I owned a Piper which had a “loadmeter.” Essentially, the discharge half of a + / - ammeter … a nutty thing to have in an airplane. As you say, looking at the regular ammeter ought to tell you if there’s enough excitation to power the alternator to provide charging. But that’s only half the story.
Voltage regulation is part of the story, too. In my 172, I have an over voltage protection circuit which removes excitation from the alternator and lights up a light that says HIGH voltage if bus voltage goes above ~16v. HUH! If that light comes on, the alternator is – for some reason – not producing power therefore you have LOW voltage. SOME Cessnas did say low voltage. Musta been a different design engineer? Only a voltmeter will tell you the static (at rest) state of a battery and the charging voltage when the airplane is operating.
As an A&P who can change batteries in my flashlight without electrocuting myself or using black electrical tape, I espouse putting both a full ammeter AND voltmeter in every airplane. That way, you can tell what’s going on with the charging system fully. Also, the charging system is not designed to fully charge a battery; it’s designed to bring it up to a point where it won’t overcharge the battery thereby boiling electrolyte (flooded lead acid) or ruining an AGM type. Occasionally, it would behoove EVERY owner to have the proper charging equipment and fully charge the battery while watching battery case temp. This can only be done with a charger capable of providing constant current mode for the specific voltage of the battery. Anything less – like the charging system – will keep the battery mostly charged but NOT fully charged; translated, battery longevity is less. Battery manufacturers don’t want you to know this for the obvious reason. Sulfation begins the instant a batteries voltage falls below it’s 100% fully charged voltage. Keep it above that and sulfation won’t occur.
I PREACH this to everyone who will listen; and it applies to vehicles, too. My batteries in all my vehicles last years and years because I do that about once a month. I have a big '97 Ford F250 w/ 460 V8 that is now on only its second battery which has lasted … are ya ready … 15 1/2 years! I don’t drive it much but it’s always on a trickle charger that supplies 13.4v. It’s now down to a game to see how long it’ll last. I wouldn’t trust it to go cross country but the battery is still happy because it isn’t allowed to sulfate (due to the charger) AND it was a good pure lead battery to begin with … not ground up WalMart battery lead. These days, many chargers have desulfation modes (high peak voltage with very narrow pulse width of a proper frequency riding on the DC charging voltage). I’ve been able to salvage batteries that otherwise would have been tossed by using these.
Bottom line … every airplane owner should occasionally use external charging system capable of constant current (full out) output and give the battery a good ‘drink’ of electrons. In between, use of a trickle charger will keep sulfation from occurring. I shudder to think of what will happen if electric airplanes ever become de rigueur and aren’t given proper care.
Keeping your battery happy will prevent you from one of PB’s ‘statistics.’