Leaving the engine ("chassis") battery master switch in the ON position didn't cause your problem. The engine starting "readily" (as you said) is an indication that the chassis batt's were not drained all that much (i.e., they were adequately healthy).
But...
A charging system should not exceed high 14 / low 15 -volts range, no matter the RPM.
Perhaps run your tests again: at regular idle, what is the DC-volts measurement?...and is the alt light extinguished (and no alarm). Then increase the idle until the light comes on and the alarm sounds....now take the DC measurement. Increase the idle further...and at this time, is the DC-volts even higher? Where I'm headed with this: failure of the alternator's regulator...which will overcharge our chassis batteries is not corrected.
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Robi & Dale ---- Go Navy ----
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or: '08 Buick Lucerne
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