The common laptop battery is a 300 cycle affair. The exact definition is elusive with one being that at 300 cycles the capacity is either half or 80% of new. Either way the lesson is clear. Use the battery sparingly, use AC whenever possible.
There's a lot of old bad advice still out there. Most of it is from the days of NICD and NiMH days but for today's LiON and such we can let it stay charged up all the time.
My DV7 series HP laptop was hooked via HDMI to hotel tv and I was streaming netflix and running on the battery.