Does the photon voltage regulator allow low voltage detection?

I’m planning on using a smart-phone (5V) backup battery to power my photon based project. I would like to be able to detect from the onboard voltage regulator of the photon that the battery is getting low so I can signal the user to replace the battery. Is this possible?

Alternately, I believe I could use something like shown in the image below where I connect the incoming line between the two 3.3KOhm resisters to an analog port of the photon. Is this the recommended option?

@harley, the Photon’s onboard regulator has no such detection and is not connected to the Photon’s GPIO. Your circuit would work just fine assuming you connect the tap one of the Photon’s analog input pins. You could use 10K resistor to reduce the current flow from the battery. Another approach is to use an INA219 to measure both current (if you wish) and voltage using I2C. Either way, you can use the measured voltage to warn the user but also put the Photon in deep sleep essentially turning it off if the voltage gets too low.