Why do three series connected 1.2 V NiMH batteries read 4.16 V when charged?
No, that’s perfectly normal. See the charge curve for NiMH below. It’s about 1.45 V / cell fully charged.
NiMH are tolerant to overvoltage as long as the current is kept low enough. This is commonly referred to as slow charging where the cells stay hot when fully charged until you remove them. Medium solution is higher charge current and thermal cutout. Any fast charger would need to sense zero delta V or negative delta V for charge termination. EDIT: Since you did measure that the charger bridged the two 3.6 V packs in series, forming a 6S1P configuration during charging, the 9 V and 0.2 A makes perfect sense. It’s actually so low “overvoltage” (9-6*1,45=0.25 V) I’m suspecting the charger is delta V sensing or “smart charger”.
At least 1.1 Ah/0.2 A = 5.5 hours since they are charged in series to reach 80 % SOC. Probably +50 % more for 100 % SOC.
Complements other answers:
7.2/6 cells = 1.2V/cell - which is the NOMINAL loaded voltage of NimH. So they are operated in series - unless they are lying.
9V/6 = 1.5V cell - which is slightly on the high side for fully charged and they MAY keep on charging indefinitely, which will risk destroying them.
'Once upon a time' NimH would tolerate C/10 trickle charge - which here = 1100 mAh/10 = 110 mA or less. Cells over about 1800 mAh removed the recombining chemicals and would accept NO trickle charge. Whether low mAh cells still retain the O2+H2 recombining material is TBD.
NimH charged at <= C/10 will automatically stop accepting current if V/cell is <= about 1.45V, say 1.4V to be safe.
1.4 x 6 = 8.4V
1.45 x 6 = 8.7V.
So if their claimed 9V was a little low it could do quite a good job BUT the cells getting hot is a sure sign of end of charge. IF they stay hot then the battery pack is being roasted.
Reducing the charge voltage with say a series diode MAY be enough to make a useful difference.
Or ALWAYS monitor temperature of cells and stop when they get hot.
Nimh charge terminate methods:
Negative voltage inflection - see winny's graph.
Absolute temperature
Increased rate of temperature rise.
Threshold voltage (varies with charge current)
Timed (must start empty)
I longish ago built solar portable lights using NimH cells. In an environment where charge current is variable and intermittent and the cells are solar heated the ONLY method that works is one based on the cell's terminal voltage.
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4.18 volts for three cells is about 1.4 volts per cell, which is reasonable for a fully-charged NiMH cell straight off the charger.
Since the battery pack has separate connections for each group of three cells, the charger can connect the two groups in series for charging (and the tool may connect the groups in series or parallel for use)