This whole thread is an illustration of how useless ammeters are in modern (or modern electricals) cars. They had something useful to tell in the generator days, but not now.
The basic instrument for monitoring a car's electricals is a voltmeter. It will tell you more useful information about your electrical system's health than any other single instrument. Voltmeters also do not have the downside of needing to route 40-50-100 amps through the wiring to the gauge.
Ideally, the voltmeter should be wired straight to unswitched battery voltage - it draws only a few microamperes, not enough to drain a car battery in months. Connecting it to unswitched voltage lets you monitor resting and cranking voltage, both important.
A simple guide to automotive voltages:
- At rest (engine and all accessories off): 11.5-12.5 volts. Any lower and you have a charging problem. It will sometimes drop to about 11 volts in very cold weather.
- Engine running (at any speed): 13-14 volts. Any less, and your alternator or voltage regulator is faulty. Any higher and your voltage regulator is faulty. This voltage should stay fairly stable even when you switch on and off every accessory including headlights and fans. If all of your accessories on pull the voltage down below 13 volts, you probably need a larger alternator.
- Cranking: Varies quite a bit with engine size etc. but should go no lower than 7.5-8 volts while cranking. Any lower and you've got a failing battery, a charging problem or (rarely) a starter problem. If it doesn't drop at least a volt or two, the starter may not be engaging properly or may have other problems.
Alternator, VR, battery, starter - that's the whole electrical system and this gauge is telling you the health of each item. Ammeters... may as well be an idiot light.