Definitive answer on using 60w or 85w power adapter with Macbook Air?
I've seen internet postings on various sites saying that it's safe to use the 60w and 85w magsafe power adapters with the Macbook Air (which comes with a 45w magsafe power adapter). But I can't find any official statement from Apple confirming that this is so. Just curious if anyone can definitively confirm that it is indeed safe.
(I know the 60w and 85w magsafe power adapters have a different form factor from the and that the Macbook Air can't sit flat on a surface if you use them -- I'm just curious if it's safe for the electronics to use them.)
here's the definitive answer: YES, you can use a higher rated power adapter with your MBA (or whatever) with no ill effect.
see this KB article from Apple: http://support.apple.com/kb/HT2346
the power rating of the adapter is simply the maximum power that it can supply. the voltage of these adapters is all the same (I think around 18.5volts?), so it simply means the higher rated adapters are *capable* of providing higher current if necessary. some of the models require the higher current to charge their batteries, therefore those machines ship with higher wattage chargers. Your MB Air will just draw the same amount of current whichever adapter is used, as long as the minimum wattage is provided.
in the reverse case, if you use too low a wattage adapter (e.g. 45W one on a MB Pro), there won't be enough current to charge the battery, although you can run it off AC power while plugged in.
With some testing, it is definitely safe for the charger if you are charging a macbook that needs more wattage.. The charger will never provide more than the rated wattage, and as such, there is no higher risk of failure.
On the other hand, if you use a 45w on your MacBook Pro, which requires an 85w charger while you are playing a game or editing video (using 100% CPU and/or GPU), your battery will still drain but at a must slower rate. I have read that if you let it reach zero, it'll keep on going and kill your battery as the voltage will go too low.
I personally use a 60w charger with my MacBook Pro Retina which comes with an 85w charger. It charges a tad bit slower but it's perfectly fine. Here are the screenshots:
In this image, I am using my 60w charger while low use (medium brightness, no CPU intensive tasks) but it is charging. It is only pulling 56.04 watts, well within the limits of the 60w charger.
Here, I experimented with a 45w charger while I am stress testing with geekbench and full brightness, as you can see the charger is pushing out 40w, again, well within the limits. On the other hand, it is now draining the battery (but at a slower rate)
So many explanations, but none cites simple technical point. As some mentioned, yes lower rated charger supplying higher load runs risk of burn out. Now, if apple makes "super quality" chargers which are capable of supplying to higher loads (although it would be surprising if thats true, as it increases manufacturing cost for usually unrealised "quality") thats a different story. If my charger is rated 60W I would NOT use it on a machine rated 85W. If someone did and didn't blow up the charger then s/he must be lucky, but its only matter of time before its toast. With that said, if the circuitry employs current limiters then you wont blow up the charger by using it on higher load, but it might well be incapable to charge battery at the rate its draining.
@Gabe, so it falls inline with expectations. It would charge slower. How long have you been using 60W on MBP? Did you ever try using your mac with battery charging from less than 25% while watching flash video with full screen brightness? (the idea is to draw max power).
If you did, and the charger is safe that confirms presence of current limiters which ensures safety. Thanks for sharing your experience.
Another indirect evidence that it is OK to charge your MBA with an 85W magsafe is that the Thunderbolt Display has a magsafe cable that Apple claims can charge up to 85W for either MBP and MBA, suggesting that the display is outfitted with an 85W magsafe that can be used for a lower watt MBA. If that is true, I would deduce that you can use an 85W magsafe adaptor to also charge a MBA. I've just got a MBP and MBA and swap the adaptors depending on convenience (i.e. I carry only the 85W around and use it to charge either laptop), and no ill-effect. Just my 2-cents.
Using a lower power adapter (45W for a 85W laptop) it's not a good idea because the adapter will be forced to work trying to provide more power than it is capable. In specific cases could overheat or get burned.
Otherwise, as explained above on the Accepted Answer, is perfectly acceptable.
This is an answer following the same advice than I've posted here: