Series connecting NiMH batteries of different mAh ratings

[DamoRC]

I can't seem to get a definitive answer on this.

Suppose I have three identical NiMH rechargeable batteries. I take the first two and make a parallel "pack" (both positives connected, and both negatives connected). I will end up with a pack that has the same voltage as one of the batteries, but twice the mAh and twice the current discharge capacity.

Now lets say that I connect this pack in series with a single additional identical NiMH battery. Under load (a load that is close to or at the maximum discharge rate of the batteries), I assume that the voltage will drop as current flows through the batteries. Then I additionally assume that the voltage drop across the parallel pair will be less than the voltage drop across the added third battery due to the differences in their internal resistances. This would lead to a voltage imbalance between the parallel set and the third 'in series" battery which would, I assume, cause additional current to flow from the parallel pair to the third battery.

Then my head starts to hurt as I try to figure out what this scenario means for the third battery.

In some ways it could be imagined as connecting 2 NiMH batteries in series with one of them having twice the mAh rating as the other. What would the actual mAh rating of this pack be and what damage, if any, could I inflict on either battery.

Thanks in advance.

 

[BrownOut]

You might as well connect just two batteries in series and forget about the 3rd one. You'll get the same performance. That said, you won't hurt the batteries by doing it the way you said.

 

[mneary]

The way you said will destroy the batteries quickly. As soon as the single battery is discharged, the twin batteries will charge it in reverse and destroy it. Some will say you can avoid this, but it will happen eventually.

Conversely, if you use a conventional NiMH charger (designed for 2 cells in series) then the twin batteries will never be fully charged and the single battery will always be overcharged.

Use just two batteries.

 

[smanches]

As mneary said, damage will ensue. Never mix batteries of different types, values, or even age as it will cause damage MUCH faster. Especially at max discharge rates.

Anytime you use multiple cells, you are going to have to worry about these kinds of issues, even when they are brand new.

 

[Nigel Goodwin]

I agree, you should NEVER mix different types of batteries, they WILL get destroyed if you do. There's no way to connect the three batteries like you asked, just place two in series and ignore the other one.

 

[DamoRC]

As always, sound advice. Thanks to all who responded.

 

[crutschow]

Now lets say that I connect this pack in series with a single additional identical NiMH battery. Under load (a load that is close to or at the maximum discharge rate of the batteries), I assume that the voltage will drop as current flows through the batteries. Then I additionally assume that the voltage drop across the parallel pair will be less than the voltage drop across the added third battery due to the differences in their internal resistances. This would lead to a voltage imbalance between the parallel set and the third 'in series" battery which would, I assume, cause additional current to flow from the parallel pair to the third battery.

Just an added nit. You can not get additional current to flow from the parallel pair to the third battery since they are in series. Only current that flows through the third battery from a load will flow through the parallel pair (which will then divide between the two depending upon their relative internal resistances).