Actually....an ammeter is the more accurate measuring device because it tells you not only which direction the current is flowing...ie into the battery or in the case of discharge flowing out of the battery...(which means your electrical load is greater than the charging system can overcome so, some some current is being borrowed from the battery) and in what quanity.
By contrast a volt meter just measures the amount of voltage or pressure behind the current. A volt meter does not tell you the volume of current being used and where it is coming from or going to.
The amp gauge requires that the heavy current travel thru the amp gauge in the dash on the way to the battery. With the introduction of alternators charging systems became more reliable and could easily keep up with the electrical load so the switch was made to volt meters which allowed the use of smaller wiring.
Many industrial applications today still use amp gauges in part so they have a way to measure the volume of current being used. It is the amps that actually do the electrical work.
The volts is equal to the pressure behind the current.
Thanks Randy
