Overcharging, or just optimizing?

[Fallingwater]

I charged a 18650 I have at a charge current of 800ma. The charger (hyperion 5i) put about 850mah in it, then signalled end-of-charge.
I was not surprised: that cell is very old and very used and it's remarkable that it can hold any charge at all. Besides, 850 is still good to run my Cree light and my hacked mp3 player.

On an inspiration, however, I lowered the charge current to 400ma and started the charge again. I was expecting it to refuse, or finish the charge very quickly. Instead, it charged for another 83 minutes, putting another 325 mah in the cell. I switched it off at this point, because I had to go out and I'm not leaving lithium cells to charge when there's nobody at home.

I came back and further lowered the charge current to 300ma. It's still charging and as I write this has put another 70 ma in the cell in 27 minutes (although at this point whatever charge current I set is ignored, I think, as it instantly goes CV: right now it's charging at 100ma and will probably signal end-of-charge soon).

My question is: am I overcharging the cell after the first 850mah charge, or is the gentler current just allowing it to absorb more energy without damage?
Thanks.

 

[VidPro]

as long as it doesnt go over the voltage, your not overcharging it, well unless its junk.

when you hit it with 800, and there is resistance in the curcuit, be it your battery or your wires, the charger will see the voltage go up further than the battery itself is.
so improve your wiring and or connections, if it is poor.
if its not the wires and the cell has that much resistance, and that poor of capacity, recycle it.

to Test, what is going wrong, see what voltage the battery really made it to before cutoff. or its rested voltage after cutoff with a seperate DMM. if you have a good battery to charger connection, the cells voltage should be close to the termination voltage of the charging.

Ya see, cause you got 4.2 (say) volts at the charger end of the wires, and the wires drop .2 volts (because they are crud), and there is your battery at 4.0.

when you reduced the rate, the wiring, and/or any resistance in the cell, the voltage will not drop as much, so the 4.2 (or whatever) is not seen by the charger so quickly and it doesnt cut off so fast.

other notes: if the battery is old and unused, and has developed high resistance, a few cycles will change that some.
and
if you go TOO Fast for charging a li-ion, and its a good cell still (not heating) the cell will not accept the charge, the resistance will increase, and the charge will terminate. the speed is somewhere above 1C of the cells capacity usually.

Disclaimer: if you cell heats a lot, or doesnt have the capacity, dispose of it properly, bad cells are bad news, especially with fast chargers. and if it was deep discharged below specs or self discharges just dont charge it. like if you can visably see the voltmeter click down showing a self discharge, when there is no load, that would indicate a terrible li-ion battery