Well, trying to deliniate whether it is the alternator or battery which provides the electrical power to the aircraft systems is kinda' like trying to determine if the propeller pulls into a low pressure area, or pushes against a higher pressure area, to propel the aircraft. The right answer is it's either and both.
Without an alternator/generator, a battery is a wonderful source of electricity for a finite period. Without a battery, an alternator is a troublesome source of electricity for a much longer period of time. They work in harmony, each doing their own rather different job. Ammeters are basically the same as to how they work, be it center zero, or end of scale zero, the means of operation is the same. If you wire an end of scale zero ammeter differently, it will indicate backward off the scale.
So why does the referenced Cessna POH say that the alternator maintains the battery's state of charge? because the aircraft load is constantly discharging it otherwise! If you flick on the landing light, with the nav lights on, and operate electric flaps or an electric hydraulic system, you will momentarily exceed the capacity of the alternator. Do you pop the breaker? No, 'cause the battery takes the load for that time. When you turn final, and pull the power to idle, with the landing light on, you can be assured that the battery is powering everything, and will continue to do so, until you add power, because at idle, the alternator is doing very little. Indeed, this condition is very hard on alternator trives, for reasons a little beyond the scope of this post. My habit is to turn off the alternator side of the master switch when landing at night, to ease the load on the alternator drive. My battery is very useful at that time. Of course, a bit of ammeter monitored ground high idle afterword, brinngs the battery back up to where it should be.
So, the subject can be argued either way, but the bottom line, is the battery should be treated and respected as the source of electricity for the aircraft, with the alternator charging it. If I thought that the battery were no longer providing, or able to provide power in flight, I would most likely turn off everything electrical - certainly all avionics.
For the discussion about the position of the ammeter in the circuit, I again refer posters to the EI document I linked earlier. If you wanted the perfect indication of the function of the electrical system, you would actually use two ammeters, each connected in the circuit in a differnt place. The EI dcoument makes it very clear as to why.
The battery dosn't power anything when the alternator and rectifier are online.
The battery is purely for emergency power and starting the engine.
Disagreed.
The battery, in addition to the foregoing, acts as a giant capacitor in a way, absorbing and returning power to the system as needed. