3 D cells in series but one drains before the others

I have a LED flashlight that uses 3 D Cell NiMh batteries in series. Since the current must flow through all 3, I am wondering why ONE of them always seems to go dead before the others. All 3 are the same brand and capacity and age. All three are discharge capacity tested and are within 10% of each other.

I would think they would all discharge nearly uniformly, with their voltages all declining together. But that doesn't seem to be the case.

Is there some reason one of the three batteries in the series would carry more load than the other two?

As the charge becomes exhausted, I end up with the following.....

Battery 1 - 1.29v
Battery 2 - 1.29v
Battery 3 - .80v

I have marked the batteries and will begin noting the location of the battery which discharges first to see if it's the same battery each time and or the same position in the flashlight.

Thanks

Apothecary said:

I have marked the batteries and will begin noting the location of the battery which discharges first to see if it's the same battery each time

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I bet it will be. Even cells from the same batch won't have exactly the same capacity. The one with the least capacity will charge/discharge first.

I'm sure you're right. I'm running another full discharge capacity test now on all three. When they were charged yesterday, two absorbed about ~1500mAh of charge while the one that was lowest absorbed ~4300mAh. They are 2500mAh batteries.

If they are not brand new, it could be memory effect.

Does the cell showing low voltage recover to over 1.1V if left for a while?
If so, try discharging each cell on its own with a lamp or resistor, leaving them for several days after they are apparently dead.

The so-called memory effect means the internal resistance of the cell suddenly rises drastically, making it appear dead, while it still actually has some charge. A Total discharge "resets" it to normal state.


If it is really flat when down to 0.8Vand holds at a low voltage, it's just a faulty cell..

Apothecary said:

I'm sure you're right. I'm running another full discharge capacity test now on all three. When they were charged yesterday, two absorbed about ~1500mAh of charge while the one that was lowest absorbed ~4300mAh. They are 2500mAh batteries.

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Are you sure.. The difference here seems to be adverse.. It would appear the one of the cells is bad.. When Alec said "wont have the exact capacity" I don't think he meant doubled...

Are all 3 inline in a battery holder?
If not maybe the connections resistance is slightly lower on one causing it to burn out first.

dr pepper said:

Are all 3 inline in a battery holder?
If not maybe the connections resistance is slightly lower on one causing it to burn out first.

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Yes, they are in series with the positive of the battery beneath it directly touching the negative of the battery above.

When the Discharge test is completed, I'll post the results......and the IR of each cell.

I would expect that each cell would be at least reasonably close to one another in voltage at the exhaustion of the charge of the cells.
Like 1.25, 1.20, 1.18 etc. but not 1.29, 1.29. 0.80 unless there is a bad cell as mentioned.

You (OP) get tricked by the expectation that remaining voltage (not loaded) equals remaining capacity.

Also, voltage of batteries reduces faster when almost empty (most led torches cannot be compared to a simple resistor), so that alone contribute to faster voltage drop on the battery with least energy left.

dr pepper said:

Are all 3 inline in a battery holder?
If not maybe the connections resistance is slightly lower on one causing it to burn out first.

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The current must be equal through series-connected cells.

Apothecary said:

When the Discharge test is completed, I'll post the results......and the IR of each cell.

I would expect that each cell would be at least reasonably close to one another in voltage at the exhaustion of the charge of the cells.
Like 1.25, 1.20, 1.18 etc. but not 1.29, 1.29. 0.80 unless there is a bad cell as mentioned.

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Chuck them away and buy three new ones - it's hardly difficult!.

You seem to have swapped from rechargable to nonchargable... ?? The 1.5v D cells are Alkaline? surely..

Here are the most recent test results for the 3 D cells in question. They are HarborFreight Thunderbolt 2500mAh NiMh D cells
(The illustration above was only to illustrate how the batteries are configured and are NOT the batteries in question)

Keep in mind they are advertised as 2500mAh batteries so they are ALL performing far better than advertised.
However, Cell D2 seems to have significantly more capacity than the other two.
I do not know at this time if that's the cell that was most depleted as mentioned in the OP.

Of course, an "easy" solution is to ensure that all 3 batteries have similar capacities and Internal Resistances. However, this is more of a learning exercise for future reference and general knowledge. As always, thanks for your input.

My question is....
If the capacities below are correct, and the batteries are in Series under a load of approximately 250mA, does electronics law dictate that the battery with the lowest capacity will discharge either first, deepest or both? How would Internal Resistance possibly affect the discharge of the batteries?
I've since moved all three batteries to different slots on the test equipment and am re-running the discharge tests to see if they remain consistent in alternate test slots. The batteries could probably benefit from the cycling anyway.

The IR (internal resistance) values don't make sense. How can a 1.25V cell discharge at 250mA if its IR is 15 Ohms or more?

alec_t said:

The IR (internal resistance) values don't make sense. How can a 1.25V cell discharge at 250mA if its IR is 15 Ohms or more?

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those are Milli ohms

Nigel Goodwin said:

Chuck them away and buy three new ones - it's hardly difficult!.

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This isn't so much about the batteries (which are performing significantly above expectations and advertising). it's more about learning some fundamentals.
This is probably far below your level of expertise, but for me it's interesting and useful.
I would feel honored to one day know a tiny fraction of what you know.

Apothecary said:

This isn't so much about the batteries (which are performing significantly above expectations and advertising). it's more about learning some fundamentals.
This is probably far below your level of expertise, but for me it's interesting and useful.

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It's VERY important that batteries in series discharge 'reasonably' together, otherwise one can end up been reverse charged by the others. If one is discharging considerably faster than the others, then you should replace all three rather than messing about with them.

This is also why you should NEVER mix different makes or types of batteries, or mix old and new ones.

Harbour Freight sells cheap Chinese "Thunderbolt" batteries don't they? 2500mAh is the same as a Duracell (Name Brand) little AA size Ni-MH battery cell. Duracell do not make a D size Ni-MH battery anymore but Energizer does, it is also 2500mAh, the same as their older AA size one, their new AA one is 2300mAh but holds a charge for one year.

I think your "new" D batteries have an older little AA cell inside. Tenergy (sold at Amazon) D size Ni-MH cells are 10000mAh. Their head office is in California. I never tried them.

audioguru said:

Harbour Freight sells cheap Chinese "Thunderbolt" batteries don't they? 2500mAh is the same as a Duracell (Name Brand) little AA size Ni-MH battery cell. Duracell do not make a D size Ni-MH battery anymore but Energizer does, it is also 2500mAh, the same as their older AA size one, their new AA one is 2300mAh but holds a charge for one year.

I think your "new" D batteries have an older little AA cell inside. Tenergy (sold at Amazon) D size Ni-MH cells are 10000mAh. Their head office is in California. I never tried them.

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These are too heavy to have AA's in them. I do have some Tenergy 10,000mAh D cells on hand also. These newer HF 2500mAh D cells all test out around 3600mAh or better. When I'm ready to dispose of one I'll open it and see what's inside. I was surprised to find that 9v batteries have 6 LR61 cells inside that are just 3.5mm shorter than AAA batteries.

Did you see the video about what is inside a cheap Chinese lithium battery cell? A tiny little battery drowning in lots of rice flour: