First confirm it isn't a faulty gauge by putting a Voltmeter on the Battery or the Jump Start stud in the engine compartment.

 

Cayenne 2014 SD4.2 V8 Serotronic stopped working but comes back after a long run. 100 miles. The battery show a charge of 14.9 volts for the same period of time and then drops to 13.3 at which point the servotronic is fine. Next day is the same issue. Are the two related? The engine starts first time no problems

 

Alternator has Rectifier Diodes which prevents over voltage conditions. Max reading on healthy Alternator is about 14.8v, while running in ideal ambient temperatures.