Charging circuit - reduced load on ammeter circuit
... at some point the voltage drop across the meter becomes great enough to turn on one of the diodes, depending on which way current is flowing. ...
Correct, except the diode doesn't suddenly turn on, but starts conducting current progressively more as the voltage drop across it increases. Think of it as a "current relief valve" (opens proportionally like a back-pressure regulator instead of popping open). Before picking a diode, I measured the voltage drop from ALT+ to BATT+ (which the diodes would see) with the alternator maxed out (by applying full-field). The drop was ~0.5 V (or whatever I wrote). I recall I did this in my 1965 Chrysler, which might have had a round-back alternator then. Regardless, its ammeter was almost pegged. No worries if your alternator outputs more, as the diodes quickly conduct more current as the voltage drop increases, up to their rating of ~200A. The rating depends on cooling, so might check how hot they get at max load. Heat is conducted away by the thick copper cables (6 awg, I recall). I used a different PN on my 1965 Dart (lower rated germanium diodes, I recall) before I found this PN on ebay which installs easier (cut in two, flip and bolt together), which I put on my Valiant and Chrysler.
Re over-charging, as in olden days, if you see your ammeter needle staying on the "C" side for what seems too long, you have a problem. As I recall my 1969 Dart long ago, when driving it daily, the ammeter would run ~1/8th scale after starting for ~5 min, then return to center. But, depends on how long you must crank. If you also want a voltmeter, you can buy cig-lighter ones cheap from ebay or bald-guy. Some are integral w/ a USB adapter.
