gauss163 said:
Such AC 1kHz tests arethe wrong tool for DC loads like powerwalls - see my prior postabove and its links for why. It would be much better to get a charger with a well-implemented DC IR test, e.g. a SkyRC MC3000. Unfortunately this point is widely misunderstood - but the links in my post will lead you to literature explaining it further (including remarks by experts in the field).
gauss163,
In principle I agree with you that a DC IR test is more indicative of the SOH of a Li-Ion cell. However even the SKYRC MC3000 falls short on a proper test. Also most of us that test high volumes of cells don't have the time to squeeze the contacts and wait 10 seconds or so to see if the cell is good. Additionally repeatable readings are definitely a misnomer as you can never duplicate the exact pressure and contact position every time.
Additionally the only proper way to really measure m? resistance is with a 4 wire kelvin measurement device. I do not see any OPUS, SKYRC, or any other reasonably priced off the shelf tester offer that. The XTAR Dragon comes close but still is lacking.
So we move on to an easy "reasonably priced" and simple way of measuring the "SOH" of a Li-Ion cell. The 1 kHz 4 wire Kelvin methode. It is a preliminary test to determine if the cell is even worthy of testing.
I have developed a cheat sheet of IR values for 18650 cells of various manufactures and modelsthat if the cell falls within those values it will perform well in a test.
Additionally as the graph from Endless Sphere so eloquently demonstrates that the AC IR does not change much with the SOC, so we are able to determine if the cell is good by this measurement alone and not necessarily have to charge the cell to 100% SOC.Also most all of these "scientific" tests are done on new batteries not ones that have been used.
(Additional note: the cell in the chart at 14m? is more than likely a INR, IMR or NCA chemistry hence the low m? value.)
Once the cell passes that test then the cells get C/D/C at 1A (Note: Only OPUS and SKYRC offer a 1A discharge) after ?25 to 30 days the V gets checked for any signs of SD and either accepted for the build or not. As far as I am concerned if a cell performs well at a 1A C/D/C cycle it will work in my powerwall just fine especially when most "properly" built walls will draw maybe 250mA from each individual cell. In my case the worst case scenario is a 625mA draw if my inverters where to run 100% which rarely happens. 90% of the time I am drawing <200mA per cell.
Also if the cell performs within 80% of its rated capacity at that C/D/C level then I fell very confident the cell will work just fine.
Expert or not who am I to dispute the "Experts". I am in no way disputing anyone other than reporting my own findings.
After testing and recording over 6000 18650 cells of varying manufactures I have a pretty good idea of what cells will and won't produce satisfactory results based on 1kHz AC IR.
Additionally I performed and recorded the "Testers" IRresults OPUS, Zanflare, and LiitoKala where recorded.
SKYRC MC3000 was not because it was to cumbersome. Also Foxnovo does not offer this feature.
Snippet of the sheet.
You can view the wholesheet anytimehttps://drive.google.com/file/d/1NujY1eO6MKwGrpyEm185m6vpkMdb_Gp9/view?usp=sharing
If you have any personal experience to share with us that would be great.
It would helpin expanding our knowledge base on testing LI-Ion batteries.
Pictures and spreadsheets gladly accepted of your own
personal results.
Wolf