2005 Mustang GT Coupe intermittent battrey light
 

I have a problem with my Battery Warning Light coming on. The car has a new battery (1 month old) , positive battery cable assembly (1 week old) and alternator (2 days old). The battery tests good and the alternator tests good. Upon starting the warning light goes out but comes on after 30 to 45 seconds. While driving the car, the battery light will go off after about 3 minutes then comes back on after 3 to 4 minutes later. There is no code being set in the ECM but it will show a fluctuation between low and high. The charging rate will spike at 14.7 to 14.9 volts the drop to 14.2 to 14.6 volts.
Does anyone have any ideas?

It sounds like your new alternator has a faulty voltage regulator. What alternator did you buy?

This is the second alternator in 30 days. Both came from Oreillys Auto Parts and both checked exactly the same.

May be a stupid thought, have someone flex your new cable while you ogle the dashboard? thinking this malady started after the new cable was installed.

I wouldn't expect a faulty B+ cable to cause an overcharging issue; more like the opposite.
The problem with buying remanufactured alternators is that you don't know the quality of the parts that went into them, and the quality of the workmanship. Often those that come with a "lifetime guarantee" end up being a pile of dog ****e. The best solution is a 2010 Denso unit. Yes, it's more expensive but I haven't yet heard of anyone who bought one and said they regretted it.

Figure it out?

I still have the problem. I'm starting to think that the PCM is the problem. I've checked all the connections for corrosion and loose, and or broken wires. I have removed, cleaned and reinstalled all of the grounds, of which, none were loose. I even removed and reinstalled the instrument cluster and checked the ignition switch wiring and still have the problem.

try and alternator for a 2010, auto or manual- whichever applies. should bolt right up. good luck