Hey DG, John Muha would be ticked to see this kinda post not in the electrical forum
Before starting... well it depends... Typically 12.6-13.2, but AT least 12 volts.
During cranking, should not fall below 10 under any so-so conditions...
Stock alternator/external voltage regulator...
At approx. 1000 rpm's, you should have AT LEAST 13.8 volts, and no more than 14.8 volts. 14.2 volts is the IDEAL charging voltage regardless of load or RPM, but it will of course fluxuate as it isn't a "perfect world". Bottom line, 13.8 minimum, "15 and over too darn high!"
I always suggest swapping the voltage regulator before the alternator. If turning on the headlights reveals a "pulsing" action in the light intensity, give your voltage regulator a proper burial.
ALWAYS CHECK CONNECTIONS AND CLEAN GROUNDS before you spend $$$. Always insist on genuine AC/Delco parts, and never throw original stuff away until you know it is defective.
Oh yeah, the so called "alternator testing" at most parts places is well...worthless...
Have fun DG.
(Yup, John, just checking in...)