Looking at your diagram and numbers, it appears I can use a bit more alternator and not have to bypass the ammeter, or have I left the reservation again??
Yes it always depends on the numbers.
As long as the alternator can supply the power at something over 13ish volts, the ammeter carries no current except for starting and charging.
We've all been sitting at a stop light at night in the rain and seen the ammeter swing over to discharge. What was happening here is the alternator couldn't produce the power demanded for ignition, field, lights, brake lights, wipers and blower at 600 rpm. Voltage drops to the point where its equal or less than battery voltage, so current starts flowing out of the battery.
An alternator that can produce more power at idle will avoid that problem, if its a problem. In the old days, we sit at a light in the rain for a minute, battery drains a little, we get moving and it recharges. No big deal.
But the way people can sit in traffic these days, with A/C on and so forth, it can run down a battery. When recharging, the lower the battery's state, the more current it will draw at a given voltage. So with a battery getting really drained, recharging at 14 Volts maybe its drawing 10 or 15 amps, and doing it every 3 or 4 minutes. An alternator that is better at idle will solve that problem because the battery won't get run down even in those conditions.
(The flip side is if the battery does get really run down, the better alternator may be able to supply the battery
the 35 or 40 amps it wants at 14 Volts. That's not such a good thing as discussed in the link.)
The ammeter will show exactly what is going on, so this is where it is useful.
There's lots of ways to abuse the charging circuit including the ammeter. Some years/versions can take the abuse better than others.
One way to abuse it is a long period or repeated high rate of charge. Typical example is someone gets a jump start and then immediately driving on the highway. The one saving grace of lowest output alternators was that even at maximum output, it was generally less than 40 amps. But even 25 or 30 amps through those connections is probably going to have resistance enough to build heat.
Another way is to tap in at the battery for more circuits. We've all seen this - many of us have done it. Plow, winch, stereo, lights, are typical additions. Some of those have a pretty big draws. If the alternator can provide it, its still the highest voltage power supply - so that's all going through the ammeter to get to the tap. If the alternator can't provide it, the battery does and then the alternator recharges the battery. In these situations its not really surprising that ammeters fail if something else doesn't go first.
Hope that make sense. Didn't intend to write that much. dang, turning into A/J LOL
