USB On-The-Go

I have a power bank that is PD capable, but I cannot charge it from my MacBook even if the MacBook is plugged in to power. I get around it by using a USB-C/A dongle and corresponding USB-C/A cable. Presumably this "downgrades" the connection and since the MacBook doesn't support traditional USB charging it has to charge the power bank. Does USB-C not have a way to indicate that a potential power source is a battery so that the MacBook can charge it if it's plugged in to power, and reverse roles otherwise? Is this a fault how the power bank or macOS implements power negotiation, or is this scenario simply unaddressed in USB-C?

Funny enough, if I plug the USB-C/A dongle on the end of the power bank and the cable into the MacBook, it also won't charge.

I also have a Philips One toothbrush with a USB-C charging input. Similarly, I can't charge it with a USB-C cable directly from my MacBook but have to use A in between (I unsuccessfully tried using either a thinner "lower speed" or a thicker "higher speed" USB-C cable). I'm assuming the toothbrush doesn't support PD, so then why can't it fall back to traditional charging with a C-to-C cable?

> Does USB-C not have a way to indicate that a potential power source is a battery so that the MacBook can charge it if it's plugged in to power, and reverse roles otherwise? Is this a fault how the power bank or macOS implements power negotiation, or is this scenario simply unaddressed in USB-C?

USB-C does support this. It's known as a DRP (Dual Role Port). USB PD can be used to signal switching between downstream and upstream facing ports depending on state of charge of the battery for instance. The problem is that many devices do not support this, and I strongly suspect your power bank is the issue here. iPhones and Apple stuff in general supports DRP renegotiation quite well. They tend to be USB-C compliant as much as possible, which can lead to issues with interfacing with 'USB-C' devices that are not actually properly compliant.