Are 60w & 85w power adapters safe to use on MacBook Air?

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)

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.

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.

Chandrakanth.

Update

@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.

-Chandrakanth.

How to Locate the Power Rating?

To find the right charger for Macbook ,there are three kinds of power rating Macbook power adapter in the market,45 watt, 60 watt, or 85 watt. It should be noted that higher wattage chargers can be used on the lower wattage laptops, but not vice versa.For example,If you have mac laptops that have used all three wattages. you could buy the 85w ones as my back up. Why? Because the 85w will work on any laptop, but the 45 and 60 will not. Further, the cost difference between the 45w, 60w and 85w is virtually nothing so you might as well “go big” and never worry if you have the right adapter for the right laptop.You can also find the Power Rating details on your orignal macbook charger and get the same Power Rating as your origanl macbook charger .

For More:http://goo.gl/1rNhIA

85 W may use dynamic negative resistance, but be stable with load to provide variable voltage. Can use 60 W on 15" MBP which may be constant voltage, but battery will run down if using discrete display. Discrete display is required for external monitor or projector.

A related issue - these macbook power supplies are 'class 1' They have two parts: the supply itself and a lead/plug that connects it to the mains/wall socket. The slotted bit that slides onto the supply has metal sides that make the earth connection to the round metal stud on the supply. That's fine however ipad/iphone power supplies use the same connection method except ipad/phone supplies are class 2 and don't have an earth. On the mains plug part that comes with these the slot is all plastic ie no earth connection is possible. The above macbook supplies will fit these just the same. They need an earth but they won't get one!!!!