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I have built a battery pack using some old (but working) 18650 cells I had.
The 4S4P battery pack diagram follows: 4S4P battery pack I tried to balance charge it with my IMAX B6AC v2 (4S, TVC = 4.2V, i=2.5A, w/ balance connector).
It fails to charge parallel #3 and parallel #4 cells. While parallel #1 and #2 reach 4.2 V during CC charging, parallel #3 and #4 stay < 4.1 V and they heat up a lot while the others sit at 4.2 V.

I am wondering if this is some kind of limitation of the current bypass method the IMAX uses to balance charge batteries: Current bypass applied to balance connector
Full schematic: https://www.rcgroups.com/forums/showthread.php?1362933-IMAX-B6-Schematic

As far as I understand, it would try to bypass the parallel #1 and parallel #2 cells to charge only cells at lower voltage (parallel #3 and #4).
I ran a quick simulation to see how effective the current bypass would be: enter image description here It seems to be quite limited due to the 20 Ohms in series with the transistor, being much higher than the internal resistance of the Li-ion cell.

Questions:

  1. I fail to understand how it would heat the cells (parallel #3 and #4) close to 50 C.
    Any ideas on that? Overcurrent even if the IMAX shows ~1.5 A?
  2. Is there a way to correct that besides charging each parallel row separately?
    I have already tested that, and if I charge parallel #4 as an 1S4P, it goes to 4.2V fully charged without a problem.
    Maybe an external balancing IC would be more effective?
gstorto
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  • If the cells get hot and don't reach full charge, at least one of them is damaged and should be disposed of. Disassemble the pack, let each cell sit individually, and check which ones discharge themselves on their own over the course of a few weeks. – Jonathan S. Nov 14 '22 at 22:51
  • @JonathanS. That only happens when I try to balance charge the whole battery bank. Charging the "faulty" row (1S4P) individually does not present any problems. All the cells have been tested individually and they charge to TVC = 4.2V without a problem. They discharge normally to 2.9V/2A to the capacities present on the diagram. – gstorto Nov 14 '22 at 22:54
  • When you charge them individually, the charger can compensate for them leaking charge by simply pushing more charging current into them. When the other cells in a series string are full, however, it can't compensate for it anymore since the charging current is limited by the full cells. You really have faulty cells. Charge them, let them sit, and observe their self-discharge rate. – Jonathan S. Nov 14 '22 at 23:07
  • @JonathanS. Fair assumption, agreed. I will check that. Wouldn't it be possible though the parallel rows have discrepant capacities (more than I have predicted on the diagram), and a few rows will just take longer to charge? How would I correct the problem if this is the case? – gstorto Nov 14 '22 at 23:16
  • The two nearly 50% higher capacity 3xxx mAh cells look like a bad idea regardless of anything else. – Russell McMahon Nov 15 '22 at 07:05
  • @RussellMcMahon Could you elaborate more? My reasoning is as far the total capacity of the rows are similar, the current bypass mechanism should be able to compensate for a slightly higher capacity (+100 mAh) on parallel rows #3 and #4. – gstorto Nov 15 '22 at 10:02
  • @gstorto It's not certain to be bad, but it's generally a bad idea. If the 4 cells are hard paralleled then the higher capacity cells dominate the voltage increase during the CC mode - they always take longer to reach a given voltage if current is shared in proportion to capacity, and they will tend to take more current than the low capacity cells. BUT the currents taken is somewhat undefined. When you get to the CV point (4.2V typically) the high capacity cells will start to taper well after the lower capacity cells. If you are in series with strings where all cells are low capacity, all ... – Russell McMahon Nov 15 '22 at 10:39
  • ... low capacity cells start to 'taper' at about the same time whereas the rows with high capacity cells have the low cap cells tapering and the high cap cells not. What proportion of current relative to C capacity goes into high cap cells is uncertain but the voltage may be OK but current well above usual rating. || You say that they get hot - is it all cells or just the high cap cells or (less likely) just the low cap cells ? – Russell McMahon Nov 15 '22 at 10:41
  • @RussellMcMahon Alright, but that can be compensated by charging the pack at the highest current acceptable for a single cell, no? If I charge it at 1.5 A, which seems reasonable for cells with C > 2Ah, there won't be any single cell receiving a higher current than it is acceptable. I noticed it quite late, but it seemed all cells within the row were heating up, including the 3265 mAh one. I will try it again paying close attention and setting the IMAX to cut charging at a lower external temperature. – gstorto Nov 15 '22 at 11:08

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