the aging process slows down at low SOCs.

For LFP, degradation is less than 1% per year at 20°C (68°F) and 10% SOC. Even for temperatures ranging from 20°C to 45°C (68°F to 113°F), LFP show less than 10% capacity degradation per year.

https://uwspace.uwaterloo.ca/bitstream/handle/10012/12177/Catton_John.pdf

Cells stored 35°C, though, do not reveal a major decrease in capacity. Despite the long duration of storage, the capacity charge and discharge voltage curves only showed a small decrease in capacity.

whereas cells stored at 35°C (95°F) only experienced just over 10% loss in capacity despite being stored for over two years.

 

[Martijn]

I'm NOT using them in an RV/truck/whatever(?)
They're sitting in my basement powering my house

Good