Hi, I have a new photon and a power shield. I powered the photon for the first time from the shield and performed initial setup. I then plugged a USB into the photon for serial debugging. All worked fine. Later I tried powering the photon directly from its USB and and the led flashes and flickers a pink-red and the device is not responsive. It however does continue to work correctly when plugged into the shield.
Has connecting the photos USB whilst plugged into the shield damaged the photons regulator?, or perhaps it was faulty to start with.
When you say pink-red, could it could be that the Photon is pulling a firmware update, that should have been part of the initial setup, but got interrupted for some reason.
try a different USB power source (wall wart, USB port, computer)
try pressing on the USB connector/gently wiggle the cable
The shield adds better DC filtering, the USB connector could have bad contact (either in the solder pads or the contacts to the cable), the battery helps overcome current spikes, ...