Accelerated degradation of LiFePO4 batteries when stored fully charged

[Cpt Pat]

There's a common belief that storing LiFePO4 (LFP) batteries in a fully charged state is a poor practice. Linked below is a good thesis on factors that degrade the life of LiFePO4 batteries, with a detailed explanation of the chemistry involved.

Here's an excerpt from the abstract (emphasis is mine): Capacity loss in cells stored at high temperature and fully charged conditions resulted in faster degradation rates. Temperature had the most significant role in the degradation of the cell and then the cell’s SOC. Comparing capacity losses between cells stored at the same temperature, but with different SOCs, found that the cells with higher SOC experienced increased rates of degradation in comparison to their fully discharged counterparts. In addition, storage at high SOC and high temperatures promoted such severe losses that the cells in question were unable to recapture capacity that they had lost reversibly.

This calls for a storage regime that is the opposite of lead/acid batteries, which should be stored fully charged to prevent sulfation. It also indicates that simply "dropping in" an LFP to replace a lead/acid battery - without changing the storage regime - will result in degraded life.

I personally never charge my LFP battery above 80% SOC or discharge below 20% SOC, and I store the battery at 20% SOC; based on a combination of terminal voltage measurements and Coulomb counting (counting amp/hours in and out). I charge my battery on shore power just before going out to avoid storing it in a charged state between sails. Coulomb counting is especially important to prevent over charging: at very low charge currents, you can overcharge an LFP battery with a terminal voltage as low as 13.65 volts.

https://uwspace.uwaterloo.ca/bitstre...pdf?sequence=3