I have a 2011 15" MBP, that uses a 85W power supply. My old 2008 MBP 15" also has a 85W power supply, but I cannot get it to work. Shouldnt it? Its the same watt power, and the adapter fits.
The way Apple power adapters are designed is that you can use any charger on any laptop that has the same, or lower, power rating. For example, you can safely use an 85W power adapter (for MacBook Pro) on a MacBook Air (which requires only 45W) or MacBook (which requires 65W).
However this does not work the other way around, you cannot use a MacBook Air (45W) adapter to power a MacBook Pro.
A 2008 MacBook Pro adapter rated at 85W should power a 2011 MacBook Pro with no problem. If this is not the case, then the adapter is faulty.
Apple sells and markets the Cinema Display with an 85 watt adapter and it's an accessory for all MacBook whether your portable shipped with an adapter of 45 watts, 60 watts or 85 watts.
Using your higher rated adapter with a Mac that draws less power is safe to do. Going the other way is also safe - but the Mac will either not charge, charge slowly or work fine depending on how much power it needs from moment to moment.
After 2 years my Macbook Pro battery life has seen better days. It will only last 2 hours max. However, I recently visited friends and forgot my 85W power supply. Instead I charged up with the 13" Macbook 60W adapter. Yes, it took longer to charge all the way. However, I found a seemingly rejuvenated battery - with life extended to 4-6 hours depending on what applications I'm running.
To test this phenomenon, I have since tried multiple charge cycles with both a 60W and 85W adapter. Sure enough, every time I used the 60W charger I had a much longer battery life. And when I switch back to my brand new 85W power supply, I'm back to about 90 minutes battery life.
I've discussed this with several physicist and engineer friends, who can't offer an explanation. I assumed that the battery life was running down with time (even though I maintain proper battery training protocol) but I can't argue against the reality that using a 65W adapter has yielded a bigger charge and longer life. Go figure?
I have only just found out that adaptors are universal. I saw the new shape attachment on my MacBook Pro and thought it would not fit my old Macbook. The old 65W adaptor works well on my upgraded 2011 Macbook Pro and vice versa. The battery life is good. I'm so happy to discover this.