Options for getting 4.5-6V out of (mismatched) AA alkaline batteries?
You can test V with a 1A analog  current meter for a blinking fraction of a second and bin them, as this will allow you to measure (almost) short circuit current and thus capacity.  Combine say 10 bins into fewer bins as needed to optimize matching.




Mismatched cells are useless as the series capacity and ESR is controlled by the weakest cell.  Car batteries have 6 cells and if not matched within 0.1% when new, it will become 100% mismatched sooner than expected lifetime. This is measured by specific gravity measured with 4 significant figures which is an indication of both SoC and ESR from sulphation.

The same idea is true for LiPo's in Tesla cars and many battery recycler's on Youtube videos yank out the bad 18650 cells from modules and measure the fully charged , settled Voc and bin them into similar tight ranges to maximize the useful total capacity.

Remember :  Series cells follow the weakest link and dead cells can become reverse polarized with high ESR as another Op reported in a 3x AAA situation.


Fresh AAA cells are equivalent to xxx Farads with xxx. mOhm ESR in small batteries and scale this for large capacity cells like lead acid and LiPo.
Dead cells are equivalent xxx microFarads and xxx Ohms ESR with no capacity except as a very lossy capacitor.


All others are in between so matching series cells is very important.

I suggest &lt;10% Voc for AAA but pref &lt;1% and for LiPo I suggest &lt; 1%Voc and pref. less. variation for longer life.

Also understand


the cell with the highest ESR gets warmest and ages faster when used high high motor current applications.
otherwise they age or deplete linearly and if you can't afford new cells just replace the dead one with a similar one or a new one and expect life to be limited by the weakest cell.