I was noticing that my battery voltage was around 13.0 on my first WOT pull. Then as low as 12.1 when the car was fully heated up. All of my pulls are at night so the lights are on. The computer can freak out when it drops below 12.0.

So I replaced my 5 year old battery and put in a new Start and Charged 160 amp alternator.

I fire up the car and the voltage is 14.3 on the voltmeter at idle so I think great.
So I start data logging with livelink and that reads 13.8 while the voltmeter reads 14.3.
.5 volt difference between the battery and pcm???

So I go do the first pull and the voltage is about 13 as usual.
Second pull with a fully heated up car (low speed fan temp had been reached) the low point was 12.5 but for the most part about 12.7-12.8 which is much better than the old battery and alternator.

Did some cruising around with livelink running the voltage is around 13.3 for the most part which is most likely 13.8 at the battery. This is with a coolant temp of 190-194 which is commanding my low speed fan on below 48 MPH.

Well I am really curious about the .5 voltage difference between the data logger and voltmeter. So I get out the video camera and shoot this video.
YouTube - batt volts

Being the voltage difference is evident on multiple mustangs it makes me question the quality of the voltmeter in the pcm.

After analyzing many of my data logs it is very apparent that when I reach Low/High speed fan temps the fan operation places a high demand on the electrical system. Even though the fans are commanded off above 50-60 MPH it appears that WOT voltage will be lower as the battery has not had enough time to get replenished.

I checked the operation of both the low and high speed fans and they are working correctly based on my tune settings.

I have never had any issues with the A/F going lean, but these findings will make me more cautious about limiting WOT when operating in the Arizona Heat.

Maybe someone can chime in regarding the voltage drop between the battery and pcm as I am by no means an electrical engineer.