A "12 volt" "maintenance free" battery is closer to 13 volts when fully charged. To reach a full charge any battery requires about two (2) volts over and above the resting or open circuit voltage. Temperature also plays a big role in the proper charging voltage. Colder temperatures increase the internal resistance of the battery. Depending on the state of battery charge and the weather, well over 15 volts is not only normal but required for a full charge.

 

I am at 14.68 most of the time. It drops to 12.8 with A?C (running two 12 inch fans for condenser) all lights on and a turn signal running. That is in drive with parking brake on. Higher if in park or neutral.

 

One thing that is oftentimes overlooked, remember the wiring itself here is 30 to 35 years old. Replace those cables! A high output replacement alternator is a waste of money, if the increased capacity cannot be utilized, due to rust and corrosion. This is the big problem in all classic rides. Here's why:

"It doesn't take much of an increase in resistance to cause trouble. Let's say a 120 amp alternator operates in a circuit that has a normal resistance of 0.11 ohms. If that resistance were increased to 0.17 ohms because of a bad wiring connection, the alternator's maximum output would be limited to 80 amps. In other words, an increase of only 0.06 ohms (almost nothing!) would reduce the alternator's maximum output by almost a third! Under light load, the drop in charging output might not even be noticeable. But in a high load situation, the alternator wouldn't be able to keep up."

http://www.aa1car.com/library/voltage_drop_testing.htm

Before spending money on an alternator, it is wise to take the time to really make sure every connection and circuit is at least as good as new. There are three main cables from battery to block, block to frame, block to firewall. Grind down to bright, shiny metal and tighten securely. It pays to do this. Let's get all the performance out of what you already have! The reason you're seeing a weak charging voltage under load is likely because of corrosion.

As a quick test, connect a heavy jumper wire or cable from the negative battery post to the alternator case and measure charging voltage under load again - at 2000 RPM say, If you now see normal 14.x volts, the problem lies in these cables and connections. Test the positive side in the same way.

Even the alternator itself is grounded to the engine and depends on a clean, tight connection through the block and hardware, a nearly invisible layer of skunge or paint will increase the resistance of any circuit or connection. Not so much that it's measurable, but plenty enough to cripple it. Bad grounds, loose, missing, or corroded cables, rust on the frame, paint on the firewall, or engine - these are the enemies of a healthy battery and charging system, and also play hell with starters and solenoids, reduce the potential ignition output, contribute to dim headlights, poor heat in the winter etc.

The irony (heh) here, it doesn't really cost anything to clean up and maintain the electrical grounds and connections, but the consequences of neglect can eventually get very expensive.

 

Reps to Tedster for that post, thank you! I try to explain same concept (keep your wiring and its connections in good shape) to people but I must be doing it wrong because people invariably come at me saying I'm FOS, that all original wiring must surely be decayed at this point resulting in it being unusable (or barely usable), all sorts of stuff to ostensibly try and justify the installation of shiny, new components... I just shake my head and move on...

 

The older alternator regulators seem to be set around 13.8 volts. At sometime they seemed to have changed this to around 14.5v.