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17" Magsafe Adapter:

Input: 100-240V - 1.5A 50-60Hz

Output: 16.5-18.5V - - - 4.6A Max

13" Magsafe Adapter

Input: 100-240V - 1.5A 50-60Hz

Output: 16.5V - - - 3.65A

The Answer:

I am looking for a yes/no answer here, but most importantly, I want to know why it's (not) ok.

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up vote 6 down vote accepted

As the voltage is the same, you should have absolutely no problem using it. Note that you should only use a power adapter that is spec'd higher.

So this is fine: 17" adapter on 13" model.
This is not: 13" adapter on 17" model.

See also this Apple FAQ:

Although you should always use the proper wattage adapter for your Apple portable, you can use an adapter of a higher wattage without issue.

For instance If you have a MacBook (13-inch Late 2009) that normally uses a 60W adapter, you can also use an 85W adapter with that computer. You would not use a 45W adapter with that computer; it would not provide enough power for that MacBook. Using an adapter of higher wattage than the adapter that came with the computer will not cause the computer to charge more quickly or otherwise operate any differently than using the adapter that came with the computer.

I think there's not much to add to this. From my own experience however it seems that you can still use a lower wattage adapter and your Macbook will work. I sometimes used charged my 15" on a smaller 13" adapter and didn't run into any notable issues, but who knows. It will probably not charge that fast. The fact that you have a battery also means that the Macbook won't just power down. For the sake of not voiding the warranty, you should just follow those guidelines.

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Pretty good answer, but I think you should edit it to make it clear that there's nothing harmful about using a lower-wattage MagSafe adaptor on a machine that needs a higher-wattage one. The lower-wattage one won't harm anything, it just won't charge the machine as fast, if at all. It may just slow the rate of discharge if you have it plugged in during use. I have often grabbed an under-watted MagSafe adaptor in a conference room to "get me through" a meeting if I accidentally went to a meeting with a nearly-flat battery. – Spiff May 21 '11 at 18:08
@Spiff Yup. I did that just a sec ago :) And yes, I also switch adapters like crazy and haven't had any issues whatsoever. – slhck May 21 '11 at 18:09
I can emperically verify this: I used a 85W MBP charger on a Macbook 2,1 (white, late 2006) and on a Macbook Pro 8,1 (unibody, 2011) and have not had any problems. – Tim Apr 9 '12 at 12:20

"Why" is because both chargers put the same voltage across the labtop's charging circuit. The laptop will draw the current it needs. That the charger is capable of supplying more is irrelevant. In the reverse case (using a less capable charger), the charger could overheat. From what spiff says, I'd guess the Magsafe chargers have current limiters in them to prevent that, but I wouldn't otherwise assume it.

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I absolutely ruined the battery on my MacBook Pro 17" early 2007 by using a lower voltage power supply. I just bought a new battery. It is now at 14 cycles. Two of them - 8 and 9 - were with a lower voltage power supply.

My newer battery as well is completely ruined.

It is not necessarily fine to use a lower voltage power supply!

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Did you mean wattage, not voltage? – bytebuster Nov 12 '12 at 8:20

as long as the VOLTAGE is correct, or slightly higher, there is no harm to using any adapter that has a lower or higher wattage.

using one with a LOWER VOLTAGE can cause problems.

the proper voltage is BEST, with spec WATTAGE or higher

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Using an adapter with too low of a wattage can damage the adapter. – macek Oct 6 '14 at 4:04

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