Hello all,
New to this forum and new to the DIY battery universe. I try and do my research but don’t have hands on experience yet. I need to test 18650 cells before using them, I am therefore trying to come up with a good 18650 testing methodology for the packs to come.
Context:
I am building a few 3s6p packs made of protected 18650 cells ( product ref. “LiitoKala 18650 3400mAh battery 3.7V Li-ion Rechargebale battery PCB Protected For NCR18650B 18650 3400
“ : ”404 page ). These packs will be used to power a device using at max 3W and most of the time 2W (not much power) but for a long time (like 90 hours or so).
The reason I am testing the cells is to avoid ruining the pack with one bad cell and to arrange them efficiently using a calculator like https://secondlifestorage.com/repackr.php for example.
The tools I used for the testing:
Capacity testing : one LiitoKala lii-500
Resistance testing: one SM8124
What I did/my results :
I am starting by testing a batch of 20 18650-cells, I labeled each one with a number to keep track.
I did several capacity tests using the LiitoKala lii-500 (in vertical position with a small external fan cooling the charger because it can get hot at 1000mA ). I also did resistance tests using the SM8124 because the resistance test of lii-500 doesn’t seem accurate (fluctuates too much).
Results raw data for the capacity test: http://www.sharecsv.com/s/1a0b21ef6c122ad0c5721d9f77dcef9e/testscapacity.csv
Results raw data for the resistance test: http://www.sharecsv.com/s/ca161a8b0f6419f84d45ee9202737dc0/testresistance.csv
Interpreting the resistance results:
Not much to say, the tester seems fine, cells have an average 55mOhm Resistance which seems ok for protected cells.
Interpreting the capacity results with a graph:
The capacity test fluctuation is too inconsistent (in some case 8% deviation like for cell 19) so something isn’t right.
Criticizing my results/methodology:
Not sure how the PCB affects the results.
The resistance test using the SM8124 seems consistent.
The capacity test fluctuation is too inconsistent (even 8% for cell 19) but maybe 1000mA is too much for parallel testing on lii-500 and not all tests are under 1000mA.
What I plan to do:
I ordered 4 more lii-500 a few days ago when I saw it took so long to test, so I will be able to test my 20 batteries at the same time and reduce to 500mA test so it won’t heat up and cool passively. Then I will do the same 3 or 4 tests and compare data.
What do you guys think ? Any help is appreciated.