I'm being a little facetious- - - -there's no such thing as an onboard hydrometer, at least not on cars/trucks. The submarine electrician who used to hang out here and try to impress everybody with his knowledge might have had something like that when he was a swabbie.
The point is, neither a voltmeter nor an ammeter paints the whole picture of what's happening with a battery or a charging system. "Normal" voltage in a "12V" system can be anywhere from 9 volts on a hard start to 14.5 Volts or more against a fully-charged battery. "Normal" amps can be from -300 to +100 or more, depending on what's happening in the rest of the circuit and the size of the alternator.
The only way to tell if a battery is discharged, half charged, or fully charged is to check the specific gravity of the acid in each cell- - - -something 90-something percent of the people reading this post will never do.
Jerry
