This post was originally posted at: http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=7512&st=0&sk=t&sd=a&start=15#p116416
I partially duplicate it here because of it's relevance to the subject.
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After many trials and errors I found a way to measure internal resistance of the cells with great repeatability, and hopefully accuracy.
The thumbnails below show all that is needed (click to enlarge):
A CBA2 with upgraded cables, a DMM with min/max hold function, and a timer.
It's basically a 4-cable measurement setup for the CBA2. I believe it would work just as well with the original cables, but have not tested it.
The most repeatable and most plausible results for IR were obtained with this procedure:
Connect CBA2 and DMM, set DMM to minimum voltage recording.
Set CBA2 to 11A, hit start, hit timer to start countdown from 35s.
When timer sounds the alarm, stop the CBA2, save the file, reset to 1A, record minimum DMM voltage during first part of test, reset the DMM minimum voltage memory, then hit start on the CBA2 again and start the timer again.
All as fast as possible.
Stop after another 30s (10s to 15s would probably be enough for the 1A testing, but not for the 11A part.)
Then note the second minimum voltage value from the DMM.
Subtract the two voltages from each other and divide by ten, there is your resistance result, with a resolution of 0.1mΩ .
Ignore whatever voltage the CBA2 displays, it is not important at all.
I did a large number of tests on many of the 102 cells in that way, and got repeatable results (when SOC, time since last charge, cell temp etc. were similar).
I did not find the results very useful, though.
I did not test all the cells because the results were almost identical between the worst and the best cells, and the testing is time-consuming.
At full SOC the IR is about 1.2mΩ.
The only difference between higher capacity cells and damaged cells (in regards to IR) seems to be at the 20A / 1.1V cut-off level:
The damaged cells showed higher resistance than the better cells at that (practically empty) SOC, i.e. about 4mΩ to 6mΩ . The better cells were about 3mΩ at the 1.1V cut-off level. I am not sure yet if this is cause or result of the reduced capacity of these cells, or both. I have not tested IR at lower SOC than 1.1V cutoff at 20A.
Otherwise the IR values were identical for all tested cells (at 10Ah SOC, 20Ah SOC and fully charged), always about 1.2mΩ to 1.3mΩ.