The background is I wanted to be able to put 18650 cells in a charger until I needed them. Please refer to earlier posts where I describe the charging circuit used and the lower charging voltage being used. A set and forget lifestyle choice. Given the sheer amount of nonsense promulgated by self appointed experts repeating each other's gospel I decided to test if such a charging regime was going to damage cells.
I started with rubbish 18650 cells, the kind promoted as holding more energy than a hydroelectric dam. These cells had been abused in my torch and seemed worthy of sacrifice.
I noticed by accident that cells left resting see an increase in terminal voltage over time. I started testing how much additional capacity could be obtained by resting for a day at a time and if this different discharge regime changed the trend in results to date.
Battery A 3.93V Float
I started with this battery, one of a batch of the worst 18650 batteries I have ever touched. Sacrificing one for testing was a given. Over time there is a clear downward trend in the tested capacity, having fallen by 17% when left in my CC/CV charger at 3.93 volts. Observation 26 represents the capacity when fully charged to 4.2V and confirms I was giving up a reasonable amount of capacity compared to my CC/CV charger.
Battery B 4.03V Float
This 18650 battery appeared to be deteriorating until I tested after resting. Inconclusive results at present.
Battery C
Appears to have been lost.
Battery D
Battery D does not appear to have suffered any significant degradation by leaving it in the CC.CV charger at 4.0 volts.
Battery E - Pouch Cell Salvaged from Cheap Android Tablet
Downward trend can be observed.
Battery F - Pouch Cell Salvaged from Cheap Android Tablet
Downward trend can be observed.
Interpretation of results
Because the batteries were already end of life status the results are inconclusive. At best there is tentative support for the claim that leaving Li-ion cells on charge with a reduced float voltage does lead to reduced cell capacity. Given all the possible sources of error in my approach I'm not yet convinced that floating at 3.9v damages the cells.
Perhaps Battery D, which showed no deterioration, was a relatively new battery. There was no way of knowing. But I cannot rule out the deterioration where noticed was simply the result of testing with damaged or end of life cells.
The move from summer into an unusually cold winter in this location did not appear to make any significant difference. Spring type battery holders have unreliable contact resistance and plugs with solder connections would give more robust outcomes.
Next steps
This initial testing revealed where the shortcomings in my approach were. To bring more rigor to the testing I will run future trials as follows:
Take a pair of brand new batteries. One, let's denote it as x1, will be subject to the CC/CV regime at around 3.9V. The second, say x2, will serve as a reference. The capacity of x1 and x2 will be initially tested fully charged.
I then put x1 in my CC/CV charger and left there until the capacity is tested, about weekly. After testing x1 goes back into the CC/CV charger.
Meanwhile, after testing x2 will be charged to 3.6 volts before removal from the charger and stored. Every couple of months it will be then fully charged and tested before re-charging to 3.6 volts and storage.
I will revert to testing capacity by discharging to 3.0 volts since the capacity tester defaults to the value. If I don't see any degradation in the first new pair of batteries to be tested then I'll automate the process and extend it.
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