It has been over 60 days since I started this experiment. To recap the aim is to see if a CC/CV charge with a terminal voltage of a nominal 4.0V degrades cell capacity if the battery is left floating in the charger.
To date I have not seen any consistent reduction in cell capacity. At first glance two cells have got worse, two cells have improved. However, ambient temperatures have increased perhaps 10 degrees celsius as summer kicked in e.g. today is was 37C, the workshop interior was perhaps 43C.
My suspicions is the swings in ambient temperatures are creating drift in the cheap battery tester being used. The reference voltage being used is the regulator output. A 78L05 has a drift of -1.1 mV/°C at 5ma of output but this assumes the junction temperature is unchanged. I would not be surprised if the drift in the supply rail was over 10mV.
This is compounded by the LM317 used in the constant voltage section of the charger drifting. As temperatures rise the output voltage falls. This might be a reduction in the order of 25mV for the change in ambient temperatures in the workshop. It is a bonus the voltage is not increasing given the high workshop ambient temperatures at present.
These temperature effects would explain the change in capacity of a few mAhrs. In another 60 days things will have cooled down again and I will post a more detailed update.
I should have been more rigorous in my testing and recording regime. My observations are not at known points in time so I cannot see a way to statistical test any change. Something I need to take on board when building an automated tester.
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