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Hi I am quite new to electronics, I had a project powered by 3xAA batteries in series (~4.5 V) with an LDO dropping it to 3.3 V. It went flat quicker than expected but what was very surprising to me was that the voltage of the three batteries was so different ~0.5 V, ~0.6 V and ~1.5 V total ~2.6 V. Is it normal for a battery pack to discharge so unevenly? Can someone explain why?
If it is relevant it sat most its time in the μA range with hourly very short spikes to ~150 mA.
You should use 3 cells from the same manufacturer, same model and same chemistry, out of the same batch or the same package. Of course the cells should not been used before.
If all cells have the same history and manufaction tolerances of capacity are small, such an uneven discharge should not happen.
But when the history of those 3 cells was different, uneven discharge is possible.
Assuming alkaline / zinc chloride / similar chemistry:
If you got eg 0.5, 0.6, 1.1 then it could be explicable by the fact that there is very little energy in the area under about 1V, so that if one cell had say 5% more energy content than the other two it would still be on the "final approach" to fully flat while the other two were essentially completely exhausted.
However, a cell at 1.5V has the majority of energy capacity remaining - probably 90%+ .
For one cell to be at 1.5V while the others are fully exhausted then they would have had only 5% - 10% of their new energy content at the start of discharge.
SO this is not a batch variation - two of the batteries were very close to dead at the start of discharge OR something else has happened not mentioned in your question.
If the batteries are 'alkaline' they would retain a substantial portion of full charge for many years. If they were Zinc Chloride or other similar chemistries then they may well have very little capacity after say 2 years of shelf life.
The different voltages on the batteries is a little unusual but not really important. You say you have a LDO but don't give it's number or the amount of quiescent current. Many/most beginners don't realize that voltage regulators continue to draw current even when the load doesn't require any. This is the most likely cause of the rapid discharge of the batteries.
This is common and apparently no viable study explains exactly why. Getting the batteries to discharge evenly is essentially impossible in a 'real world' application. In my flashlight test experiment the battery closest to the bulb always discharged soonest, the other batteries discharged inconsistently sooner/later. Using rechargeable batteries and changing their position didn't affect this result. Flashlight is an old incandescent type. My experience has been the engineers that dismiss me the quickest with a question are the youngest and the least imaginative and don't ask such questions themselves, I guess because it inhibits simply getting results.
