I have researched threads here on what battery voltage should be and now I am more confused than before. I have a 77 280. We all know it is externally regulated. I get 13.76 volts at the battery during idle. Now where I get confused is when you increase the engine rpm's shouldn't the battery voltage also increase? Iv'e read here that it should be 13.8 at 2500 rpms and I also read here where it should be 14.7 at 2500 rpm's. Which is correct? My battery holds pretty steady at 13.7 volts through out the rpm band, no changes. Help me understand. Thanks

The voltage regulator will limit the system voltage to somewhere between 14.0 volts to 15.0 volts. At idle the alternator does not have full current capacity and might not be able to keep up with the electrical loads place on it. This is especially true when the headlight or heater are in use. In this case the voltage will fall a bit because the battery is providing the part of the current the alternator cannot. When the engine is running at higher RPM the alternator output rises and is more than required to power the loads. As the alternator output increases so does the battery voltage. This is when the regulator begins working and limits the voltage to the set voltage of 14 to 15 volts.

In your case, 13.7 volts seems a bit low but is not necessarily a problem. The internally reglated alternators have a lower output voltage spec. Is it possible your car has been converted but the external regulator was not removed? Are you experiencing any problems such as non-starting or dim lights when driving?

beermanpete, that is a negative on any problem. The reason I even decided to check voltage is because my alternator is making a noise now (sometimes). I thought it was starting to go out on me. 100% sure no conversion was ever done. I have owned the car since 1985. 13.7 volts at WOT could be a problem right, when I need a hotter spark to burn the extra fuel? Or am I just being paranoid. Thanks

Any alternator and voltage reg that keeps the system voltage over fully charged battery voltage will keep things generally running. 13.8 at idle is wonderful. Relax.

Aaaaah, thanks I needed that!