A Ni-MH battery gets fully charged, put it in the charger again, why is it still charging?

[XTAR Light]

A customer told he got one of his NiMH AA batteries fully charged on the MC4S charger. Then he took off the battery, and put it in his VC2S charger again. It showed the NiMH battery charge again in the VC2S. Was the battery not got fully charged? Or Was there anything wrong?

If the battery and charger are all in good condition, it's also normal to see this situation. As you know, the common full-charge detection for a Ni-MH/Ni-CD battery is the 0ΔV and -ΔV method in many chargers. It means observing a slight voltage drop after a steady rise, then the battery gets fully charged. When a full charged NiMH battery is reinserted into the charger, the -ΔV may be not so noticeable. And the charger will continue charging, until it detects the 0ΔV and -ΔV. This process may takes around 20 minutes.

And it's not suggested to reinsert a fully charged NiMH battery in the charger again. Some chargers may not have adequate protection mechanisms. It could lead to overcharging and overheating of the battery.

 

[alpg88]

Yes, that happens, but it is not the only issues that can pop up while charging nimh/nicd with smart chargers, I have about a half a dosen of smart chargers. Sometimes you insert a cell and it terminates the charge too fast, if you are using a smart charger that shows how much was poured in, you can see that you only filled 1/3-1/2 of the capacity, but the charger shuts off telling you the battery is full, It happens regardless of the charger and charging current.
I have no clue why some cells do that, even from the same manufacturer, some line of cells charges fine, others only partially. Hopefully you as a cell manufacturer can tell us why it happens, the mechanics of it.
At first i wanted to throw them out, then i decided to charge them "manually" with a dumb charger and a timer, and to my surprise those cells perform just fine once they are fully charged manually. one such light I build over 10 years, ago still works and batteries still have around 80% capacity left.
Back in a days i used to rebuild nimh\cd packs for power tools, as well as building custom packs for my projects, so i had to cycle and match the nimh/nicd cells. those are very tricky animals, especially when in packs,

 

[XTAR Light]

It seemed there might be something wrong. What kind of NiMH batteries and chargers did you use? Were all of those NiMH batteries in good condition? There may be different full-charge detection ways for the NiMH batteries on your different chargers.

 

[letschat7]

I bought a new packet of Ansmann rechargable aaa batts. I use them in a Led Lenser till they go empty. I charge all four at same time and two get charged before the others. Why?

 

[MidnightDistortions]

I bought a new packet of Ansmann rechargable aaa batts. I use them in a Led Lenser till they go empty. I charge all four at same time and two get charged before the others. Why?

It's hard to say without seeing more data, like resting voltages and a full recharge discharge recharge test in a charger analyzer. What may have happened was 2 of the cells were more empty than the other 2 and when your light dimmed 2 of the batteries were empty and the other 2 were still partially charged.