[Nitestick]

Maybe on older (NiCd or NiMH) battery types this was the case. Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) batteries suffer from capacity loss if they are recharged before they fully deplete their charge. If you unplugged your NiCd or NiMH laptop and used it for like 5 minutes and then plugged it back in, this could damage the battery.

that in itself is a bit of a myth, you shouldn't completely discharge NiCd or NiMh batteries either. they should be run low, to maybe about 5% for recharging but not completely discharged.

Lithium batteries pretty much all have some internal voltage/charge monitoring circuitry, additional circuitry is including in the charger or laptop to sense when to stop charging the battery. as has been said. once the battery is charged it is almost as if it is disconnected, though they generally remain on a trickle charge. i guess in essence there is some truth to this but in practice it should make little difference.

Put it this way, why would the battery me so easilby removable if you would only have to take it out to replace it...the companies that make the laptops don't tell you how to properly treat the battery for you damage it and buy another one from them eventually.

what about second batteries? i have two batteries for my laptop. i hibernate and swap them when the charge gets low and i'm away from mains power.

summary: there is some truth in saying that leaving your laptop on AC power with the battery in it will reduce the lifespan. this however is generally only noticeable (from experience and what i've heard) when the laptop spends most of it's life on AC. it should be ok if the battery is cycled fairly regularly.