Discharge of batteries connected in series

[Kian]

Hi,

I have a circuit that is powered by 3 AA batteries and has been running for a while. I noticed the battery is getting weak and I took them out for replacement. I measured the voltage of each of the battery and noticed that they had different voltages remaining, 1.3V, 1.1V and 0.8V.

I always have the impression that when they are connected in series, they should all discharge at the same rate. I wonder what caused this.

 

[ronsimpson]

they are connected in series, they should all discharge at the same rateries .

The same amount of current passed through all the batteries.
BUT
If the batteries had a different amount of charge they will be different at the end.
If the batteries have a different amount if internal leakage.....
If the batteries were made slightly different....... (a 1% difference will get you to this point)
If the batteries came from a different lot..... (made on a different day of the week)
If the batteries came from a different company.... (all ways use the same batteries in a group)
If the batteries were of a different age......
different temperature
different (anything)

With the exact same amount of electrons flowing in all three batteries; one of the batteries will run out of "go" before the others.

 

[dknguyen]

They

Hi,

I have a circuit that is powered by 3 AA batteries and has been running for a while. I noticed the battery is getting weak and I took them out for replacement. I measured the voltage of each of the battery and noticed that they had different voltages remaining, 1.3V, 1.1V and 0.8V.

I always have the impression that when they are connected in series, they should all discharge at the same rate. I wonder what caused this.

Sort of, but not really since no battery is ever identical in capacity or internal resistance to every other battery. They will all have the same current draw, but will have varying % discharge and different voltages per cell while loaded (and unloaded also).

 

[schmitt trigger]

Another problem with series connected batteries is a deep discharge.

As you have noticed, individual cells discharge unevenly. If you deep discharge the pack, there is the possibility that the weakest cell voltage goes to zero volts and then it becomes negative....most likely resulting in a ruined cell.

 

[MikeMl]

The capacity of series connected battery cells is no better than the weakest cell in the string. As pointed out, since that same discharge current flows in all of the cells, the cell to completely discharge first will reverse polarity, totally ruining that cell. Especially true for NiCds, NiMh, and lead-acid automotive and storage batteries.