Background and Method
More fully explained in this post but briefly I am using a control battery to confirm if any degradation is due to float charging or simply the passage of time and high ambient temperatures. Apart from the time being tested the battery remains in a CC/CV charger floating at 3.93V. This charges the cell to approximately 70% of capacity then floats at this voltage.
Initial Test Results after Three Months
Having established a base line for the cell capacity when fully charged I have now floated one cell at 3.93V for just over three months with 7 discharge tests during that period. After the third test where I had minor contact resistance issues I soldered a connector onto the cell for discharge testing for the fourth and onward tests.
Looking at the discharge tests of the battery left on float charge at 3.93V there is as yet no meaningful degradation in capacity. Also shown below is the capacity of the control cell, which was stored at about 50% capacity before being recharged for the test, and the test cell after fully charging.
Both the control and test batteries recorded an improvement in capacity of almost 5% for the second test compared to the original test.
Conclusion
It appears float charging at 3.93V, or 70% capacity, does no harm to a Li Ion battery. I will continue the testing for another 3 months to see what happens.
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