Does the MacBook Air draw power from the battery or wall socket when the battery is at 100% and plugged in? I have a MacBook Air 13" (late 2010 model). I use it both as a portable computer (i.e. not plugged in, drawing power from the battery) and as a desktop (i.e. plugged into the wall socket) regularly. I'm wondering what's happening when I use it in the latter situation.

From what I learned at Battery University, I know that you should minimize battery usage because the more charge cycles a batter has, the less charge it will be able to hold. In other words, when I use it in "desktop mode", I would ideally take out the battery and only use the wall socket as a power source. However, this is not an option on the MacBook Air.

So my question is if the battery is at 100%, the laptop is plugged into a wall outlet, and I'm using the laptop, where is the laptop drawing its power from? Which of the following scenarios is happening?


Wall Outlet -> Laptop
Wall Outlet -> Battery -> Laptop
Wall Outlet + Battery -> Laptop


If it's #1, that means I don't need to worry about keeping the laptop plugged in all the time.

I'm looking for empirical evidence of what's happening, rather than speculation made by individual observations. Here's an example of an answer I'm not looking for:


  I notice that the battery icon stays at 100% when my laptop is plugged in, therefore it must mean that the power outlet is the one supplying power.


Who said that? Where's the evidence that that's what's really happening? What if Apple is purposely hiding the fact that the battery is being drained a bit and then refilled? What if the power is going from 100% down to 99.6%, then back up to 100% constantly? Although it may seem like this wouldn't affect battery life as much as going from 100% down to 60%, the nature of Lithium ion batteries makes it so that 100 of these mini cycles is almost exactly like the one larger cycle.

Also, does heavy usage of the laptop affect where it draws its power from? For example, say I'm running a CPU intensive game (e.g. near 100% CPU usage), would it be able to draw all the power it needs from the wall outlet, or does it go into the battery reserves?
Don't worry. It will use the wall outlet if that offers enough power which it should using the original AC adaptor.


  What if the power is going from 100% down to 99.6%, then back up to 100% constantly?


That was until about a decade ago or so. Modern laptops even don't charge your battery if it is eg. at 98% for reducing unnecessary cycles. Don't bother about your battery in your computer.

If you want to care for your battery, just remember every cycle shortens battery life, no matter if 10 minutes on battery or completely emptied it. The battery has also some natural aging, so it will die after some years even if you never touch it.