Figured I'd lump this post into my a123 thread, as the 7s module thread has devolved into tangents.
I've been buying a bunch of the 28s3p a123 modules from victpower. The latest one came without the "Active Cell Balancers" that all the other ones had - which is OK, as they are described as having no BMS, so all the others were bonus. However, the pack arrived pretty well discharged, and without the BMS's one cell was down in the 1.8v territory, and another was a little bit lower than the rest, but the other 26 were all in balance.
I've been wondering why these packs were discarded, and pondering whether an underperforming cell was flagged by the BMS. Maybe I didn't see it on my other packs, because the BMS was discharging the other cells to boost the voltage on the bad one. The two low cells in this pack makes that seem likely - and if so, that is bad news for the 7s packs, 1 bad in 7 is worse than 2 in 28.
I decided to break out my old CBA-II and see how bad the cell is - and do I need to solder on a booster cell to keep it from draining flat? I wanted to compare the discharge curve of the bad cell to one of the good cells. Bad news is that my CBA-II appears to have issues; before I start a test, it reads .08v higher than my multimeter. Once the 26.5a test is running, it reads about 0.48v _lower_ than my multimeter. During the test, it apparently runs hot enough to melt the cover off the fuse (see bonus pictures below). Rebooting my PC and replacing the fuse did not help, so my test results be taken with a grain of salt. So instead of running a WH test - which would be skewed by the incorrect voltage - I ran an AH test, and dropped the cutoff to 2.2v instead of my preferred 2.7v. This worked out pretty well, at the 2.2v cutoff, it was just above 2.7v on my multimeter. The discharge is at 26.5a - less than .5c - but that is all my CBA can handle.
The red line is a 'good' cell, and the black line is the 'bad' cell. The bad cell discharges at a lower voltage, demonstrating higher internal resistance. Interestingly, both cells discharged a full 60ah right as they started going off the 'cliff', so that gives me a lot of confidence. Interestingly, the bad cell gave more AH - but I have seen that on c-rating comparison charts, where a high-c discharge resulting in lower voltage makes up the WH by yielding more amp-hours.
This made me re-think my perspective on LiFePO4 balancing a bit. I have been comparing cells from a WH capacity standpoint for a long time. However, all cells will discharge the same number of amps, so maybe I should think of capacity in amp-hours, not WH. True, a bad cell with higher internal resistance will discharge those amps at a lower voltage, and consequently deliver less watt-hours, but if it has equivalent AH, it won't go flat before the good cell. In fact, as we have seen in the discharge comparison below, it might outlast the good cell! Of course, the bad cell means a higher self-discharge rate requiring more frequent balancing, waste heat in the pack, and less watts in the motor, but it doesn't go off a cliff early.
At any rate, I'll take this to mean that although I need to monitor the degraded cells, I can take us much as a full 60ah from this pack without really worrying about it