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(This is similar but doesn't address the fire hazard issue.)

I have a MacBook Pro with an 85W power adapter. My wife has a MacBook with a 60W power adapter.

We charge both computers with both adapters. Of course the MacBook Pro charges more slowly from the 60W adapter, but otherwise it's fine.

However, according to this comment, using the 60W to charge the MacBook Pro is a fire hazard!

Is this true? I am surprised Apple engineers would have made them interchangeable if this is the case.

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good question...+1 – studiohack Apr 22 '10 at 19:54
up vote 5 down vote accepted

It's not a fire hazard. In the technical documentation, if you plug-in a low-powered charger with the high powered macbook pros they will use the trickle-charge mode instead of the quick-charge (high power) mode.

Handling this kind of thing is part of the design spec.

It's not the best for the battery life to always charge on trickle-mode, but in a pinch it'll do.

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The wattage indicated on the adapter indicates the maximum output. If you attach a device that demands less, it will use less. (Actually, the voltage remains the same, but the amperage is less.) This is why you can find universal notebook adapters that only have the maximum wattage indicated. – Paul Lammertsma Apr 22 '10 at 18:58

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