G'day pjs

Luvya drainometer - I made up a similar device years ago for much the same purpose too [funny how things go ain't it ]

The charging & discharging of batteries is a series of chemical reactions that [are useful to us by giving off electricity, but] over time fade in efficiency as the chemicals lose their potency [same as me too]

However, there is another side to battery recharging that you can do something about -
As you recharge the battery, the internal chemical reactions cause the battery to heat up, then causing an electrical resistance to occur (call it a blockage if you like)
Your recharger has a light or series of lights indicating the charging in progress &/or when it's finished. If you allow the battery to cool, then plug the recharge in again, you will "top up" the battery

Taking Rick's comment further ... when new you might get a 95% charge, topped up to 100%, but when a year old you might get a 70% charge, topped up to 90% etc etc.

I do this all the time with my rechargeables - I have 1/2-dozen sets of AA rechargeables, (one set is over 10 years old and still going strong) - used in the motorhome for torches, trannies, cameras, flashguns etc etc

Hope this helps
Regards, Phil