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My girlfriend has a Macbook Air. The power supply is smaller and lighter than the Pro. It has an output of 14.85V and 3.05Amps. My Pro Retina has a power supply that is 20V and 4.25 Amps.

Both supplies have the same Magsafe adapter, so the power supplies seem interchangeable at a physical level, but what would happen if one of us plugs in using each other's adapter?

  • Could damage result from under-powering my MacBook Pro? (like a brownout)?

  • Could damage result from over-powering her Air? (like a power surge)?

  • Will the Macbook Air charge faster with a Pro adapter?

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For what it's worth, I've got a 2012 13" MacBook Pro (non-Retina) and my girlfriend has a 2011 MacBook Air. I've used her charger a lot and she used mine, no issues so far (4 months in and battery charge capacity is the same). My MacBook Pro takes longer to charge with her charger, though. –  bogdansrc Nov 17 '12 at 17:17

5 Answers 5

up vote 7 down vote accepted

You can safely use the 85W adapter on any Mac, it will only draw as much power as it needs. The 45W adapter from the MacBook Air may just about manage to power your MacBook Pro but it won't charge it. It will not damage your Mac, but you may notice the battery may still deplete when using it.

Here is the information, straight from Apple:

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1  
This is the accepted answer, so most will only read this one, therefore I feel the need to mention as per my own answer that using a lower rated supply from say an Air to power something with higher requirements like an MBP, is potentially bad for the charging unit (not so much for the MBP) as it will eventually burn out from running at 100% capacity. –  stuffe Apr 4 at 13:13
    
"Eventually burn out" is a fairly vague term. I imagine that the OP will only use the MBA charger once in a while in a pinch to power the MBP as has been noted, the MBA charger isn't powerful enough to run and charge the MBPs battery. I do this as well when I bring my MBP home from work and use my wife's MBA charger. If I don't tax the MBP, I find that it does charge slowly.. –  Ashwin yesterday

You have 2 scenarios as I see it:

1) Using a higher rated power supply than the original unit.

This is absolutely fine. The power supply will never just automatically run at 100% of it's capability, it will only supply what is requested. If you use a Macbook air that is supplied with a 45w adapter with an 85w adapter instead, it will still only draw what it needs, which is likely a whole lot less than the rated capacity of the charger anyway, which is rated to be able to not only keep the computer going at full bore if required, and also charge the battery at the same time.

2) Using a lower rated power supply than the original unit.

Chances are this is fine most of the time but there are more risks. The same rules apply, it will draw the power that it needs, and no more. Assuming you use a MPB on a MBA charger, then light use will likely never draw enough power to max out the charger. If it's in sleep and just charging it will be fine, however the chance is there that if you thrash it it will start requesting more power than the MBA charger can safely provide. There are 2 points to note here:

  • Although it isn't stated in obvious terms, the 45w (or whatever) rating shown on the power supply isn't necessarily accurate, it is in fact the guaranteed minimum continuos rate that it can supply. Changes are that it can supply more, say (depends on the quality of the unit) +10%.

  • A high quality unit (such as an official Apple unit, not an eBay Hong Kong special) will have various safety measures to prevent all sorts of things, from overheating, over current, etc etc, and the worse you can except is the unit to fail gracefully with protected systems. Get a cheap unit made from chicken wire and chewing gum though, and you are asking for a fire.

As for charging speeds, the Air likely wont charge any faster at all, but the MBP may charge slower (or, like an iPad running off an iPhone charger, not at all when in use and merely provide enough power to keep the device going).

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If the magsafe plug (1, 2 or whatever) fits then use it. I can assure you there would not be any problem except for the slower charge rate. It wont burn anything. Apple's engineering is topnotch and they wont design the magsafe plug to fit if something will go wrong. You can count on that. Bogdansrc has even attested to it already.

Never mind the wattage (45W, 60W or 85W) the load or the apple unit being charged will vary the voltage and ampere depending on the processing tasks on hand. As Nike says Just do it" Cheers !

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There is little risk of physical harm occurring if you use her adapter on your computer. You will likely just get a slower charge since there is less voltage going across the line. There is always greater risk of using a higher powered adapter, though. Current MacBook airs use a 35 Watt power supply, whereas MacBook Pros use a 60 W power supply. I'm just throwing those in as an addition to the amperage and voltage specs that you reported. Bogdansrc is not reporting problems doing this, but I would air on the side of caution in this case. The last thing that you want to do is replace a fried logic board when you could've avoided it. I have, in the past, fried an external HDD by using a 1.5 mA supply when it called for 1.0 mA.

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Interesting. What would this mean for the Thunderbolt Display? It has a MagSafe connector and appears to be intended for all Mac types...Pro/Retina/Air ... –  makerofthings7 Nov 17 '12 at 20:37
    
Good question. I'm not sure. My best advice would be to treat it like finding wild mushrooms: unless you're absolutely, totally certain, about what you're doing, leave it alone. Like I said, I've been adventurous before, and later wished that I hadn't been. –  soxman Nov 17 '12 at 20:49
    
I believe this answer to be wrong, it's a misconception that using a larger PSU is dangerous, the opposite is true. The important element is the device using the power. Imagine a home made PC with a 1000w power supply in it, powering just a motherboard and some RAM because you have yet to fill it with hard disks and graphics cards - perfectly safe. However try to run a PC stuffed with power hungry cards like that on a 250w supply, and you can expect major issues. –  stuffe Nov 17 '12 at 23:23

-Will the Macbook Air charge faster with a Pro adapter?

Don't ever do this, it could permanently damage MBA.

-Could damage result from under-powering my MacBook Pro? (like a brownout)?

Actually, there's a few things interesting with how power system work on MBP.

As far as I know, my Retina MBP runs at 12392 mV (12.392 V) while I'm just browsing, and bumps up a lot when I'm gaming. So, it is possible to give it only 14 V, but it will charge extremely slow, and "Please Don't Try That If You're Ready To Take Risk! More Importantly, Don't Blame Me For Any Damage!"

Also, I want to mention one thing, that is the MagSafe Airline Adapter:

http://store.apple.com/us/product/MB441Z/A/apple-magsafe-airline-adapter?fnode=5a&fs=m.manufacturer%3Dapple%26m.productKind%3Dadapter

This adapter by Apple connects to Airplanes, and they are basically one end MagSafe, another end this EmPower thing:

http://en.wikipedia.org/wiki/EmPower_(aircraft_power_adapter)

It's 15V, and at the end of that Wikipedia page it reads,

Apple offers an EmPower Magsafe power adapter for their MacBook, MacBook Pro, and MacBook Air lines of notebooks, and also includes a cigarette lighter socket adapter. However, this system only runs the computer and will not charge the computer's battery, and Apple indicates that users should not plug this device into a car's cigarette lighter outlet.

So my guess is, it will work, but not well.

It's ultimately your choice, and if I were you, I'll test it secretly on my friends' laptop.

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By the way, on Apple's product page, it says that MBA power supply is only for MBA. So... –  Shane Hsu Nov 23 '12 at 17:08
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Per the link in Justsomeguy's answer: "Although you should always use the proper wattage adapter for your Apple notebook, you can use an adapter of a higher wattage without issue." (emphasis mine) Please don't offer FUD without citing some source that backs it up. In this case, Apple contradicts your warning. –  Dan J Nov 23 '12 at 17:11
    
@DanJ On the bottom of the 85W product page, it reads, "Compatible with MacBook Pro with Retina display." So at my personal point of view, I don't really think it's a good idea. –  Shane Hsu Nov 23 '12 at 17:20

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