an ammeter will tell you if some unknown amount of current is flowing into or out of the battery, if the gauge shows "neutral" you have no idea what the charging system is doing - a voltmeter
tells you the condition of the battery all the time, which is what you need to know, how much voltage is in/applied to the battery right now, regardless of its rated voltage, just as if an elf was under the hood holding a digital meter on the battery - if the voltage is less than 13.5V [12V system] and slowly dropping with no electrical load other than the minimal ignition current flow, it's not charging, and your old ammeter isn't precision enough to see that - if it's reading well above it's charging rating [~14V], the charging system is defective .... aside from safety issues [not a big problem on an A-OK totally original design system], as Mike points out in the Tip, voltmeters are particularly useful for modified systems with high output alternators and heavy battery use
Bill
