LiIon cells have a steady decay of it's capacity from it's day of manufacturing!
Even without being used.
In theory, after 5 years the capacity is down to 50 % approx., but mostly they fail after 3 years in my experience.
Leaving a device in a charge connection always, usually seem to reduce the capacity sooner.
They have no memory effect, according to the descriptions of the cells, but do decay rapidly if the are badly manufactured (Ref. the Sony's China manufactured portable cells that ignited and banned Sony portables from aircrafts a couple of years ago!).
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