A voltmeter will tell you the voltage at the battery. An ammeter will tell you the current through the system, but not the state of the battery.
As every ammeter I know of requires that the full current in the system be routed through it, I agree with George that doing so is to introduce more hazard than is neccessary.
Modern vehicles use voltmeters exclusively. Last week I drove a brand new Western Star semi-tractor with the DataStar digital monitoring system. Every thing you want to know can be dialed up on that thing. But not amps. Just volts. It works perfectly fine, and the current load in that A-train combination is more than any five Volvos put together.
Do yourself a favour and stick with a voltmeter.