Pack / Cell compression Optimized By Using Springs.

[rickst29]

..... 300Kgf calibrated at 15-20% SOC seems like the way to go (for us bottom-balancers…).

I’ve got a single row of 16 280Ah cells and once the pack has settled in (over 5-10 cycles),the total travel I get is minuscule - less than 1/8” total between my depleted SOC and my highest SOC.

Since my 300kGf springs can accomodate more than 1/2” of travel within +/-10% of 300Kgf, it’s a non-issue…

Is the new spec for the LF280K with correct clamping force now 8000 cycles???

That’s 22 years to 80% capacity…

I suspect calandra aging will reduce cell capacity under 80% before then,

Your plan (to re-calibrate to the minimum 300 kgf at 15-20$ SOC) seems perfect, although I would add additional force to account for issues or of accuracy in my spring force calibration process. To the extent that their Engilish words (they provide the translaqtion themselves) are well-chosen, they are trying to recommend a minimum of 300kgf, with a substantial range for "higher but still safe and beneficial" compression values also RECOMMENDED, and not merely allowed. Why not target 320 kgf instead?

The working length of your springs (between uncompressed at "completely flattened") is more than 5 inches? My springs are are much shorter, with a fairly high rate (of additional pounds applied per additional inch of compression). 1/8" of movement by my cells would move each spring from about 110 lbs to around 145 (nearly 900 lbs total, up from 660), because my spring rate is nearly 300 lbs per inch. But of course, with only 4 cells in the row I never see even 1/16" change in the distance of the outside cell faces - and 900 lbs of total force would still be within the recommended range anyway.

They DID specify 8000 cycles as the test requirement - but don't forget, they also adjusted charge and discharge rates downards at extremly high and extremly low SOC (voltage) values, and your charging system is probably not able to make those adjustments.

 

[Hedges]

They DID specify 8000 cycles as the test requirement -

8000 cycles / 1c / 2 directions / 365 days = 11 years.
[edit: left out 12 cycles per day, except JustGary said it's 0.5c and half hour rest. 4.8 cycles/day and 3.4 years]
Most [electronic] life tests are done with an accelerating factor. Can be load or temperature. [maybe none for batteries]

Wonder what the source of their data point is.

but don't forget, they also adjusted charge and discharge rates downards at extremly high and extremly low SOC (voltage) values, and your charging system is probably not able to make those adjustments.

What range of SoC supports the 0.2c to 1.0c people might typically do?
Can charge voltage settings avoid going higher than max SoC which supports that?

 

[justgary]

8000 cycles / 1c / 2 directions / 365 days = 11 years.

They specify continuous cycling, not daily, so their cycles take roughly 5 hours (~two hours charge, rest a half hour, ~two hours discharge, rest a half hour), or 4.8 cycles per day. That's 6,000 cycles in about 3.4 years or so. The cycles are a 0.5C, not 1.0C.

Yes, they say you should get 8,000 cycles but the test says to cycle 6,000 times (3.15.1d).

 

[rickst29]

They specify continuous cycling, not daily, so their cycles take roughly 5 hours (~two hours charge, rest a half hour, ~two hours discharge, rest a half hour), or 4.8 cycles per day. That's 6,000 cycles in about 3.4 years or so. The cycles are a 0.5C, not 1.0C.

Yes, they say you should get 8,000 cycles but the test says to cycle 6,000 times (3.15.1d).

Yes, the test procedure called for only ~6000 cycles with short rest periods, although the specification says 70% energy retention after 8000 cycles. The power rate for charge and discharge within each cycle was varied to even lower watts as the test battery cells become worn. I assume that the verification at the end of their test showed somewhat more than 70% energy storage capability remaining, they possibly used assumptions and historical graphs of earlier versions leading to the 8000 cycle figure.

 

[rickst29]

What range of SoC supports the 0.2c to 1.0c people might typically do?
Can charge voltage settings avoid going higher than max SoC which supports that?

They did not test higher charge rates in the in cycle tests, but they did test discharge "energy efficiency" one time, comparing the .5c result to a 1.0c rate (896 watts). Their document only reduced the limits only in relation to lost capacity as the test cells became worn out: SOH limits, not SOC limits as I misstated in my earlier post.

I did not see a particular value required for the cell under the 896 watt test (1.0C), which was run all the way from 3.65 volts down to 2.50 volts. My own BMS is set for lower maximum voltage and higher minimum voltage. I have a personal SWAG that discharging @ 1.0C for extended times, over many many charge/discharge cycles, is probably bad for overall lifespan, but they didn't list a product requirement or test for that kind of usage over multiple cycles.