Powering the Photon from a USB battery

What is the maximum voltage the Photon can accept without being damaged? For example 5.1 volts. Will the Photon be damaged if the voltage drops to low?

http://docs.particle.io/photon/photon-datasheet/#2-2-power

The photon locks up the FLASH areas and generally should not be corrupted due to low voltage.

The maximum recommended voltage for the Photon, depends on the pin, and it’s usage.
Normally, 3v3 is expected, but some pins are 5volt tolerant.
Are you referring to the VIN, 5 volt power source? It should be good up to 5.5 volts.

I am referring to the USB port on the Photon.

Yes, 5.1 on the USB port is fine.