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Check the voltage across the battery terminals with the engine running. A properly functioning alternator will maintain battery voltage between 13.9 and 14.8 volts (14.4 is optimum). Worst-case (with all the car’s accessories turned on) there should be at least 13 volts at the battery. Be sure that the engine is running at a high enough rpm for the charging system to function (especially if running a one-wire alternator). 11.5 is not enough. If the belt is not slipping any engine speed above 1,000 RPM should produce the necessary voltage. Good news is that GM alternators are fairly reasonable to replace in terms of cost and effort.

Let us know what you find and once again, Welcome.
Glenn