Slightly stupid question… (2013 MBA)

Discussion in 'MacBook Air' started by Lexical, Jun 20, 2013.

  1. Lexical macrumors regular

    Joined:
    Nov 8, 2011
    #1
    I've just order an 11" base MBA for portability so i can easily study and i just wondered…

    I already have a 15" rMBP, what would happen if i plugged in the magsafe from my rMBP into my new MBA? Would i blow up / cause it damage etc? Or would it just charge faster?

    Very excited to see these new MBA's and having something so portable will be great.
     
  2. waw74 macrumors 68030

    Joined:
    May 27, 2008
    #2
    http://support.apple.com/kb/HT2346

    but your MBA adapter will not have enough power for your rMBP
     
  3. Lexical thread starter macrumors regular

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    Nov 8, 2011
    #3
    Ah ok, thank you for that. Thats good then as i can just use my rMBP cable to charge it when i need too.

    Hopefully i won't find it to terrible looking at a small non-retina display after using the rMBP!
     
  4. falconeight Guest

    falconeight

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    Apr 6, 2010
    #4
    You will! I did but after a couple of days it went away.
     
  5. luisito macrumors regular

    luisito

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    Nov 15, 2012
    #5
    The higher wattage adapter will only get VERY hot when you use it with your MBA. Since it has to dissipate more energy to the ambient to compensate for the lower wattage requirement.

    Even using the proper wattage adapter gets hot, I can't imagine how hot a higher wattage adapter could get.

    In short, yes, it will get the job done, it will charge it faster, but it will get hotter than normal.

    I said the word hot way to many times.. oh dear..
     
  6. Mikael macrumors regular

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    Aug 4, 2005
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    Gothenburg, Sweden
    #6
    The dissipated heat is simply a function of the output power and the efficiency of the power supply at said output level. Unless the rMBP adapter's efficiency is much worse at the MBA's load level, the beefier adapter will not get any hotter.
     
  7. luisito macrumors regular

    luisito

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    Nov 15, 2012
    #7
    Thus, the adapter will get hotter.

    The adapter for the rMBP is efficient for the rMBP only. That is why Apple uses different wattages for each of their models, so they can engineer specific adapters based on how hungry the devices are, all adapters are based on efficiency.

    Otherwise they would only sell a "standard" adapter for all of their models. Otherwise they wouldn't make the fine print saying how much wattage is appropriate for a specific model.
     
  8. Mikael macrumors regular

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    #8
    Of course they make a specific model based on actual wattage. They do that because they don't want to make unnecessarily large adapters for small and less power hungry machines. It's also based on manufacturing costs, since a higher power adapter uses more expensive components.

    Anyway, I've not heard of a switching power supply where efficiency drops offs so sharply that actual dissipated power increases at just ~70% of max output. Do you have a link to where this has been specified or tested? Otherwise, I have to wonder where you're getting this information, as I find it peculiar that Apple would ship such a bad switching PSU with their Mac Books.
     
  9. luisito macrumors regular

    luisito

    Joined:
    Nov 15, 2012
    #9
    Lol. Really?

    Like I said before, that is why Apple ships out a particular adapter for a particular device. It becomes much less efficient when you use the product on a different manner specified by the manufacturer, such as swapping adapters with different wattage.

    Why don't you try using the MBA's adapter to power up a rMBP? You will find out that the efficiency is so bad, that it won't even have enough power to feed the rMBP wattage needs due to its low wattage, reason why it was specifically design for the MBA. The same happens when you overfeed a smaller device with a high power adapter, the power not being used has to go somewhere, where you may ask? To the ambient, that is the purpose of the adapter, to dissipate the extra electrical energy into heat.

    :)
     
  10. Lexical thread starter macrumors regular

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    Nov 8, 2011
    #10
    So ummm… Having received my 11" MBA this morning, i have a tiny yet annoying problem…

    The laptop doesn't appear to sit flush with the desk when its open? The bottom left corner seems to be slightly raised so when i rest my palm on that bit it knocks against the desk ever so slightly. However when its closed it seems ok? Is the 11" prone to this because its a bit top heavy?

