When you say categorically that it doesn't get warm, it gets me worried here. Mine always gets a little warm, about +5F to +10F over ambient temperature at 1C charge rate. So unless I am totally wrong, getting a little warm is ok, it doesn't (shouldn't) get hot.
Sure, "a little warm" is fine. But it should be a steady temperature that does not keep increasing, and for sure not hot.
Hello,
IanB, I finally encountered exactly what you described.
As a relatively new newbie, I do not have much experience. I was exposed to just:
(a) works fine,
(b) slight warm, stay slightly warm but otherwise works fine, and
(c) an old StarTac phone lithium cell that heat up instantly to scary temperature - too hot to touch by hand. within a minute or two max.
I have a new crop of lithium 18650. One got warmer than usual. So, I stop the bugger on the TP4056 and dug up my old setup. My old setup is an Arduino controlled LM317 with 16 bit ADC to read voltage and 12 bit DAC to set voltage. I added an LM35 as temperature probe to see what is going on relative to temperature.
The bugger(s) would charge the CC phase normal. During the CV phase, its operated normally with voltage held constant and current decreases. Normal batteries would reach end-of-charge which is a preselected "saturation current XXXmA". With these buggers, when it decreased to around a certain mA, current starts to increase as much as CC limit allows. Temperature starts to rise at the exactly moment current switch from decreasing to increase. My "normal" batteries gets slightly warm, but it does not couple with a current increase.
I took some charge data (room temperature was about 75F):
1st time I cut off when after it risen to 101F. Discharged it using a plan resistor.
2nd time I cut off charge as soon as it reach 92F. Discharged it using the same resistor.
3rd time I cut off charge at 85F. Discharged it using the same resistor.
All were discharged to battery-protection cut off at 2.6V. The difference in cut off temperature translate to about 30 minutes more between 85F to 92F and about 25 minutes for it to rise to 101F. So in theory, the 101F was charged about an hour longer.
Since it is the same resistor, time_to_discharge alone can compare relative charge. With all 3 tries, there was no appreciably difference in run time - that suggests to me that the power going in after the inflection point is just heating rather than charging. Had I allow it, current would have risen increasing the heat, and the temperature might have kept rising.
I also tried two separate charging (charge to inflection point, wait, charge again). That seem to "get past" the inflection point with no fanfare, but does not result in any increase in run time (ie:charge stored).
It is interesting. While I did not doubt your words, but seeing it happen in real life is interesting. Now with this crop of 6x18650, looks like not too many are useful. This "bad yielding crop" part is disappointing.
Rick