I have been charging 18650's(TF flames 2400) individually for several weeks with the iCharger 106B+. At the end of the charge they always have 2.08 V 4.08 V.
Today I'm unable to charge a battery higher than 3.7 V. The current falls and the process stops prematurely. A picture:
(Termination charge voltage is set to 4.2 V, firmware is 3.14, charge rate is set to 1.6A)
Now I have finished the same test with a 1.2 A:
The final voltage is 4.1V...
Why????
Sounds like very high internal resistance in the circuit. Could be a very bad cell, bad contacts. Graph above looks like a very bad cell.
Is that meant to say 4.08v, rather than '2.08v'?
Assuming that you do mean 4.08v, presumably you are asking as to why the cells don't stay at the 4.20v that they are being charged to?
Ouch!, yes 4.08 V. (And 4.10 V in the second graph) The cell is the same in the two graphs.
Basically, your charger is reading voltage under load - you are pushing current in to the battery, against the internal resistance. That will no doubt lead to heat being generated, which will alter the internal resistance. As the voltage and current changes during the charging process, the temperature will change, and hence the resistance will change, and so on.
What I'm getting at, is that during the charging process, things are changing constantly. No surprise then, that once the battery is removed from that process, you are looking at an entirely different set of circumstances again, and if you measure the voltage with a DMM, you are now seeing a different value - the voltage when the battery isn't under a load. It isn't unusual for this value to be lower than the CV voltage that the charger is set to.
Basically, voltage under load and voltage with that load removed are going to be different.
As okwchin stated, how big a difference there is between the two values can be related to cell condition. V=IR, so the higher the internal resistance, the higher the voltage will read for a given current. Likewise, the higher the charge current, the higher the heat generation, which could also have an effect.
So long story short, your charger is charging to 4.2v, as you can see in your charts, but that voltage is being measured under load, and that load will have an effect on the voltage being measured. Remove that load - remove the battery from the charger in other words - and the conditions change, which in turn leads to a different voltage being measured. A lower resistance cell, or a lower charge rate, would most likely lead to there being a smaller difference between the voltage under load and the voltage off the charger.
So... I need to buy a 8$ charger to fully charge my batteries?
Try charging at 0.5A.
Is this bad for battery?
1.6A is a high charge current for a little old 18650 and will not do it any good!
I am charging a TF flame at 0.5A right now, it's at 996ma and 3.95v at the mo'.
Yep, as ANL stated, you can just adjust the hobby charger to a lower current, which will reduce heat build up, and hence reduce to an extent the negative aspects of a cell with high internal resistance. Obviously a better cell would be an advantage, but this at least makes the most of what you have.
The other advantage of charging at a lower rate with a true CC/CV hobby charger is that the charge generally cuts off at 1/10th of the set charge rate, so with a setting of 1.6A, charging stops in the constant voltage phase when the current drops to 160mA. With a 1A setting, charging terminates at 100mA, and with a 500mA setting, charging terminates at 50mA. So with a lower charge rate, charging continues down to a lower rate, making for a fuller charge, but at the expense of longer charge times.
However, none of this is to say that you'll not still see some voltage drop after removing the cell from the charger, but it should be less pronounced. It's perfectly normal to see some voltage, particularly if your cells are not in best possible condition. It may well be that some cheap consumer type cradle chargers would leave the cells at 4.20v after removal, but if you know that your cells have a tendency to come off a hobby charger at less than 4.20v, then it would suggest that the cradle charger is actually charging the cell to more than 4.20v during charging.
I have charged the battery another time from 4 V to 4.14 V using 0.5 A.