Looks like our posts just crossed each other.
No, it doesn't. Once turned off, the phone cannot be turned back on other than by plugging it in the charger. And it is permanently stuck on the 20% battery status.

I tried to understand what fastboot is and how to use it, but I failed, despite being a software developer for 30 years. Perhaps that's the problem, I am too old.

I am using Emma, as per this guide. It asked me to download and install the drivers on the first start, which I did.

I can try that, as soon as I find one. But I am successfully using the same cable for file transfer from the same phone using WinSCP, so I assume it works.