There may be a variety of reasons for the voltage production of an alternator to drop whenever the RPMs are increased. In some cases, it is part of the alternator's design and function, in tandem with a voltage regulator, to have fluctuations in voltage, or even drops in voltage, during normal alternator operation.
Nearly every automobile alternator-generator unit is coupled with a voltage regulator, which determines the actual output of the alternator. The voltage regulator detects the charge level of the battery and can decrease the voltage output of the alternator once the battery is nearing full capacity. As a result, the voltage output from the alternator, as it produces electricity for the vehicle's battery, may drop once it has reached higher RPMs if the electrical load requirement is low.
If you are suspecting that your alternator may be failing, you can test it with a multimeter. Connect the electrodes from the multimeter to the positive and negative terminals of the car battery. Set the multimeter to voltage and turn off all lights and accessories on your vehicle while the engine is running. If the car has been warmed up, the voltage output and normal idling range should be between 13.8 and 15.3 volts.
It is part of an engine's normal functioning to have a higher charging voltage during the first warm-up phase of turning a car on, with the voltage running 2 to 3 volts above the normal idling voltage range. Once the vehicle has warmed up, the voltage production should taper off to the normal idling voltage production range.
Other factors influencing voltage production could include the functioning of the alternator and voltage regulator. If either are malfunctioning, then you may experience erratic voltage production or voltage drops at unnecessary times. If the alternator is failing, then the vehicle will run for about 20 to 30 minutes and turn off due to lack of electricity production, regardless of the amount of RPMs the engine is producing. The state of the battery's charge, the load on the vehicle's electrical system and temperature also influence the voltage production of the alternator. As temperature increases, charging voltage will also drop. Poor or deteriorated wiring and battery connections could also lead to erratic voltage production. Failing diodes can also lead to voltage and amperage drops.
