The laptop’s battery during these days would discharge and charge, slowly degrading the battery because only the last ~ 20% would be charged and discharged.
How, tho? Sounds like what we had for e.g. NiCd batteries (memory effect) but do not have for li-{ion,poly} ones.
Also, why would the laptop discharge the battery with ac attached? Sounds weird to me
Lithium-ion batteries these days do not have a memory effect, but will degrade when kept at 100% charge because the internal composition of chemicals will change, destroying the battery in the process. The ideal charge is between 20% and 80%. With (better) battery charge control you can extend the design capacity (the maximum charge the battery can hold when new) and lifespan. With AC attached, the battery will discharge but it will be charged when the minimum charge level is reached.
Huh, apparently some vendors kinda do it themselves (not sure if always, tho) at least for the lower bound:
cat /sys/class/power_supply/BAT1/voltage_min_designreports 10.8v for a 3s battery which is about 3.6v per cell instead of 3.2. Also the upper limit is uncertain so far