OK, after extensive testing, I am back.
A lot to digest in your report, so I’ll take it a point at a time.
‘williaty’ said:
1) The bank cannot make its rated capacity. Discharged at the 20-hour rate with the cells being at 70F, the bank capacity is only 272Ah. I repeated the test twice to confirm this number.
From what you report below, ‘cell A’ is limiting pack capacity to 272Ah or 97% of 280Ah rating (with one other cell, ‘cell C’ on it’s heels).
2) The bulging problem got worse, not better, with deep discharges. Cells A and C have gotten so bad that I had to file a longer slot into the bus bars because they wouldn't reach between the studs.
Sounds as though you are not clamping in any way, is that correct?
3) I have been consistently torquing the nuts on the studs to 30in-lbs using a CDI Torque Products torque screwdriver. The driver was last calibrated in 2019 and has never required adjustment any of the other times I sent it out for calibration over the last 15 years. Even with being at the lowest torque setting I could find recommended in any of the datasheets for prismatic cells like this, I still ripped one of the studs out of Cell A.
The datasheets don’t include any torque specs for clamping busbars or lugs - the ‘anti-torsion’ spec of 8Nm they provide is the maximum torque that can be applied to the aluminum terminal before it will twist out of the cell. The tapped M6 aluminum threads put there by the reseller will strip long before that…
The only way you can avoid ripping out threads with solid busbars is to either clamp your cells with sufficient force or position them far enough apart for each cell to fully breath / expand during the charge/discharge cycle…
I am not confident this repair will last if I have to keep taking the bank apart to put it back into parallel and top balancing it every time I take a trip.
Your battery is not properly designed if you need to keep ‘taking it apart’.
There are ways to successfully recover a stripped thread, but you’ve got greater issues to address first. 3 threads of Helicoil is unlikely to hold.
4) The problems follow the cells regardless of which position they are in the pack. Cell A is the worst. It will always hit the high voltage cutoff first and always hit the low voltage cutoff first. It's clearly lower capacity than the other cells. However, Cell C is close on its heels. Cells A and C are never more than 10mV different when Cell A hits cutoff. Cells B and D are significantly larger capacity than A or C as they will be 250-350mV away from the low/high cutoff when Cell A triggers a shutdown.
Achieving a battery delivering 97% of rated initial capacity would be a good achievement, but you are a long way from that. What did you pay for these cells and who did you purchase them from?
5) The pack will slip out of balance in 4-6 synthetic "days" of testing. My real world use pulls 300-750Wh per day out of the pack.
If there is a high-degree of variability in self-discharge amongst your cells, that is a second factor that will limit capacity and dictate the ‘strength’ (current) of balancer you will need.
The easiest way to characterize self-discharge is to do it from the ‘bottom’ up (charge needed to being each cell back to 3.65V from where it has settled), but that is tough without the correct equipment.
If your typical storage period will be overnight, the hard way to characterize self discharge is to charge each cell to 3.65V, let it settle for 12 hours, and then repeat a capacity test.
‘3% / month’ or ~0.1% per day is the level of self-discharge to expect. So ~140mAh over 12 hours is a realistic level of self-discharge and a cell that is self-discharging at 3-5 times that rate means an additional ~280-560mAh lost overnight that your balancer will need replace daily.
An active balancer can achieve that amount of balance charge in several hours easily but that degree of mismatch in self-discharge may go beyond what the typical passive balance integrated into most BMSs can compensate for over a typical charge cycle.
The pack gains 60-90mV of imbalance per "day" when doing this. Eventually, Cell A will trigger the cell overvoltage protection on charging, usually with the bank voltage around 13.5V resulting in an imbalance between the highest and lowest cells of around 350mV.
60-90mV means very different amounts of charge well up into the knee above 3.5V versus down in the flats of after the cells have settled overnight.
Above 3.5V, 0.1% per ~40mV or ~7mAh/mV is a realistic sway, so 60-90mV would correspond to ~420 to 630mAh (3-5 times the self-discharge you’d expect over 12 hours).
Below 3.375V, 0.1% per ~0.4mV or ~700mAh/mV is a realistic swag, so 60-90 would correspond to 42-63Ah or 15-22.5% of cell capacity. If your cell A is losing that much versus the other cells through a charge/discharge cycle it is hopelessly defective…
With those results, here are my current thoughts:
a) The battery bank as-is isn't functional.
Where ‘as is’ includes using cells without a clamping fixture and connected with solid busbars, you are correct.
b) The battery bank cannot be "fixed" by replacing a single cell. It appears A and C are pretty close to matched and B and D may be pretty close.
It’s not the capacity of your cells that is your issue, is it primarily the way your battery has been assembled and may possibly be one or two cells with defectively-high self-discharge levels.
Once you have more carefully characterized the difference in self-discharge, that will tell you whether you actually need to replace any cells or not.
One easy way to address the issue within the testing methodology you’ve been using is your add an active balancer to the system. A 60-90mV voltage delta will translate to a 600-900mA active balance current, so easily compensated in under an hour if the voltage mismatch you’re seeing is up in the knee. If you can achieve a repeatable voltage mismatch across multiple charge/discharge cycles, at least you know what level of usable capacity you can count on from these cells.
c) The bulging issue is a serious problem since these were listed as Grade A Matched cells.
I’m suspecting you purchased these cells for ‘cheap’ (compared to purchasing true Grade A cells direct from the manufacturer EVE) from an aftermarket reseller as most of us did. Pretty much no ‘Grade A Matched’ cells you purchase from resellers on Alibaba.com/AliExpress are either ‘Grade A’ or ‘Matched’ in the way the manufacturer EVE uses those terms.
With a clamping fixture (as EVE now recommends), your issue with cell bulging will disappear. If you don’t want to bother with a clamping fixture, you need to space your cells with a big enough gap that they can freely bulge without causing mechanical stress on your solid busbars.
The other solution is to invest in flexible busbars in which case you can allow the pack to bulge/move/expand as it needs to without stripping out terminals.
(For what it is worth, I have both a 300KgF clamping fixture as well as custom 2/0 lugged battery cables to absorb any movement without causing stress on my terminals).
OK, can you guys think of anything I'm missing here or any other way to patch the problems I've got going?
When you talk about connecting with ‘the lowest torque’ you saw specified, I’m also worried your terminal connections have higher contact resistance than they should and probably high variability as well.
So my advice would be one of two paths:
Easy Path: invest a bit in an active balancer to continue testing as you have been to understand whether excessive self-discharge of Cell A is an actual problem or not. If it is (and possibly cell C as well, you will probably need to replace those one or two cells before you can build anything usable.
More Difficult Path: characterize the self-discharge rate of Cell A, Cell C, and whichever cell you consider to be be of your ‘best’ cells at the individual cell level. To be clear, this means measuring either the charge needed to ‘top off’ each cel to 3.65V alter a 12 hour or 72 hour ‘rest’ or measuring remaining capacity using your capacity meter after a rest of at least 72 hours if not longer (since I doubt the accuracy of those cheap capacity testers is below 1% / 3Ah).
Once to understand whether you truly have any defective cells or not (meaning truly unusable) and which battery type you want to build (meaning fixture or not and connection type), there is a path to get you there.
(Again, for what it is worth, I caused cell bulging and stripped a cell terminal before I know what I was doing. I painfully learned the best technique to recover a stripped terminal, built a clamping fixture, ditched the bundled solid busbars for custom-built 2/0 lugged battery cables, added a Heltec Active Balancer and have been successfully using my 560Ah 24V LiFePO battery for several months now…).