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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?

  1. Wall Outlet -> Laptop
  2. Wall Outlet -> Battery -> Laptop
  3. 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?

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Always 1 on the 11". It's 1 with the 13" if you're not using too much resources, otherwise it could be 3. On the MBA 13" page at Appleit says that the laptop could consume more power than what the adaptor could give, but it's unclear whether it's still true if the battery is already charged. –  Gio Nov 4 '11 at 22:52
    
My experience with numerous Lithium battery PowerBooks and MacBook Pros is that leaving them plugged in all the time isn't the best for the life of the battery. Using the computer on just battery power for part of the day at least a few days a week, eventually draining it all the way and charging it back up to full seems to give the most life to a battery (three years of heavy use is what I get). Luckily for me three years is usually when I'm looking around for a new computer anyway. –  Richard Nov 5 '11 at 3:14
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Does this not say it all? "Exposing the battery to high temperature and being at full state-of-charge for an extended time can be more damaging than cycling" and "The question is often asked: Should I disconnect my laptop from the power grid when not in use? Under normal circumstances this should not be necessary because once the lithium-ion battery is full, a correctly functioning charger will discontinue the charge and will only engage when the battery voltage drops to a low level." Source: batteryuniversity.com/learn/article/… –  cksum Nov 7 '11 at 0:53
    
@cksum: I'm not sure which side you are arguing for... the 1st quote seems to say I should unplug it, but the 2nd one says that leaving it plugged in is fine. Thanks for the link though, I'll check that out. –  Senseful Nov 7 '11 at 4:20

3 Answers 3

up vote 3 down vote accepted

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.

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Well the reason I was worried was because until I realized this, I was thinking that the best strategy is to charge until 80%, let it drain to 20%, then back up to 80% (because I read that using the battery while it is fully charged is bad as well). However, now I realize that what I was doing was probably worse for the battery because it was going through a lot of charge cycles. Now that u explained that batteries use all their power from the outlet when left plugged in, I know that that is the best solution. Therefore, I should have been worried, and did need the knowledge u provided. Thanks –  Senseful Nov 4 '11 at 21:54
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Best to do with batteries is not caring too much and replacing it after two or three years when capacity is too low. ;) –  Jens Erat Nov 4 '11 at 22:07
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@Senseful: modern laptopts don't charge your battery to 100% gross capacity. The 100% shown in OS X are normalized capacity. The machine continually calibrates how much capacity the battery has remaining, compared to when it was new (that's the "Health" status you can see with e.g. iStats). The laptop will charge the battery to som XX% of this current gross capacity. –  Arne Jan 6 '12 at 14:45

I don't have an answer for you (and you should probably accept Ranon's answer, since your comment to that makes it seem like that's the answer you wanted). I do, however, have a test you can do to determine the answer for yourself:

  1. Check the cycle count.
  2. Leave it plugged in for several days straight (weeks would be better).
  3. Check the cycle count again.

If the cycle count hasn't changed, then it's not draining the battery at all.

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I don't believe there is any empirical evidence that this is related to the actual rate of chemical degradation that occurs in the battery. –  Steven Lu Aug 21 '13 at 16:49

According to Apple's Website and I quote "Apple does not recommend leaving your portable plugged in all the time."

That means your initial assumption "If it's #1, that means I don't need to worry about keeping the laptop plugged in all the time" is incorrect.

Have a read more on that website on how to look after and maintain a battery which they manufacture on your MacBook Air.

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It goes on to say that the reason is that the battery should actually be used every so often. Otherwise, they recommend discharging it once a month. –  Senseful Feb 11 '12 at 7:56

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