charginghardwaremacmagsafepower
I’m looking to understand how to test and evaluate chargers so this is a bit technical a problem. Since MagSafe is proprietary, I can’t consult a USB power distribution specification to know how things are designed to work.
What is the voltage output of an Apple 85W MagSafe 2 model A1424 without a load?
Most switching power supplies are most efficient when operating at 50% capacity. And they're definitely pretty inefficient at less than 20% capacity.
Since the MacBook draws less power than a MacBook Pro, I would suspect that an 85w power supply would work just fine, but the larger power supply for the MBP probably would draw more power from the wall socket than is necessary when connected to a MB because it is operating in a suboptimal capacity range.
Here's an example: Take for example the 45w power supply for the MacBook Air. Sure you could hook up the 85w power supply, but since the MBA might only draw 20 watts, the power supply is operating at less than 25% capacity. At this capacity, the power supply may only be 60% efficient (an educated guess), that means the power supply has to draw 33 watts of AC power just to supply the laptop with 20 watts of DC power.
Long story, short: It's a power conversion efficiency thing. I'd say buy the one that is designed for your laptop so that you save energy.
I use this for traveling as they are often cheaper than the original. You can plug any philips power plug in the adapter.
correct me if I am wrong, but I thought these were called philips power plugs
Best Answer
I believe these are smart chargers, and initially only supply about 3 volts in order to power a power management chip inside the device to be charged. Once powered by the 3 volts, the power management chip inside the device then communicates with the charger verifying that the device is compatible to be charged, and if that fact is verified, only then will the charger output full charge voltage.
Additional info provided by @MarcWilson.