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I recently had my Mac AC adapter die on me, so I just cut the end off and soldered it on to an old Lenovo AC adapter I had laying around. Since they are both 20V AC adapters, it works fine, but I am trying to understand how the AC adapters actually work. As I understand it, the Magsafe connector itself is responsible for charging and voltage regulation, whereas the power brick is just a dumb DC power supply.

Looking online, the different Macbook Magsafe AC adapter bricks 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 AC adapters work at the same voltage and simply limit the current on each charger based on the power?

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

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

Is the voltage regulation done inside the laptop? It still seems strange to me that the voltages are different since I would assume that a 4 5W AC adapters would never charge the battery up to a proper voltage, and an 85 W AC adapters would completely overcharge a smaller Macbook Air.

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

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  • \$\begingroup\$ Those are AC adapters (power supplies), not "chargers". The charger is inside the laptop computer, to charge its battery. \$\endgroup\$
    – Davide Andrea
    May 17 at 19:49
  • \$\begingroup\$ The reason for using higher voltages for higher-power supplies is that the current doesn’t increase too much, which would otherwise require a thicker cable and would impact the life of the connector itself. The computer has its own switch-mode supply that can accept a range of voltages. \$\endgroup\$
    – Frog
    May 17 at 20:07
  • \$\begingroup\$ @Frog I have a feeling that this is the right answer. They increased the voltage on the higher powered chargers in order to avoid thicker cables. I am thinking this might be done as a cost saving measure more than anything else. It is probably cheaper to have high quality regulators inside of the laptops that can accept a wide input voltage range instead of having thicker cables. \$\endgroup\$
    – Saaif
    May 18 at 19:25

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Because there are DC-DC converters inside the laptop computer to reduce the voltage at the DC input down to a variety of lower voltages, such as 5 V, 3.3 V, 1.8 V, and whatever voltage the battery requires at the moment.

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    \$\begingroup\$ Loser voltages? Is the "in" crowd down at 1.8 V and 1.2 V now? \$\endgroup\$
    – John D
    May 17 at 19:53
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    \$\begingroup\$ One little typo and your message is completely urined. \$\endgroup\$
    – Frog
    May 19 at 19:56

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