Battery Wattage vs. Charger Wattage

Discussion in 'MacBook Air' started by RandomQuestion, Jul 26, 2013.

  1. RandomQuestion macrumors newbie

    Jul 26, 2013
    Why is it that the battery wattage is higher than that of the charger for some MacBook models.

    1) Would the computer charge faster if you were to use a higher wattage charger?
    2) Would the charger not overheat, since its not constantly working at full load?

    Note: In terms of physical dimension Smallest<45W<60W<85W<Biggest. The 45W may be supplied with the MacBook Air simply due to the fact that its more portable.

    MacBook Air 11"
    38-watt-hour lithium-polymer battery
    45W MagSafe 2 Power Adapter

    MacBook Air 13"
    54-watt-hour lithium-polymer battery
    45W MagSafe 2 Power Adapter

    MacBook Pro 13" Retina
    74-watt-hour lithium-polymer battery
    60W MagSafe 2 Power Adapter

    MacBook Pro 15" Retina
    95-watt-hour lithium-polymer battery
    85W MagSafe 2 Power Adapter

    MacBook Pro 13"
    63.5-watt-hour lithium-polymer battery
    60W MagSafe Power Adapter

    MacBook Pro 15"
    77.5-watt-hour lithium-polymer battery
    85W MagSafe Power Adapter
  2. scaredpoet macrumors 604


    Apr 6, 2007
    You're comparing two different units. A watt is a unit of energy, while a watt hour is how much of that energy can be delivered by the storage cell over a set period of time.

    So, a 54 watt hour battery isn't delivering 54 watts to a macbook air. If it did, not only would it get really hot, but if the MBA didn't burn out completely first, the battery would be completely dead in about an hour. On the other hand a 45-watt AC adapter is just that... it can deliver up to 45 watts over an indefinite period of time, so long as it's connected to a power source.

    I haven't actually measured whether a Macbook Air will charge faster with a higher-wattage AC adapter, but it's unlikely an MBA will ever consume a full 95 watts, even if connected to a 95 watt adapter.

    The reason for the differences is probably size like you said, but also cost (cheaper to make lower-power ac adapters when a higher wattage isn't needed).
  3. filmbuff macrumors 6502a


    Jan 5, 2011
    A Watt is a unit of power, a Joule is a unit of energy. But otherwise you're right ;)
  4. Jefe's MacAir macrumors 6502

    Nov 21, 2010
    Would a higher watt rated charger charge a MacAir faster? Could or would it damage either the battery or the computer?
  5. RandomQuestion thread starter macrumors newbie

    Jul 26, 2013

    From my understanding, Mac's only draw as much power as required from the charger.

    So if you charge your MacBook Air with a 85W charger, your Mac will only draw as much power as needed. Thus not damaging your Mac (the Mac draws power vs the charger pushing power).

    But question is: what happens when you charge a MacBook Pro 15" Retina with a 45W charger... besides charging slower. Doing sp repeatedly will the charger overheat and die sooner than later?
  6. scaredpoet macrumors 604


    Apr 6, 2007
    I've actually done the above. What happens is, the battery won't charge while the MacBoook Pro is turned on, but will charge, slowly, when the MBP is off. During high CPU loads, the MBP might also draw a small amount of power from the battery if it has a charge, to supplement the low power AC adapter. If the battery is dead, the CPU will drop to a low speed and refuse to go full power.

    So, basically the Mac and AC adapter will make do with what they have. The power consumption won't go beyond what's available.
  7. throAU macrumors 601


    Feb 13, 2012
    Perth, Western Australia

    Depending on what you are doing on your MBP, you may not get any charging at all. For example, if you're running a game that cranks up both the GPU and CPU, they may be consuming 90 watts. You'll simply drain your battery at half speed.

    The charger will still only supply 45 watts....
  8. scaredpoet macrumors 604


    Apr 6, 2007

    I just tried this. My MBA's battery doesn't charge any faster when hooked up to an 85W Magsafe charger (with MagSafe 2 adapter) from my old MacBook Pro, as opposed to the 45W AC Adapter that came with it.
  9. gnasher729 macrumors P6


    Nov 25, 2005
    If you use a 25 Watt light bulb, it draws 25 Watt of power. If you use a 150 Watt light bulb, it draws 150 Watt of power. The MacBook Air will use a certain amount of power and not more. If you use a higher watt rated charger, it will not draw more power. It will not damage the MBA or the charger, but it won't charge faster either.

    The other way round, if you used the MBA charger to charge a 15" MBP, depending on how carefully the charger is designed, it will either not supply the power you want, or it will get really hot, beyond what it is designed for, and the life of the charger will be reduced.

    A fully charged 15" MBP connected to the MBA charger would be no big problem, because on average it doesn't draw that much power. Charging while turned off is worse. Charging _and_ using it would be the worst case and should be avoided.
  10. gianmz macrumors newbie

    Aug 22, 2013
    Check this out, it was very interesting and i think it may answer your questions:

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