As already mentioned ammeters, which measure the current both going into and out of the battery (hence centre zero scale) generally went out of use when higher current alternators took over around the late 1960's. If you know what to look for they offer more immediate information on your cars electrical performance than a voltmeter. Downside is that all the battery current has to pass through the instrument or its shunt which is usually integral to it. This is an extra link in the chain involving higher current handling cables so generally not practical with modern car alternators putting out 50 + amps for reasons of reliability and cost. Higher current ammeters with remote shunts are available, the ones on my boat measuring up to 500 amps, but not practical for use on cars.
A voltmeter on the other hand is very easily fitted without interrupting any of the existing car electrics, allowing an approximate guide to battery state of charge before engine started and also if it is being charged to some degree afterwards.
