Lihium ion battery storage SOC vesus capacity loss study

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Lihium ion battery storage SOC vesus capacity loss study

Postby karrak » Tue Feb 21, 2017 12:43 pm

Found a link to this scientific paper while looking through the latest posts in the Cruiserforum thread lifepo4-batteries-discussion-thread-for-those-using-them-as-house-banks, http://jes.ecsdl.org/content/163/9/A1872.full.pdf which I thought might be of interest.


These graphs are from that paper.
SOCStorage.jpg


One of the respected posters on that forum posted this in this post http://www.cruisersforum.com/forums/f14 ... ost2312168, he was testing CALB cells.
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. Remember this is just sitting there at full charge with no float....

I am not sure how this impacts on those of us who are floating our batteries at close to 100%. I still can't detect any drop in the capacity of my battery after nearly four years of which a couple have been floating at nearly 100%SOC.

Simon
Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger https://github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller
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Re: Lihium ion battery storage SOC vesus capacity loss study

Postby Gordon-Loomberah » Tue Feb 21, 2017 1:09 pm

Thanks for posting that interesting study Simon.
I've had my CALB battery operating for over 4 years now, and although I have not done any discharge to flat tests, think that I still have at least 90% of capacity remaining. One cell was taken out last year due to reduced capacity and replaced with a Winston, but apart from that the rest of the cells seem ok, albeit with some voltage variation across the operating range of SOC. My cells are subject to >45C ambient some of the time in summer, but also <10C in winter, and on average, probably a bit under 25C. They have had a range of Vabsorb over their life, early on it was 3.65V in order to make use of the EVPower balancers, but later it was lowered so that they were not activating (currently 3.425V average), and I currently have active balancers, although they don't appear to work very well near the end of the absorb stage when net charging current is under 10A, as there can be over 0.5V difference between the 2 halves of the battery.
Operating efficiency (energy available for loads / energy from charge controllers) of my system when charging to ~54.8V is still around 99%.
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Re: Lihium ion battery storage SOC vesus capacity loss study

Postby tom rickard » Wed Feb 22, 2017 3:17 pm

Thanks for posting that paper.

I know a couple of people that have been charging their Winston cells to 3.75V every cycle for over 4 years now and they are still going strong, although they may not get to the last 25% of their cell capacity, so I guess until you do you won't know if it's still there! At least the SOC section of the cell that they still use is providing the same performance that it ever has..

It is good to see that there are people taking the time to seriously examine the specific behavior of the different chemistry.

Even between different types of LiFePO4 cells, there will be a large difference in degradation. Really, after 5 years of these things being widely used in off-grid type power situations, it's becoming clear how they should be treated.

The full story will be available in another 5 years time!
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