Any charging system- - - - -generator, alternator, or even a hamster running in a wheel, regardless of system voltage- - - - -will only produce enough voltage to push a charge into the battery. Against a very low battery, the charging voltage might only be 3/4 to 1 volt higher than the "engine off" voltage measured at the battery posts, but the charging AMPERAGE will be quite high. As the battery builds up a charge and its internal resistance rises, amps go down, and terminal voltage goes up.

You MUST know the state of charge of a battery before passing judgement on charging system voltage. Back in the dark ages when batteries had removable cell caps, we could stick a hydrometer in a battery cell, measure the specific gravity of the acid, and know for sure the state of charge of the battery. Now all we can do is check the open circuit voltage at the posts of a sealed-top battery and make an educated guesstimate about a battery's state of charge.

IF:

The battery is fully charged

The charging system and voltage regulator (internal or external) is functioning properly

AND

The engine is running at a fast idle speed or above and minimal accessories such as headlights, heater blower, etc. are turned off- - - - -

THEN

Battery terminal voltage should be around 7.5V on a 6 volt system OR 14.5V on a 12V system.

Under any other circumstances, you're making a SWAG ("Scientific Wild-Donkey Guess") at the very best!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!