Well, here's a different "opinion": It's not so much about "over charging" to the point of where a battery would explode.. because that wouldn't happen under any ordinary circumstances.
You can however, shorten the overall life of your battery if you keep it plugged in all the time. Pigoo I feel, has been fairly fortunate, if it is indeed true that he's still getting about 4-5 hours worth of battery life when it's not plugged in. Though I'm sure he's really good about discharging the battery every so often, which is very important.
While it's true that today's battery technology is "better" than it used to be (don't ask me to quantify that in terms of relevance to years/time), it's still not perfect, and "memory effect" is still a very real thing. What Pigoo said is true... the cells in these batteries need and want to be in a state of recycled motion most of the time.
If you were to leave the battery charging 24/7 for a few years, I can absolutely guarantee that as soon as you run the MBP on the battery alone, it would die in a matter of minutes. Literally. This then should prompt us to figure out when, and how often it is good to run off of battery and when to charge.
I suppose that if one was to run off of battery alone for each session at their computer, and only charged until it was topped up again, then ran it down again, the argument could be made that you'd only be able to get so many charge cycles out of the battery before it died. So then, a healthy compromise should be made.
I personally wouldn't leave my battery plugged in all of the time for months at a time, or even just a month. The key here is to "condition" your battery. And by condition, I do actually mean "get it accustomed to" something. Even if getting accustomed to, happens to mean making sure that it charges and runs on battery power alone, at random times.
Maybe that should mean leaving it plugged in for a few days, and then unplug it and run it down a bit, or more than a bit. Then maybe the next time leave it plugged in for a week, and then let it drain down almost all the way then recharge etc.. .
My point is, that it is NOT true that leaving it plugged in constantly will do no harm to the overall life and longevity of the battery. Apple stores leave their machines plugged in constantly, but they can afford to, because they don't have to replace the batteries. They simply replace the machines when a new one comes out!
Ironic timing, this thread. I had a friend come over the other day, and I helped her upgrade to Lion. Her battery read 78% when we turned it on, and within about 20 minutes or less, the MBP died. No warning, nothing.. just died. It wouldn't turn back on until I plugged it in.
Funny thing is that it recharged to 100% in less than half an hour. I unplugged it again just to see what was happening, and the battery was draining like water down a plughole. I asked her what kind of battery life she usually got, and her reply was that she had no idea because she has not unplugged it in the 2+ years she has owned it! Well that's just great...
And this is definitely not the only time I've seen this happen, trust me on that.
I think that one easy solution might be to shut the machine off when you go to sleep, and unplug it. There are obviously other ways to go about it... but I can not recommend keeping it plugged in all the time. And to keep it all "pc", I'll follow up with an "IMO".
Doug
Nobody wants a laptop that can over-charge the battery!
