I'm curious. I put a 60 amp alternator on the 53. The original max output with the OEM generator was 45 amps. I've had absolutely no issues with the 60 amp output damaging the OEM ammeter. I was just thinking about how much current the ammeter can take without being damaged. Could one use a 100 amp alternator without killing the OEM ammeter? I put a 60 amp alternator on the 38 and damaged the OEM ammeter on it because it was designed for a 30 amp charging system. If, sometime in the future, I decided to put a 100 amp on the 53 would it damage the ammeter?
I doubt if you will damage the ammeter on the ”˜53 with a 60 amp alternator. Remember that the ammeter reads the net charge going to the battery. So any electrical load such as the ignition reduces the current going to the battery.
About the only scenario that could put a lot of current through the ammeter is the short time after the engine is started when the battery might require a heavy recharge. That should only happen a short period of time and then quickly taper down to a very nominal value.
I can understand why the ammeter in your ”˜38 failed if you installed a 60 amp alternator in it. That ammeter is only rated at 30 amps. I expect you would have a similar failure if you installed a 100 amp unit in the ”˜53. Except I have never seen an ad for a 100 amp 6 volt alternator.
Remember that when you reduce the voltage on a 12 volt alternator to 6 volts you also cut the amperage in half. That is why you only see 35 amp and 60 amp 10si model 6 volt alternators. They started as 70 amp and 120 amp 12 volt alternators. There were no 200 amp 12 volt 10si alternators.
I have read some discussions about putting a shunt in parallel with the ammeter to reduce the current going through the ammeter. Another idea would be to hide a second same model ammeter in parallel with the one in the dash. Each ammeter would read half the current going to the battery I have not spent much time researching either solution.
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.