Battery Charging twice. Is this bad for my cell?

LED Flashlights – General Info
1

I have been charging my NiMH(AA) and LiPo(18650) at 0.5c-1c on a hobby charger.

Then when they are finished they are a 0.100-0.200 off of max voltage (1.6v/4.2v).

I know with every use the battery loses capacity from normal wear and a used one will never hit the max voltage ever again.

But what I do is charge the battery, say 18650, at 0.5c (@1500mA) and the charger detects the battery is fully charged and stops.
I then switch over to view it's voltage and they say something like 4.155v.
So I put it back on the charger at 0.1Ah (100mA) and can gain some extra calculated capacity and the end voltage increases more twords 4.18v-4.19v.

The charger uses CC/CV algo for LiPo battery charging and what ever the smart algo for NiMH is.

tl;dr:
Charged Cell at 0.5c (1.5Ah) on smart/hobby charger CC/CV
Checked voltage to be 4.15v.
Put back into CC/CV charge at 100mA.
Charging stops.
Voltage is now 4.19v and calculated an additional 100+mAh capacity.

Am I harming my cell or by doing this second low current charge?

2

The 18650 cell is safe doing a charge like that. Is the extra time waiting worth the 100mAh?

3

Probably not, but if you have them already hooked up and have nothing better to do, then why not? haha.

How about my NiMH batteries? That's not going to harm them either?

4

NiMH are a different story. At the end of their charge, there is a little dip in voltage. If you pull off the NiMH and start recharging again, the charger will be looking for that dip and it doesn’t come (or takes a long time to come again) so you end up overcharging the cell.

5

is that called Peak Sensitivity? Would raising or lowering the mV on that second charge be better? Or just avoid double charging NiMH entirely?

6

Just don’t double charge NiMH.

7

Sorry, I can’t remember much terminology. I remember delta V or dV as the small drop in voltage. Avoid double charging NiMH. With Li Ion, you fully charge the cell but with NiMH, you overchage the cell and it will degrade much faster over time. MUCH FASTER. If you choose a charge current that’s too low (using a hobby charger for eg), sometimes the dV is not detected so the cell charges indefinitely.