I use rechargeable batteries for my keyboard, mouse, alarm clocks etc and have noticed a puzzling phenomenon. My recharger is probably a standard model, with an indicator light that turns off when the process is "complete."
But... if I then test the recharged batteries in a battery tester, sometimes the "recharged" ones are only perhaps 70% max. It is not the battery's fault, apparently, because when I pair up a couple 70-80% ones and reinsert them, they recharge to the 100% level.
I'm guessing that when two weak batteries are inserted, the recharging process proceeds until one of the batteries is at max, then shuts off, without continuing for the second battery. If I insert a 75% and a 25%, the 75% reaches 100% but the 25% might be only 60-80%. They would seem to be charging in series, not in parallel.
It's easy to workaround the problem by using a battery tester, but it's a nuisance. I don't know whether the design is necessary to avoid overcharging the better one of the pair, or whether my (old) recharger was suboptimally designed.
Someone out there will know the answer. Thanks in advance.
(Just to clarify, my tester does not give exact percentages; I am extrapolating from a red/yellow/green non-numerical readout.)
Addendum. Wow. Lots of useful information in the Comments section, including this link to a comprehensive relevant website.