a7S III USB-A power at <2W vs >6W depending on cable

[kcimc]

I would like to power my Sony a7S III from a USB-A power bank, but I get very different behavior depending on the cable and adapter that I use. What are the minimum requirements for getting full power so I don't have to recharge my camera battery? From reading the documentation it seems that there are no guarantees that USB-C power will work indefinitely.

Before testing with the battery, I tested with wall power. I am using:

- RAVPOWER 61W AC adapter
- Cable Matters USB-C to USB-C cable
- Cable Matters USB-A to USB-C cable
- Syntech USB-A male to USB-C female adapter
- Kavalan USB-C power meter

In all tests I have the power meter plugged directly into the camera.

- USB-C to USB-C is 8.7V 1.5A 13W when camera is off, 8.8V 0.7A 5.9W when camera is on
- USB-A to USB-C is 5V 0.2A 0.8W when camera is off, 5V 0.3A 1.6W when camera is on
- USB-A to USB-C adapter to USB-C to USB-C cable is 4.4V 2.6A 11.5W when camera is off, 4.7V 1.4A 6.5W when camera is on

I also noticed in some cases that the camera would use less power if the cable was plugged in after the camera was turned on, and more power if the cable was plugged in before the camera was turned on.

I also tried using the USB-A to USB-C cable that is included with the Sony a7S III and I had the same results as the Cable Matters camera.

Why would the adapter plus USB-C cable work better than a USB-A to USB-C cable? Is there any way to make a single cable work correctly, or do I need to use an adapter?