Will batter life decrease if I constantly leave my macbook plugged in 80%-90% of the time?
If you are leaving your laptop plugged in for extended periods of time, it's sometimes best to remove the battery. Also, in general, it is healthy to fully charge and discharge your laptop battery every so often, i.e. do not keep it in a fully charged or discharged state for long periods of time. What you are describing should not harm your battery life too much though, but it still is not good for it.
You can also monitor your battery's health and stats via Coconut Battery.
Whatever you do, don't run your MB straight on AC! It slows down your processor and increases the risk of your motherboard getting burned out. Apple advises this way..
Also, you should be fine plugging in for 80-90% of the time.. as long as it's not 100%! Make sure you use your MB on battery a couple of times a week, and do the calibration once a month, to keep the battery running correctly.
This is straight form Apple...
http://support.apple.com/kb/HT2788
http://support.apple.com/kb/HT1490
that will kill your battery pretty darn fast so i wouldn,t do that if i were you i would unplug it and discharge it fully at least once a week![]()
Its easier by just using the laptop as a laptop. I've gone through over 23 charge cycles in 2 weeks so far. I just use it. Probably the only time I plug it in is when I'm at my desk or the battery is under 20%.