2001 Ford Mustang GT electrical problems

Recently the check battery light has been coming on at different times throughout the day. In the morning at startup, on the freeway, or idling at a stop light. I checked the original Ford battery, and the glass "eye" was
red. This led me to believe the battery was bad. I replaced it with a yellow top Optima, but the check battery light still lights up. I took the car to Advanced Auto parts, Auto Zone, and Pep Boys to see if they could find the cause. Their machine couldn't find anything wrong. At idle the alternator was putting out 13.2-13.5 volts, and 20 amps. I have Steeda underdrive pulleys, but the techs said this was still an acceptable output for the alternator being underdriven. I have checked the cables, grounds, and alternator connections. I haven't had any problems before, up till now. I have had the underdrive pulleys for about 2 yrs now. All connections are clean and tight. I don't know what else to check. Anyone ever have this problem. Any help is appreciated.
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============================Did they do a diode check on the alternator? They should......It can have a bad diode and still charge, but the light will come on..
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it has to be in the alternator either bad diode/stator or an intermittent regulator problem. probably the regulator problem good luck rusto

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No Diode check that I am aware of. No one brought it up either. May have to go back and specifically ask them to test. Is it user serviceable or does the alternator need to be replaced?

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====================Nowadays you just replace the alternator....the diodes are inside.
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