18 volts is the safe norm with 16v as an absolute min in my experience. Have seen them handle below that as far as 12 volts, but it all depends on the state of the battery and what is the reason for it not being able to sustain voltage. If the battery for instance was completely rundown at some point and then just recharged instead of deep cycled you are playing with fire as it could be battling to produce sufficient amps(voltage might still be fine) to drive the starter at a rate at which the N1 can increase sufficiently and thus airflow to what the FCU expects inducing the fuel. If it is a first time for the volts to run that low cause the batt was just on too long before start or other equipment driven, you can getaway with it and then sometimes you might see the warning when the gennie takes over as the batt draws the current from it to reinstate it's charge-not electrically inclined so technically the explanation will not be correct but this is how I have understood it in the field.