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An ammeter shows you energy in and out of the battery and that the battery is connected and IS being charged. The voltmeter shows you a system voltage, but generally not the battery terminal voltage, so under charge the voltmeter rises faster than the battery voltage, IF you can tell the difference between 13 and 14 volts which is a tiny part of the scale on many meters. And a voltmeter won't show if the battery is disconnected while running. The starter solenoid will tell you about the state of the battery charge. If its charged it cranks, if not, it clicks. The difference on the voltmeter is the difference between 12 and 11 volts at rest, again a tiny part of the typical automotive volt meter. I've run both, at the same time. Even with a laboratory grade voltmeter, the useful information came from the ammeter, not the voltmeter. It is harder to pass a 100 amp alternator output through a simple ammeter and I'm sure that's why my truck has a volt meter, but with calibration points at only 8 and 18 volts it tells ME less information than a lightbulb does by its glow. I'm not too sure it is anything more than a pointer that moves to a fixed position when the ignition is on, just like the Ford "oil pressure" gauge that is controlled by a switch. Gerald J., Electrical Engineer.
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