In a 3-cell series battery, is it normal for 'middle' cells to have lower voltage vs. outer cells when removed and measured individually?

Question

I have a battery pack consisting of 6 alkaline 'D' cells, the packs are build with 3 cells in series and then the 2 series strings are put in parallel for a 4.5V output. The individual cells are connected with spot welds at a factory and the entire thing is heat-shrink wrapped, so it's not a spring loaded pack or anything.

The battery pack is connected to an electronic device that typically draws current in the 300uA range, but will a few times per day will draw ~1A for a few minutes while active.

Most of the time the battery packs will last for 2-3 years, but a few packs are lasting for only 3-4 months. The activity of the electronic device connected to the pack is apparently the same.

I have disassembled several fast-depleting packs when they are at ~2.9V no load, what I'm finding is that the "middle" cells, 'E' and 'B' in my diagram, are consistently 20% lower voltage than cells 'A', 'C', 'D', and 'F' in all packs.

For example I'll take the cells apart and find 1.06V at A-C-D-F and at E-B it will be 0.83V.

The packs have had days/weeks to rest with no load before I disassembled them.

My admittedly low understanding is that in a battery pack like this all cells must have equal voltage, if they do not then the higher voltage cells will discharge into the lower voltage cells to normalize, but the consistency of the 'middle' cells being lower makes me think this can't simply be an issue where the battery pack factory is putting a few mismatched charge cells into packs.

My question: In a multi-cell series battery, is it normal for the 'middle' cells to have lower voltage vs. the outer cells when they are removed from the series and cells measured individually?

Answer 1

The reason for the mismatch may be heat as Jen suggests, since the run time and current is long enough to cause the cells to heat.

However I don't think that's causing the short pack life- maybe the cells are old or substandard. Check the date codes, if any, and other indicators or hints as to origin. Compare with the 'good' packs.

Compare the weight of the cells in good and bad packs. Substandard cells are often considerably lighter, like an AA cell in a D case. (though there have been reported cases of 18650 lithium cells with sand added to increase their weight). Or perhaps they are cheaper "heavy duty" zinc-carbon cells subsituted for alkaline (in which case they would have poor ability to supply the 1A high current). The internal construction is quite different- zinc-carbon cells have a carbon rod connected to the + terminal, which alkaline cells do not have.