Car charger for my MBP?

Race Johnson

macrumors member
Original poster
Jun 13, 2007
44
0
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.
 

Zel

macrumors regular
Jul 7, 2007
172
0
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.
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?
 

jsw

Moderator emeritus
Mar 16, 2004
22,819
41
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.
 

clickclickw00t

macrumors regular
Jun 28, 2007
186
0
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?
Isn't 85W * .7 in the 60watt range?

I say anything in the 90W range should be fine.
 

clickclickw00t

macrumors regular
Jun 28, 2007
186
0
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
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:

85*.7=59.5W

again, i could be wrong, but this is how I see it.
 

Race Johnson

macrumors member
Original poster
Jun 13, 2007
44
0
I have a 320 watt adapter already, with a plug in where i can plug my charger into... will that work?
 

Zel

macrumors regular
Jul 7, 2007
172
0
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:

85*.7=59.5W

again, i could be wrong, but this is how I see it.
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.



I have a 320 watt adapter already, with a plug in where i can plug my charger into... will that work?
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.