I have an iPhone, iPad, and a Mac and when I connect my phone or iPad to the Mac, it automatically starts charging. But why? Why not the other way around?
My thinking is that Mac’s have a bigger battery? But still what about that makes the Mac a charging source in the presence of an iPhone/ iPad?
In: Technology
If you use regular old-school USB on either side, the USB standard especies that the primary devices with type A connector (the one on your computer) will always supply power, and the secondary device with a B connector (what people refer to as mini and micro USB, common on older smartphones) will always* receive power.
(There is one rarely-used exception, USB OTG, which allows a device with a B connector to send power to power small peripherals with a special adapter, that way you could do things like connecting a keyboard to your phone.)
If any one of the two sides has a type C connector while the other has one of the other ‘classic’ connectors, the type C end will figure out that the other device is only power send or power receive (almost all type C cables have a small chip to tell the devices what the cable can do, which includes what way power can be delivered and how much power).
If you use type C on both end, in theory both devices negotiate which way power is distributed. By default, computers will preferentially ask to be a power source, while phones will ask to be power sinks (to receive power). This can be reversed in software: some Android phones have a setting to allow the phone to send power through USB-C instead of receive it, useful if say if you’re charging your wireless headphones with a type C to type C cable.
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