1966 c10 alternator: How many amps?

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Originally Posted by vince1

I think the charging voltage across the board for most alternators is about 14 to 14 1/2. I don't know what happens with a lower rated amp alternator. Does the voltage just drop off when it can't keep up to demand?

Yes. If the alternator can keep up, the whole truck runs at the regulator voltage (probably about 14.7 volts). Any leftover current capacity (amps) goes to charge the battery. The battery takes what it wants. If the battery is low it might take all the leftover current available, but usually not.

When the alternator can't keep up, the voltage in the truck will drop fairly quickly to the natural voltage of a 12v battery (about 12.6 volts). The voltage will continue to drop as the battery discharges. Eventually the battery would go dead if this went on for a long time, but usually it doesn't.

When the alternator catches back up, the voltage will go back up to 14.7 or so, usually instantly, but if the battery is about half discharged it might draw all the current available for a short period of time. In that case the voltage would head back up to 14.7 a little slower as the battery charges.

Even a 37 amp would keep up with most trucks just fine. Huge 100+ amp alternators are unnecessary 99 percent of the time. Some people want them. Nothing wrong with that, but the wire has to be big enough, because it could melt, and it won't happen until some cold morning when the battery is half dead.