Way back a few pages (post 5057) I made a comment about the voltages on my Winston 700 ah bank being some 60 mV different from the published voltage and that I "should" do some testing. Well, I did do some testing, And here is what I found.
I charged my bank up to what I thought was 100% (but now realize was only about 80% as explained in post 5127). I have not ever run a full capacity test on the bank and generally cycle around 60% to 40%.
Starting the test I charged the bank up until it reached 13.750 volts at around 30 amps. Then letting the bank rest for a day I measured a pack voltage of 13.358 volts. The cells were (3.340, 3.339, 3.338, 3.341)
Then I would run a space heater drawing 100 amps or so for the length of time that it took to drop the bank by 70 AH (as measured on a victron BMV-602s). Then let the bank rest 24 hours measure the voltage again then draw another 70 AH.
0 AH 13.358v (3.340, 3.339, 3.338, 3.341) Assumed 100% SOC
-70 AH 13.257v (3.317, 3.308, 3.310, 3.321)
-140 AH 13.202v (3.302, 3.299, 3.300, 3.301)
-210 AH 13.185v (3.296, 3.295, 3.295, 3.295)
-270 AH 13.152v (3.290 3.284, 3.283, 3.291)
-350 AH 13.036v (3.262, 3.253, 3.254, 3.266)
-420 AH 12.890v (3.226, 3.214, 3.216, 3.232)
-490 AH 12.090v (3.090, 2.937, 2.924, 3.143) Terminate test
Based on post 5127 and on fitting the data above to the open circuit voltage curves I would say that I was only charged up to around 80% and that the curves I've seen fit the published curves closely in the 20% to 60% SOC range with my (used) cells rising faster above 60% SOC.
I did note that the charge voltage did rise significantly at higher charge rates (100 a) as I got to the 80% SOC area. This of course defines the transisition point from CC to CV.
I do not have a power supply that can do more than 30 amps in CV so I was manually adjusting my inverter/charger to mimic a CV charger
. I did this with a 3.5v max cell voltage. It there a better cell voltage to use for the CV range? Cell #3 was at 3.475v while cells #1 and #4 were at 3.408v. This was at 30 amps or so.