A voltage guage doesn't tell you the status of the battery. A dead battery can still have 12 volts. The thing is that the dead 12 volt battery cannot provide enough amperage to crank the car.

With the amp meter, it will show the charging status. (If the amp meter isn't resistering, it isn't charging. A volt guage can show 12 to 14 volts, but that doesn't mean that the battery has enough amperage to crank the car.)