I've got 16 lishen 272 cells on order and I'm trying to better understand the matching process and options. I'll be building 4 12v battery packs using 120a overkill BMS's.
My understanding is that I'll want to match cells by capacity as much as possible.
Here is my current plan to figure out capacity:
-Top balance: Group cells into 4 separate 4-cell parallel packs for top balancing. Do this initial grouping/matching based on internal resistance of each cell
-Reconfigure in series: After full charge/top balance, reconfigure each pack in series, add BMS
-Capacity test: Run test on the pack with inverter/shunt, watch cells as they drain and note any outliers dropping voltage faster than others. Compare BMS voltage vs. voltmeter readings to make sure BMS is properly calibrated and all connections are good.
-Gather results: At completion of capacity test, record final voltage of each cell. Santity check that any outliers above are the lowest voltage cells.
-Rinse, repeat for the other 3 packs
-Test individual cells if needed: If I see any dramatic outliers, run capacity test on that individual cell to make sure there wasn't a testing problem, poor connection, BMS calibration issue, etc.
-Match Cells: Review data and sort all cells based on ending voltage of capacity test. Rebuild packs as needed to line up final voltage and call it done.
I've seen some posts from people individually capactiy testing each cell and matching them based on that. I could go that route also, but that would take a lot of time and I'm wondering if the process above could get you pretty close to same results. I've got one of those cheap capacity testers coming, so this is an option, but I don't want to waste a bunch of time if it's not valuable.
Any feedback on the approach described above? Am I missing any important steps? Any opinions on whether it would be worth doing individual cell capacity testing?
I'm also questioning whether capacity matching is even important if all 4 of my packs are going to be charged and discharged at the same time. I guess if I get up against a BMS shutdown limit, it's nice that all the lowest capacity cells are together so it might keep the other 3 packs running longer.
My understanding is that I'll want to match cells by capacity as much as possible.
Here is my current plan to figure out capacity:
-Top balance: Group cells into 4 separate 4-cell parallel packs for top balancing. Do this initial grouping/matching based on internal resistance of each cell
-Reconfigure in series: After full charge/top balance, reconfigure each pack in series, add BMS
-Capacity test: Run test on the pack with inverter/shunt, watch cells as they drain and note any outliers dropping voltage faster than others. Compare BMS voltage vs. voltmeter readings to make sure BMS is properly calibrated and all connections are good.
-Gather results: At completion of capacity test, record final voltage of each cell. Santity check that any outliers above are the lowest voltage cells.
-Rinse, repeat for the other 3 packs
-Test individual cells if needed: If I see any dramatic outliers, run capacity test on that individual cell to make sure there wasn't a testing problem, poor connection, BMS calibration issue, etc.
-Match Cells: Review data and sort all cells based on ending voltage of capacity test. Rebuild packs as needed to line up final voltage and call it done.
I've seen some posts from people individually capactiy testing each cell and matching them based on that. I could go that route also, but that would take a lot of time and I'm wondering if the process above could get you pretty close to same results. I've got one of those cheap capacity testers coming, so this is an option, but I don't want to waste a bunch of time if it's not valuable.
Any feedback on the approach described above? Am I missing any important steps? Any opinions on whether it would be worth doing individual cell capacity testing?
I'm also questioning whether capacity matching is even important if all 4 of my packs are going to be charged and discharged at the same time. I guess if I get up against a BMS shutdown limit, it's nice that all the lowest capacity cells are together so it might keep the other 3 packs running longer.