An ammeter is nothing more than a shunt with a voltmeter-like gauge across it. The two-ammeter solution is really the same solution as the second shunt, just more complicated because the interconnecting wires will affect the reading. Make shunts out of a straight piece of wire, crimp and solder bare ring terminals on. The wire gauge will have to be selected by making some different shunts of different gauge all the same length and trying them.The bigger the wire, the more current the shunt takes, and the lower the ammeter reads.
A properly sized ammeter, including it's shunt or shunts should have full scale current (amps) set the same as the maximum output of the charging system.
Having the full scale current higher on the ammeter than what the charging system can do is harmless, and seen in some factory systems, just less than ideal. And yes, having the full scale current lower can peg the ammeter or burn it out when the battery is really low on charge. As Rusty 37 Master mentioned, normally with a full battery the charge tapers off right away, so you could get away with being a little bit wrong, but maybe not a lot wrong.
That all assumes the car is wired correctly and no accessories (except maybe the horn) are connected to the battery side (wrong side) of the ammeter.
One other thing to consider is that the battery will still draw the same amount of current on charge as it always did, so on a normal day, the ammeter will read much lower at a normal charge rate on a 100 amp ammeter than it did on a 30 amp ammeter. For instance if a battery is drawing 10 amps on charge, that's 1/10 scale indication on a 100 amp system, and 1/3 scale indication on a 30 amp system. The one or two amp maintenance charge that occurs when the battery is full is going to be difficult to even see on a 100 amp system.
Another thing to consider is that more current needs bigger wire, so don't get carried away with alternator (or generator) size when using stock wiring.
Last edited by bloo; 06/25/21 04:21 AM.