Automotive text books claim that a 6 cell automotive battery produces 2.1 volts when healthy. 2.1 x 6 = 12.6 volts - the sign of a charged and healthy battery.
My '91 300-SEL has a yr. old AC-Delco battery designed for this car. After sitting overnight, a multimeter indicates a reading of 12.3 to 12.4 volts. After driving the car 'til good and warm, it reads 13.25-13.30 volts at idle. I see the same reading if I rev and hold the engine speed at 1500 rpm.
I've read elsewhere that I should be seeing 13.4 - 13.9 volts at idle. Are the readings I'm seeing an indication that the brushes are growing tired? Thanks for your time.