If you are having issues with voltage peaking, lights fading in and out as your driving, or your battery keeps dying despite the car running, It could be the result of a faulty voltage regulator. Voltage regulators control the amount of power coming from the alternator.
If the output is too high, the regulator prevents the excess power from damaging any sensitive electronics in the electrical system.
If the output is too low, the regulator will cause the alternator to provide more power to the system to prevent excess drain and strain on the battery.