Here's the situation, I have an alarm clock that is powered by a barrel plug DC connector that says "5V" next to it. Out of the box, it has an AC plug into a figure-8 connector going into a brick, then from that a DC cable going into the clock. So I got the idea, since it's 5V, I can declutter by powering it via USB-A.
I can't really buy a custom cable because 1) I can't confirm the size of the barrel plug, but more importantly 2) it is a custom L-shaped connector that fits into a molded hole going up into the clock, so if I get a generic plug it won't fit correctly. So buying a ready cable or buying a PD-adapter are not possible.
So then I thought I'll just cut off the DC cable where it goes into the brick and just solder the two wires into a USB-A 5V plug and power it via USB.
So finally my question. The clock is clearly not designed to be powered via USB. Now, the power brick can feed it up to 5V 4A, but that's because the clock has 2 USB charging ports, one can deliver 1A, the other 2.1A - I never use more than one, so it'll never actually need that much current. The clock itself probably uses less than 0.5A.
So now, if I make this cable and plug it into a 2.4A USB-A port, will the clock actually be able to draw 2.4A? Isn't 2.4A a proprietary standard that needs negotiation between the charger and the device? If so, what is the max current that a "dumb" device not designed to be charged with USB will draw?
And final question, does USB include overcurrent protection? That it so say, let's say I use the alarm clock's USB charging port to charge something at 2A, then the clock, not knowing it's powered via USB, may try to draw 3A from the 2.4A port I've connected it to. Will the port just limit it to what it can do or will there be some risk of overcurrent? And if the port limits it, will the clock know to prioritize powering itself vs. powering the USB charging port.
Sorry lots of questions, at this point I doubt anyone will bother to read, but I had to go through everything...