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I bought a £16 smartwatch just because it used USB-C
(shkspr.mobi)
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this post was submitted on 06 Aug 2025
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too powerful? what do you mean? USB PD by default supplies 5v the same as USB A and increments from there
5v is pretty low - 3v is pretty common logic voltage, but i doubt anyone would use voltage that low for battery charging?
do you mean you don’t like to “waste” a perfectly good powerful USB C port? you can get some pretty low watt USB C plugs, but honestly i much prefer to just have a brick with 7 big ports
The person you replied to is referencing findings made by the author, in the article.
The author tried plugging a PD charger into the watch to charge it, and it wouldn't work. It's probably not PD as a specification couldn't work, but that the watch failed to negotiate with the charger.
Whatever the reason, the findings were that plugging your PD laptop charger into this cheap little watch does not result in any charging.
And the author wrongly said
The correct way ro ask for 0.8 W (5 V, 0.16 A) is to request 5 volts, any current. Doesn't matter if the charger is capable of 500 mA (legacy USB), 1 A or 3.1 A. The PD standard can accomodate the watch, it's just that the watch lacks active electronics that are required to talk to the charger (and even the supplied C-C cable is non-compliant by being power-only).
Edit: apparently PD allows 0.1A steps between 0.1 A and 3 A for 5 volts so it's technically possible for a PD charger to deny power to the watch if it's VERY underpowered and can't even put out 1 W. Still, it's the watch's fault for lacking correct PD implementation.
right… i think that’s less of a problem with PD chargers and more of a problem with non-compliant A chargers (and the device itself being non-compliant): wattage/amperage at these has nothing to do with the protocol (other than auto shutoff under a given current draw, but that’s not instantaneous)
i believe that the USB spec says there needs to be a resistor bridging one of the pins to receive power? i can see USB-A chargers just dumping 5v through the cable no matter what and USB-PD more reliably implementing the spec because it’s more complex, so less reason to cut corners
The device is probably just using a USB-C format connector to get power, without using the data connection at all, and a strict implementation of the USB protocol on the other side (the so called Host) would mean the device gets from the host only the minimal power levels (100mA @ 5V, if I remember it correctly) meant to merelly power enough a connected device which has no batteries (say, a mouse) for it to actually do the initial USB connection negotiation, and that current will only get increased by the host it if during that negotiation the device tells the host it requires high-current (which in different USB versions has a different value - in USB 1.0 it was 500mA but latter versions increase it), a negotiation which that device can't do because it doesn't actually do USB data at all and just treats the whole thing as a dumb power cable.
Dumb charger bricks don't care at all because they themselves only do power and not the USB protocols, so really just treat the USB cable as a power cable into which they always make available whatever current the other side pulls up to the brick's max supply capacity (usually 1A or 2A) with no "USB negotiation".
This is why even in the times of USB-A some devices would charge fine from a dumb USB power brick but charge really slow if connected to a host which is a data device that can also do charging (like, for example, a notebook).
This is even without getting USB PD into the mix.
Because USB PD is a comms and power protocol, were the device tells the host the characteristics of the power it expects to get (not only current but even voltage) the USB PD brick has a proper USB implementation were it acts as a USB host.
I expect the USB PD brick has a strict implementation of the USB protocol which, in the absence of USB negotiation, just provides that minimum current that per the protocol a USB host is expected to provide pre-negotiation, which is too low for properly charging most things.