the starter motor is the greatest power consuming device on a car. say a 2 hp starter motor will draw about 120-200 amperes. common automotive ammeters have usually a maximum reading of 100 A. usual gage have 60 A. thus the gage will blow out. afaik, ampere gages used on cars have a wire wired in parallel to the ampere gage to prevent this. exception are the alternator and starter testers used by auto and battery shops.

so my say is the voltmeter is better than the ammeter to check for a no-charge or overcharging condition. ammeters are way cheaper than voltmeters thats why most mechanics around the block use it for cars