Car charger for my MBP?

Discussion in 'MacBook Pro' started by Race Johnson, Aug 6, 2007.

  1. Race Johnson macrumors member

    Jun 13, 2007
    I am drving to Arizona where I will be attending school. Its a long drive and I am thinking about getting a car charger so I can plug my MBP in. Im thinking of something that wont cost me too much. I will only need one plug in port. Will I need a certain wattage?

    Thanks in advance.
  2. Zel macrumors regular

    Jul 7, 2007
    The MBP adapter outputs 85W and normally adapters are 70 to 85 percent efficient, so if you get an AC inverter it will need to supply 85/0.7= 122W.

    A DC Direct connection would be more efficient for everything involved, but good luck finding a mac specific car adapter.

    Personally, I'd juice up before leaving and at motels and just live without a computer for a few hours... you are the driver, right?
  3. jsw Moderator emeritus


    Mar 16, 2004
    Andover, MA
    I agree - I used a 125W inverter without any issues on a previous trip, and the built-in one on my minivan (which I think is also 125W) on a 2800 mile roundtrip.

    There are no magsafe car adapters that I know of, and the Apple one is for airplanes only.
  4. clickclickw00t macrumors regular

    Jun 28, 2007
    Isn't 85W * .7 in the 60watt range?

    I say anything in the 90W range should be fine.
  5. Zel macrumors regular

    Jul 7, 2007
    It's algebra... 85 = 0.70 * X

    read: seventy percent of X is 85.

    so divide 0.7 from both sides, you get 85/0.7=122=X
  6. clickclickw00t macrumors regular

    Jun 28, 2007
    I could be wrong, but I read it as

    If a magsafe power thingy is 70% efficient, that means from the 85W it takes in, it only outputs about 70% of that. Therefore:


    again, i could be wrong, but this is how I see it.
  7. Race Johnson thread starter macrumors member

    Jun 13, 2007
    I have a 320 watt adapter already, with a plug in where i can plug my charger into... will that work?
  8. Zel macrumors regular

    Jul 7, 2007
    Yeah, it's easy to confuse the two, the 85W is referring to how much gets over to the laptop side, not how much is drawn on the AC current side. Edit: also keep in mind that the 70 percent figure is pretty randomly chosen, only the electrical engineers at apple know the efficiency of these adapters ((and anyone with a wattmeter...)) but 70 was the lowball estimate for normal power supplies.

    Sure thing, that's more than enough power. Drawing that much power might make your gas mileage sink though, so treat it like Air Conditioning, dont use it when you dont have to.

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