Why do three series connected 1.2 V NiMH batteries read 4.16 V when charged?

Question

I have a 7.2 V Dremel. The old battery pack was dead, so I had to replace the six rechargeable batteries inside. The originals were 1.2 V 700 mAH AA; I could only find 1.2 V 1100 mAH NiMH AA batteries.

The pack design has three of the batteries grouped together, with nickel strips (shown in blue) connecting them in series:

There are two groups of these three-packs inside, but the two battery groups are not connected to each other. So the pack exposes four terminals, two plus and two minus.

The Dremel dedicated charger is marked as outputting DC 9 V at 0.20 A and indicates a 3 hr charge time (but does not shut off by itself).

I figured that since the original batteries were 700 mAH and I replaced them with ones rated 1100 mAH, I would need to charge them about 1.57 times longer. So, I charged them for 4 hours and 45 minutes. (At the end of this period, both the charger and the pack got really warm.)

When I checked the battery pack connections with a multi-meter, it read 4.16 V and 4.18 V, for each of the two groups.

My questions:

1) Shouldn’t each battery group read 3.6 V (i.e. 1.2 V per battery x 3 batteries in series)? Why am I seeing 4.16 V and 4.18 V instead?

2) Why is the charger output marked as 9 V? Wouldn’t this voltage damage the batteries? Or is the 9 V divided into two and each group gets 4.5 V? Even so, 4.5 V is still greater than the series-sum of 3.6 V.

3) Is my duration calculation correct? If not, and since the charger is rated at 0.20 A, how long should I be charging the battery pack?

EDIT PER winny's SUGGESTION:

Answer 1

1. No, that’s perfectly normal. See the charge curve for NiMH below. It’s about 1.45 V / cell fully charged.

2. 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”.

3. 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.

Answer 2

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.

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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.

.

Answer 3

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)

Answer 4

1) No, because a battery only nominally 1.2V per cell. They are empty at about 0.9V and charging voltage can be as high as 1.4V to 1.6V.

2) 9V could be the no-load float output, or maybe it charges the two stacks in series?

3) Approximately yes. One sign of batteries being full is when they start rapidly heating up. But it is impossible to say if the charger is simply resistor limited or is it a constant current charger or something more clever.