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How are Mac Chargers compatible when they are different voltages?

Hey everyone,

This is not a question specific to this Macbook, but I am just trying to understand how this works all works. I recently had my Mac charger die on me, so I just cut the end off and soldered it on to an old Lenovo charger I had laying around. It works fine, but I am trying to understand how the chargers actually work.

Looking online, the different Macbook Magsafe chargers actually all have different max voltage outputs:

  • 14.5 V DC for the 45 W units supplied with MacBook Air
  • 16.5 V DC for the 60 W units supplied with MacBook and 13" MacBook Pro
  • 18.5 V DC for the 85 W units supplied with 15" and 17" MacBook Pro
  • 20 V DC for the 85 W units supplied with 15" MacBook Pro Retina

Why is it done like this? Why not just have all of the chargers work at the same voltage and simply limit the current on each charger based on the wattage?

Even more important, how does the computer manage to accept higher voltages? I can use an 85W charger on a Macbook Air without problem and it will work just fine, but the chargers are just a simple 2 wire connection between the Magsafe Adapter and the brick so I don't understand how the laptops "tell" the chargers to supply a lower voltage.

I can hook the Magsafe charger end up a fixed bench power supply and it will happily take a tightly regulated 20V.

Is the voltage regulation done inside the laptop? It still seems strange to me that the voltages are different.

If anyone could explain how this works it would be very helpful!

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First, I wouldn’t use your hybrid charger! While applying power may appear to work, the nature of Lithium Ion batteries one needs to monitor the charge rate as you can push it beyond what it can handle! In doing so you can expect smoke and even fire as the lithium burns, also killing your system in the process if not your home!

Apple’s chargers have a feedback system to monitor the batteries thermals working in unison with your systems SMC logic.

So I very strongly recommend getting the correct charger ASAP and hopefully you haven’t killed your battery.

I would install this gem of an App CoconutBattery it will allow you to see what’s happening. If the battery has over 1000 cycles and/or is over four years old it’s time for a new battery. If you open the system and inspect the battery if it’s puffy it’s also time for a new battery.

The reason there are different chargers is related to the size and chemistry of the battery. Think of it this way years ago cars had leaded gas and quite a few grades. Overtime the cars which needed leaded gas died off and the grades have on the most part are no more than three today. That same logic holds true here.

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Hi Dan,

Thank you for replying! I do appreciate your concern, but this setup I have will not be a problem. The charge handling is actually done (partially) inside of the Magsafe charger, all I am doing is using the Lenovo charger as a DC power supply and the Laptop and Magsafe connector are doing the actual work.

I will definitely get a proper charger, but I recently replaced my battery so it is not a problem with age or safety or anything like that. I am just trying to understand the charge logic from an Electrical Engineering point of view. Regardless, I do appreciate the help!


@saaif88 - The control is via the one wire comm link between the charger and the system (the middle pin in the connector). There is nothing within the MagSafe cable other than an indicator and physical connection to the system.

The SMC logic within your system is the controller of the charging and it works with the charger to limit the power so it won’t overcharge via that one wire comm link. As your replacement Lenovo Charger has no idea how to communicate with Apples SMC logic you can’t control the charge rate.

Install CoconutBattery let’s see what it tells us.


@Dan It was my understanding too that there was some sort of communication between the charger and the system, but from what I can see, there is not. There are just two wires between the Magsafe end and the actual power brick. Louis Rossman actually has a video showing how to use a bench top DC power supply to power and charge Macbooks and there are teardowns that showcase the internals of the Magsafe adapter.

The Magsafe end of the charger is indeed what communicates with the SMC, which is what I have grafted onto my old Lenovo charger. I'm just trying to understand how the Macbooks utilize this charger voltage. It is most likely an internal voltage regulator inside the Macbook, but I still don't fully understand why it is done this way.


@saaif88 - There’s an illusion taking place here!

You see two wires so there just can’t be a third, can there be? Well there is! It’s the shield.

As to using a bench power supply that’s a different animal! A good beach supply allows one to set a limit of amperage and voltage.

A charger which doesn’t have an active feed back can over charge the battery.

The charger is where the control takes place otherwise the logic in the system needs to shed the excess power into heat and just not done.


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