Numerous questions and comments have indicated to us that a lot of drivers are interested in the service life of halogen lamps in car headlights. In this third part of our series, we discuss the influence of operating voltage.
Conventional AA batteries and many others have 1.5 volts. That’s not very high as voltage goes, so you may wonder why 1.5 volts can be a lot under certain circumstances, namely when the operating voltage of an automotive lamp is 1.5 volts too high.
Although the on-board voltage in your car is usually given at 12 volts, that is only the voltage of the battery. When the engine is running, the electric generator, also called the alternator, supplies the electrical systems with 13.6 to 14 volts, though some of that always is lost in the wiring. Headlight lamps therefore are designed for 13.2 volts. If you apply more, the lamp shines brighter, but it ages considerably faster, too. Five percent excess voltage costs you 50 percent in lamp life! A lamp that runs continuously on 14.5 volts or more will soon burn out. Excess voltage is the most common cause of premature failure. And 1.5 volts are more than five percent.
There are several reasons why the operating voltage can be too high. In most cases, it is due to defective alternator regulators. Batteries that are almost clinically dead also can be the cause. And then there are car manufacturers who push the limits of the operating voltage to speed up recharging. Cases even exist where deficiencies in the design of the headlamps had to be offset by applying high voltages to the lamps. And in other cases, carmakers reduced the voltage with a retrofit resistor to correct the problem of rapid failure.
The deciding factor is the voltage at the base of the lamp, not at the battery terminals. The reason: in some newer cars, the headlights have a separate voltage regulator, which incidentally can also be defective.