As stated in my message I have just returned to using my external display after the summer, so of course I have used my mac the whole summer on battery power, not as a desktop.
There is no good reason to use the machine on battery power at all times if AC power is available. Doing so just runs up the battery's cycle count and reduces its storage capacity needlessly.
I have often read in mobile phone, other computer, mp3 player (etc...) manuals that to maximise lithium battery durability it is best to rarely leave them fully uncharged or leave them in charge while fully charged. My interpretation of the apple support page stating "all the time" was not as drastic as yours, so I assumed that it confirmed other sources stating not to leave batteries on charge for prolonged periods of time.
No Apple product manufactured in the last ten or fiteen years has continued to charge after full charge is achieved, so there is no danger of leaving an Apple product's battery "on charge" for any longer than it takes to charge it fully — in fact you can't do so. By design, the charging circuitry disconnects automatically at full charge, and doesn't resume charging again (in the case of your MBP's battery) until the battery has gradually drained to less than 95% of full charge — which it will slowly do over a period of a few days, even if the AC adapter remains connected the whole time and the battery isn't drawn upon at all.* This is why you will often see the colored LED on your MagSafe plug lit up green (indicating that full charge has been reached and charging has stopped) when your battery's charge level is only 95%-99% rather than 100%. When the level drops below 95%, the light turns orange to indicate that charging has begun again.
* Every battery is slowly losing charge whenever it isn't being charged, even if nothing is connected to it at all. This is just a fundamental characteristic of batteries.
May I know what sources you have used to determine that batteries can stay on charge 95% of the time without ill effects?
Batteries can't be charged continuously 95% of the time without ill effects. But Apple batteries aren't charged continuously, as I've explained above. The principal authority for my advice to leave your machine plugged in as much as 95%- 98% of the time iwhen it's convenient to do so is the following Apple statement:
"An ideal use would be a commuter who uses her notebook on the train, then plugs it in at the office to charge."
If this hypothetical commuter runs her notebook for 45 minutes on the train to work, plugs it in to charge for eight hours at the office, uses it on battery power for 45 minutes on the train home, and plugs it in to charge at home, it will be plugged in 22.5 hours a day, or 93.8% of the time — not counting weekends, when presumably it will be plugged in at home all or most of the time.
Further advice about Apple batteries and their optimal management can be found in these articles (including the one I've quoted from):
http://www.apple.com/batteries/notebooks.html
http://support.apple.com/kb/HT1446
http://support.apple.com/kb/HT1490
Note in the last of these articles (on calibrating the battery) that Apple makes a distinction in its calibration-frequency advice between a machine that is regularly used on battery power (calibrate "every few months") and one that is "very rarely" or never used on battery power except when it's being calibrated (calibrate "once a month").
This said, I still do not understand the reason for totally impeding the use of an external display while on battery power and with the screen open (and deactivated) when performing heavy tasks.
You and many others who have posted in this thread. We'll have to wait and see whether Apple makes the desired change.
