UPDATE Starting/Charging issues

A volt meter will give you an indication of system voltage. So with the key on, engine off, it will read 11.5-12.5 volts. With the engine running it should read 13.5-14 volts. The reason most OEM's went away from the ammeter in the 70 is because cars started requiring greater electrical loads, and were fitted with more powerful alternators. It was not practical, nor was it safe to route the entire output of the alternator through the dash and a single istrument. Oh yeah, that big fat wire was expensive too. A red "amp" or "charge" light is all you really need, but add a voltmeter if you wish. Sooner or later the ammeter will crap out and the cheap fix is to bypass it.