    ----------

    Oh wait it does it when its closed as well just in the top right corner….

    Ughhh thats so annoying! I don't want to send it back through the mail i bought it online through Apple. Any suggestions?
     
  11. falconeight Guest

    falconeight

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    Apr 6, 2010
    #11
    You can take it into a apple store.
     
  12. Lexical thread starter macrumors regular

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    Nov 8, 2011
    #12
    The ridiculous thing is that i cant. I just phoned them and spent almost an hour arguing about it. They wouldn't let me return it and get a new one from my local Apple store it had to be returned via courier because i bought it from Apple online. No matter that i work monday-friday and can't be in for them to pick it up and i took today off work so i could get this one delivered.

    After much arguing they finally agreed to arrange pick up and re-delivery in the evening. But how stupid is it that they wouldn't just let me go into an Apple store and get another one? As i kept pointing out its all the same company. They claimed they didn't have the ability to accept stock back into the store i wanted to go… What a load of crap.

    Anyway, they already ordered a new one for me to be delivered on Tuesday. What a waste of time for both parties.
     
  13. Mikael macrumors regular

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    Aug 4, 2005
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    Gothenburg, Sweden
    #13
    Really? The age old misunderstanding that a PSU always consumes its rated power and that any "unused" power is dissipated as heat? I suggest you go read up on how PSUs work before spreading your "knowledge" on forums. I'm done here.
     
  14. dingaling007 macrumors newbie

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    Jun 15, 2013
    #14
    Im sorry but your wrong. You cant "overfeed" a smaller device with a high power adaptor (same voltage of course). The smaller device will pull what it requires and the high power adaptor will not have to work as hard to provide that smaller wattage. They dont have to run at %100 maximum wattage you know? Switching power supplies are pretty awesome. Maybe the old heavy transformer power supplies are what you are thinking of?

    Put it on a watt meter and see the watts vary depending on laptop load, after the battery is charged, the adaptor is only going to register maybe 5watts at idle, and will be cool to the touch.

    Its the same as a 12v car electrical system the 12v battery can supply alot of amperage, but devices will only draw what they require.

    As others have said apple make different adaptors for different laptops with different power requirements (business sense).
     
  15. luisito macrumors regular

    luisito

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    Nov 15, 2012
    #15
    The reason an iPad or an MBA would charge much faster when not using the provided charger that came with them is because the adaptor is overfeeding them. The battery is in a state of a supercharging mode. I think the iPod and the iPad doesn't even come with adapters, and Apple sells those 5 and 15 watts adapters I think, to charge them on the go and much, much faster.

    Don't you agree?

    Lol. At this point I think we are completely talking about two different things.
     
  16. dingaling007 macrumors newbie

    Joined:
    Jun 15, 2013
    #16
    The li ion batteries in mba's, ipads etc are very strictly controlled so they can be allowed to charge faster upto a limit depending on a range of factors such temprature. The mba/ipad etc charging circuit manages all this and may tap more power if its available from the psu.
     
  17. paulrbeers macrumors 68040

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    Dec 17, 2009
    #17
    You know nothing about power supplies. A power supply has a maximum wattage/amperage. A device will only consume as much power as It is rated for. A power supply does not supply it's entire load if the device connected to it does not want/need it. This is power supply 101. Apple specifically states that it is okay to use an 85watt power supply for all of their laptops: MBA thru 15" MacBook Pro. This is because the air will simply draw 45watts (or less depending on what you are doing) and the Pro will draw up to 85. the power supply will give the correct wattage. It will not over power and it won't give off the excess power as heat. It simply will not draw the power at the wall. If the MBA asks for 45watts, then the power supply will simply draw 45 at the wall. My 85watt adapter actually runs cooler with my MBA on it than my wife's MacBook Pro because it is converting less energy. PSU 101!!

    This is why I only buy 85watt power adapters for my house to simplify our charging needs.

    This se principal is why you can charge an iPhone with an iPad charger. Oh the iPhone will not charge any faster on an iPad charger because again the charger only supplies the amount of power that the device is designed for.
     

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