Is there a low battery indicator on the PolyTune 2. I have found reference to one on the original PolyTune but not for the later model.
Indeed, in contrast of the PolyTune Classic, the PolyTune 2 doesn't have a low battery indicator. You could measure the voltage of the battery prior to performances to ensure functionality or you could use a power supply so battery life is not a consideration.
If you do decide to only use batteries and your battery fails to provide enough power, your guitar will still pass through the pedal as it is a true bypass design.