PSA: Using a 60W MagSafe Charger with rMBP 15" can actually drain battery

Discussion in 'MacBook Pro' started by jamesjingyi, Dec 7, 2015.

  1. jamesjingyi macrumors 6502a

    jamesjingyi

    Joined:
    Dec 20, 2011
    Location:
    UK
    #1
    So today I was doing some pretty CPU intensive stuff on my MacBook and only had the 60W adapter with me. My MacBook was plugged in and supposedly charging (it had a green light at the start). Then I noticed as I progressed through the hours, the laptop actually started to say that it needed charging. The MacBook then drained down to about 20% before I stopped using it and let it charge again. I'm guessing this isn't any good for the battery but does mean that I can still use the laptop for a longer time than I was before. The charging indicator showed a charging time of 20:00 (20 hours) which is ridiculous. I think this is normal behaviour as the dGPU and other aspects of the machine obviously use more than the 60W provided and require more, which the 85W will provide. The 85W is not just included for a faster charge but actually out of usability. I'll have to purchase more now!

    (This was probably discussed way before me in multiple threads but just thought that I'd share my experience for those who didn't know) Screen Shot 2015-12-08 at 01.06.14.png
     
  2. tylamb19 macrumors member

    Joined:
    Dec 27, 2014
    #3
    This is due to the way that your power adapter and battery work together to provide the power for your system. Actually, even an 85W power adapter is not enough to power the 15" MBP while running at 100% CPU and 100% GPU usage.

    Since the CPU's TDP (TDPs are assuming you are using a 2.5 GHz 2015 Retina) is 47W and the GPU's TDP is 42W, that gives a combined wattage of 89W. Obviously, this is higher than the 85W of the charger, even when not including screen backlight and other overheads like fans and lights. Even though this is the case, instead of shutting down or throttling, your machine will actually use the battery at the same time as the charger to provide more power.

    This also is the cause of the immediate CPU throttling when the battery is removed from the system completely. By throttling the system to half speed when the battery is removed, you will not overload the charger.
     
  3. maflynn Moderator

    maflynn

    Staff Member

    Joined:
    May 3, 2009
    Location:
    Boston
    #4
    Yep, this is why if you remove the battery (if that is even possible on current MBPs), it will only run at half speed. The power brick doesn't really power the computer any longer (for the most part) it just recharges the battery as the battery runs the computer.
     
  4. jamesjingyi thread starter macrumors 6502a

    jamesjingyi

    Joined:
    Dec 20, 2011
    Location:
    UK
    #5
    That's really interesting, why doesn't Apple include a 90W adapter instead? I suppose its probably because 99.9% of the time nobody will be running the laptop in these conditions, and if so, not for a long time.

    So if my battery is at 0%, the computer will run at half speed as well? Really interesting design choice.
     
  5. maflynn Moderator

    maflynn

    Staff Member

    Joined:
    May 3, 2009
    Location:
    Boston
    #6
    If your battery is at 0%, it probably won't even boot up, until there's sufficient charge.
     
  6. tylamb19 macrumors member

    Joined:
    Dec 27, 2014
    #7
    Yep, your computer will throttle back (not to half, but it still will) until your battery is sufficiently charged. If your battery is in a "service" or "replace" condition, it throttles as well. My personal theory on why Apple doesn't include a more powerful adapter is just to keep the looks good and size/weight down for said adapter. If you have seen some of the older IBM ThinkPad 120W adapters, you will know what I'm talking about...
     
  7. snaky69 macrumors 603

    Joined:
    Mar 14, 2008
    #8
    It keeps the power adapter small and lightweight.

    I have a work supplied windows computer, and the 200W adapter on this thing is a brick.
     

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