locked Re: Batteries starting to get weak - MMM - What to do -

Mark McGovern


I think you ask a great question.   The only information that I have been able to find regarding anything remotely close to answering the question of what actually happens if you keep LiFePO4 batteries at a 100% SOC is from Rod Collins at Compass Marine.  He has been researching, using, experimenting on, and installing LiFePO4 batteries on boats since 2008 both on his personal sailboat and professionally as a marine electrician.  He is also part of the committee that is writing the ABYC safety standard for Li-Ion batteries. His extremely long and detailed treatise on LiFEPO4 batteries in marine systems is here: https://marinehowto.com/lifepo4-batteries-on-boats/

Below is the excerpt on the two long-term experiments that he has conducted on what happens if you leave LiFePO4 batteries at a 100% SOC for a long period of time:

STORAGE SOC EXPERIMENT: We recently ended a very expensive experiment regarding storage at 100% SOC. The test duration wound up being 12 1/2 months using four 100Ah CALB SE cells where they were charged to 100% SOC and then left to sit idle with no connections to a BMS or other parasitic loads. The low temp recorded over the 12 1/2 months was 46°F and the high temp was 87°F. This test was meant to be a representation of the real wold.
A min/max capture thermometer was used to record the peaks. The cells, prior to letting them sit at 100% SOC for 12 1/2 months, were regularly testing at 101.2 to 101.3 Ah’s of capacity (previous 6 Ah capacity tests) as a 12V nominal bank. After 12 1/2 months the cells were discharged to a cut off voltage of 2.9V for the lowest cell. After 12 1/2 months of doing nothing but sitting there, at 100% SOC, the cells had lost 11.6% of their previous rigorously confirmed Ah capacity. Now imagine if you additionally stressed the cells by continually float charging them. Ouch!!!!
“The cells lost 11.6% of their confirmed capacity just sitting at 100% SOC”
UPDATE: We have now completed a second round of this type of testing with a brand new prismatic cell. The difference was rather dramatic and I have no explanation as to why? The second test we did went for 13 months, under identical testing criteria, and this cell only lost 3.8% of previously verified Ah capacity. While this is quite a bit less capacity loss it still lost capacity.
How can LFP cell manufacturers suggest that the mere act of storage, at 100% SOC, is bad for the cells, which we have physically tested and confirmed is degrading them, and then suggest it is okay to float? How can they say “store at 50-60% SOC” yet then give you a “float” voltage?
I can sum up my feelings on the cell manufacturers, and their charge voltage guidance, like this:
They figured out a great recipe, they can repeatably make the recipe, but they have no idea why it tastes so good.
Once again, I will ask any and all Li-Ion battery researchers or scientists (I know many of you are reading this because I have your emails) to please send me any credible data to suggest a “need” for such high voltage charging guidance for the proposed use as a marine house bank.

I have not found any other source that has conducted a similar experiment and published the results.  If anyone else has, please post a link as I would love to read it.

Mark McGovern
SM #440 Cara
Deale, MD USA

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