Voltmeter or Ammeter the choice?
A voltmeter is a much more effective, and safer meter than an ammeter. It carries
no amperage to speak of.
Readings are approximately as follows:
battery(key on)12.6 v.
cranking 10 v or higher (less may be bad starter),
charging 14 v or higher.
Alternator is great if 14 v is held while all accesories are on. If it drops,
maybe a bad diode. (one gone drops output by 2/3)