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| 04-MAY-2016 | |
For this test I chose to use an intervolometer (see video below)to actually make a movie of the discharge. This image was taken after the battery was near fully charged because the video image was poor quality. The first frame is the charge end point of 13.8V and 7.5A flowing into the bank. The charger then turns off and the constant load discharge tester turns on. The bank was discharged at 30A and I have been using this tester for its repeatability. At a .5C or 1C load the AH capacity would be less but with the low Peukert of LFP not by a huge margin.
A guy in the off-grid LiFePO4 community (Terry) suggested that LFP banks can develop a situation where you may not see all the true capacity after repeated light discharges. I had also noted this in an over-discharged Mastervolt marine battery where I was able to see continual capacity increase in each cell as a kept cycling them to 2.7V then back to 100%. I decided to test this theory on this bank by conducting multiple 100% discharges followed by 100% recharges.
This test was conducted at cycle # 772 and done at a .075C discharge rate. This rate is limited by my constant load dischargers ability.
Here's the video:
Before you get too excited these cells have:
--Never been floated, they get charged, then discharged
--Only absorbed to a net 8A - 10A at 13.8V -14.0V
--Not charged above 14.0V unless for testing purposes (I now have a few other banks for that)
--Max charge rate at approx .3C
--Stored at 50% SOC when not being used or cycled
--Stored in 45-60F temps when not being used or cycled
--Only very rarely exceeded 80F
--Highest voltage they have ever seen was 3.8VPC while top balancing initially.
On these last few capacity tests I ran a total of seven 100% DOD tests back to back and noted that I gained nearly 15.3 Ah's of capacity from test #1 to test #7. I meant to do five tests but the video quality messed with me a few times and I am still not entirely happy with it. This resulted in two extra tests. There seems to be some truth to what Terry had been suggesting about light loads diminishing some of the real capacity.
My last published capacity test (above) was at cycle #550 (I generally conduct them every 50 cycles) and it delivered 419.2Ah. Most tests since I began using the constant load tester have been within a few Ah's of this 419.2Ah cycle 550 mark. These however have been just 1 capacity test and to a lower cut off voltage of 2.8 volts per cell. I now use 2.9 volts per cell because I noted very little capacity below 2.9V and voltage starts to drop off rapidly below this (lower knee).
I suspect I did not see as much gain with the back to back 100% DOD cycles as Terry has because I am normally discharging this bank to 80% DOD & then recharging at .23 - .3C.
With a total of seven back to back Ah capacity tests at a .075C discharge rate I did see an increase of about 15.3 Ah before the Ah capacity stabilized and stopped increasing. What does this mean? I don't know, but I did see an increase of 15.3 Ah by cycling this bank from 100% to 0% seven times back to back.
This LFP bank is an n=1.... In science an n=1 is not sufficient data. Cells vary and every boater will treat them differently. I have customers with over 12 years on GEL batteries and others who murder them in 2 years. I know of a number of owners who've murdered LiFePO4 banks. I have a dead $10,000.00 factory made "Marine LiFePO4 battery" in my shop as evidence of that.
I am treating this LFP bank as I suspect they do well with, in a PSOC environment, and it seems to be working, but an n=1 really tells us little in the whole scheme.
The frustrating part is that very few of us are actually capacity testing, with repeatable equipment, to know what is actually happening with our LFP banks. Until more fractional C data in a PSOC use situation comes out, it's learn it as we go.
© All Images property of Compass Marine Inc.