There are various "answers" to this issue, and many of them sound suspiciously like voodoo. What is causing the issue with the battery / power? That might help. In my case, the phone went from intermittently starting, even letting me root the phone and use it for an hour or so, to gradually getting worse and worse as far as even starting at all went. At present it does not start.
Here are the more believable, from my point of view, possible reasons this is happening.
1. The battery has gone bad. (This may be in part true, but it is doubtful it went this fast.)
2. The charger and/or cable is bad. (I'm open to this, but have tested the charger on another phone. I don't have another usb c type device to test the cable on.)
3. The software is bad. (I tried two completely different operating systems, the default one and Lineage Oreo; both failed to continue running.)
4. The internal usb c ribbon assembly has gone bad / has a short in it. (I am considering buying a third party one of these off ebay - they are cheap and relatively easy to remove / replace - to find this out).
5. Some other short circuit / electronic issue is at fault.
If anyone else has this issue and sorts it down to a *cause*, I'd sure like to know about it.