using 85w power brick on other MBP's

Discussion in 'MacBook Pro' started by Davidsilence, Apr 29, 2010.

  1. Davidsilence macrumors member

    Oct 3, 2005
    Brighton UK
    Iv'e noticed that the new MBP's now come with a 85w power supply last years model used a 60w. We have a few macbook pro's here and are constantly moving around our space and jacking into each others supplies. Will using a 60w on the new model or the 85w on the old model be detrimental to our machines.
  2. Jaro65 macrumors 68040


    Mar 27, 2009
    Seattle, WA
    The 60W supply won't be able to simultaneously run and charge the machines that came with 85W power supplies. The machines will run fine, but won't get charged up. I have the same issue at home. You're good otherwise.
  3. matttung macrumors member

    Apr 17, 2010
    I was thinking the same a few days ago -

    But from what I read on forums, the 85W charger will work for both 85w and 60w models without problems, as the macbook will only draw the amount of charge it 'needs'.

    Don't use the 60w charger with your 85w MBP, as it has insufficient charge to power your computer~
  4. alphaod macrumors Core


    Feb 9, 2008
    The MacBook Pros have been using 85W adapters for a while now; it was only recently with the 13" integrated into the MacBook Pro line as well at last generation 15" being integrated graphics only that we saw the MacBook Pro use 60W adapters.
  5. designgeek macrumors 65816


    Jan 30, 2009
    I've used Macbook chargers on my MBP and for some reason it will charge my machine. :confused: I believe there was no way this is at all possible but I plug it in and it says "Not Charging" at first but then starts charging my machine. I just let it do its thing despite the fact that I don't understand it.
  6. alphaod macrumors Core


    Feb 9, 2008
    If your computer is not using full power, the 60W can provide enough current to power and charge the computer batteries; however should your computer use full power, the 60W charger will not be enough and any additional power will be discharged from the battery.
  7. iThinkergoiMac macrumors 68030

    Jan 20, 2010
    Do NOT use a 60W adaptor with a machine designed for an 85W one! The MBP has no way of sensing the max amount of wattage a power brick can provide and will attempt to draw the full 85W (if need be) from it. I don't know how Apple's bricks are set up, but this will eventually burn out most bricks, since they will attempt to provide the amount of wattage needed, even if it's over 60.

    As others have said, using the 85W brick with a machine designed for 60W is no problem, it's just highly recommended that you don't do it the other way around.
  8. FearlessSpiff macrumors member

    Apr 16, 2010
    Highly recommended by you? 60W is all it will deliver. It won't burn out. The MacBook cannot "request" more power. This is nonsense. All that will happen is that the MacBook will be charged slower or not at all.
  9. miles01110 macrumors Core


    Jul 24, 2006
    The Ivory Tower (I'm not coming down)
    This has been argued before, and nobody's been able to provide any evidence that this is actually a problem.
  10. m85476585 macrumors 65816

    Feb 26, 2008
    Yes it does. Apple isn't stupid-- they wouldn't design something that will burn out if you don't carefully compare the specifications of the charger you are using to the nearly identical looking one you had. If there was something you shouldn't do, they would tell you not to do it. Otherwise they assume that you will use the product how you want. They design stuff that "just works".

    I don't know how the charger communicates with the computer. It most likely sets a voltage on the center pin that tells the computer how much power is available. There could also be a resistance between some pins. There could even be serial data sent, but that's less likely. The point is there are simple ways for the charger to tell the computer how much power is available. Maybe when I get a chance I'll try to figure out the sense mechanism.

    This is a bit off topic, but serial data over the magsafe gives me an idea: why not run USB over the magsafe too, that way one plug would be all that's needed to connect the computer. They could put a USB hub on the magsafe brick which devices could be plugged into.
  11. cluthz macrumors 68040


    Jun 15, 2004
    My old SR MBP had the 85W adapter and my room mate had a MacBook with 60W. We exchanged adapter all the time, no problems what so ever.
    Only issue was that if i used the 60W one, it took about twice as long to charge.
  12. drambuie macrumors 6502a

    Feb 16, 2010

    If the CPU gets insufficient operating current, it will either downclock to match the available power, or go into an idle state. Apparently Dell had a bit of a problem with this when it introduced the i7 720QM quad CPU laptops. The CPUs were downclocking when on AC power, (okay when on battery), and it was traced to the 90 watt power bricks being supplied with those laptops. A 120 - 130 watt supply is needed for a system with an i7 720QM or 820QM, and a discrete video card. Those i7s have no integrated video. When the bricks were upgraded to 120 or 130 watt units, the CPUs clocked at normal frequencies.

    Besides the heating and battery life issues, the increased power requirements are probably part of the reason why the new MPBs don't have quad CPUs. The magsafe connectors would need to be redesigned with additional pins to handle the additional current.
  13. m85476585 macrumors 65816

    Feb 26, 2008
    The MBP will draw power from the battery if there is not enough power for the CPU. It's theoretically possible that you could have the Magsafe plugged in and the battery will continue to drain (though more slowly than fully on battery power) if the demand exceeds what the Magsafe can put out. Someone with a 45W MBA magsafe might be able to try it out.

    If you try to run your MBP without a battery present, it will clock the CPU down to 50% (even with the correct Magsafe).

    Edit: The 720QM and 820QM are still on a 45nm process. I wonder if Intel plans to shrink them to 32nm, and if that would save enough power to be able to fit them in a MBP in the near future?
  14. scaredpoet macrumors 604


    Apr 6, 2007

    Experience (both my own and others on here) indicates that this is not the case with 60W magsafe AC adapters with MacBook pros.

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