My car starts to squeak once I put it into drive. This will drop the power on the battery gauge. The squeaking will stop if I push hard on the accelerator. The squeaking is also more common during cold weather. I have already brought my car, a 97 Jaguar xj6 to a shop where they thought it was an issue with one of the bearings and the air conditioning belt, but the problem persists.
There’s likely more than one drive belt, if not a serpentine belt.
Just keep looking,you’ll find it.
With the engine idling, quirt a little water from a spray bottle on the drive belt(s) one at a time until you find the squeaky one.
If you find the noise is not from a belt but persists, remove the drive belt(s) and turn each pulley by hand while feeling and listening for a slight scraping noise.
Ah, I should have read your post more closely first. Oh well, perhaps you or someone else can use the info for themselves.
You state the battery voltage drops when the squealing starts. Perhaps the fault lies with a loose alternator belt and/or a worn alternator pulley bearing OR possibly worn alternator brushes.