A properly designed raspbery Pi PSU seems to need to be able to sink about 3Amps to be really stable and I’m not sure the micro USB connector itself is up to handling that sort of current reliably.
I use the hifiberry amp+ which drives theraspberry pi from the amp board and is fed with 18V at 3 amps to allow it to do 60W but I still think it could do with more.
Perhaps a 12Volt lead acid cell with a decent meaty 5V regulator is the answer . . . .