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When a device (say a modern MacBook charger or GaN charger) lists modes, such as...

Max Power Output
66W
USB-C1
65W Power Delivery 3.0, 5~15V/3A, 20V/3.25A
USB-C2
65W Power Delivery 3.0, 5~15V/3A, 20V/3.25A
USB-A1
36W Quick Charge 3.0, 5V/3A, 9V/3A, 12V/3A, 20V/1.8A
Power Input
AC 100-240V 50/60Hz 1.7A

If I'm say charging a 60w device from this "65w" charger, is all I care about "syncing" the voltage.

i.e. I need the device being charged to be able to sync at 5->15 or 20v, but I don't need sync on the amps.

so the device can draw as much as it needs of the 3->3.25 amps, once it has a happy sync on the voltage?

Woodstock
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    That's correct. The charger and device will communicate over the cable and agree on a voltage and a max amperage. – nanofarad Mar 08 '21 at 18:26
  • it's a `power supply` ... the battery charger is built into the MacBook – jsotola Mar 08 '21 at 18:29
  • Not sure if I misunderstood you but the range is not 3->3.25A, but 0->3.25A. – Wesley Lee Mar 08 '21 at 18:32
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    Does this answer your question? [Choosing power supply, how to get the voltage and current ratings?](https://electronics.stackexchange.com/questions/34745/choosing-power-supply-how-to-get-the-voltage-and-current-ratings) – brhans Mar 08 '21 at 18:36
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    Indeed the **voltage** needs to be the same and the current of the device needs to equal or smaller than the current that the supply can deliver. Do realize that "Power Delivery" and "Quick Charge" devices need to be able to **communicate** with that power supply. Without that communication, all a device gets is **5 V**. So an old laptop that needs 20 V but which does not support PD or QC, will only get 5 V and will be unable to work from such a modern power supply. – Bimpelrekkie Mar 08 '21 at 18:38
  • thanks all! @Bimpelrekkie, would that old laptop just not charge slowly though? – Woodstock Mar 08 '21 at 20:25
  • *would that old laptop just not charge slowly though?* No, the electronics inside that laptop need around 20 V and aren't going to work on 5 V, they are not designed for that. The advantage of supporting only 20 V is that the battery charging circuit can become much simpler and cheaper compared to a circuit that can support 5 V to 20 V. And since the laptop comes with a 20 V adapter, there is no need to support 5 V. – Bimpelrekkie Mar 08 '21 at 20:48
  • Interesting. I ask as I’ve charged my 99w MacBook with a 61w Apple powerbrick. It seemed to work just slower...must have synced at some lower voltage? – Woodstock Mar 08 '21 at 20:50
  • Coming back to this, it seems in the 99w Mac on the 61w Mac charger, it seems to work as both support ~20volts (20.3v and 20.5v), with the 99w Mac seemingly ok with 20.3v @ 3 amps, instead of it's usual 20.5v @ 4.7 amps... seems it voltage matches, machine will try and charge with as much amperage as is available – Woodstock Mar 09 '21 at 12:51

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