Battery gauge dropping

[Mat7783]

My Thomas MVP ER was running fine on a 1,060 mile round trip to the Georgia mountains but on the way home,my battery gauge was dropping to 11.5v when I exceeds 60 mph and stayed perfecto above 13v under 60mph.there was no signs of struggle as I cruised home.batteries are new and wondering if alternator is the issue ?

[flattracker]

Years ago I had an identical failure in my Nissan King Cab pickup. I replaced the alternator and the problem went away. Probably worn out brushes on the slip rings of the alternator.

[Booyah45828]

Find the specs on your alternator and then load test it using a carbon pile tester. It's likely to fail, due to brushes/rings being worn like flattracker suggests. FYI, above 12.6 is charging, but most alternators have a voltage set point a little higher then 14, so "perfect" voltage would lie somewhere in the 14's IMO.

[nikitis]

I would also add to check your serpentine belt for wear, if it's slipping it could be contributing to it.

[Mat7783]

Originally Posted by nikitis

I would also add to check your serpentine belt for wear, if it's slipping it could be contributing to it.

The belt is in good shape and pretty tight so I am leaning towards the degradation of the Alternator as the cause.

[Mat7783]

I stand corrected as it was putting out 14v under the conditions mentioned until my ride home and now was dropping to 11.5 under full load over 2000 rpm at 65 mph.immediately rises to 14v when speed is under 50mph��

[HamSkoolie]

Originally Posted by Mat7783

I stand corrected as it was putting out 14v under the conditions mentioned until my ride home and now was dropping to 11.5 under full load over 2000 rpm at 65 mph.immediately rises to 14v when speed is under 50mph��

That sounds like a diode in the voltage regulator of the alternator could be the issue (most likely non replaceable).
Depending on the size (physical) of the alternator, you may be able to have it tested at a regular automotive parts store. If not, a motor repair shop can likely test (and repair) it.
The alternator should generally put out 13.8 to 14.7 volts. 12.6V will never fully charge a lead acid technology battery (SLA, AGM, GEL) and 12V is roughly 50% discharged.
Also don't rely on any installed gauges for reading those voltages.

[Mat7783]

Yes.I am going to pull it and take it to a rebuild shop for analysis.a side note is my tach is not running and this alternator has a sensor inside for pulsing tach.so that needs fixing too.thank you for the replies 😎