This sounds like the USB-C controller of the light advertises itself as a “preferred sink” dual role port. Smartphone should be “preferred sink” when almost empty (hence why it works there) and probably “dual role” (no preference) otherwise (in which case the phone tries charging the light).

It’s a bit tricky - if you make the light a “preferred source” it may end up trying to charge USB-C powerbanks instead of being charged by them. There’s only so many options for power direction flow you have :confused:

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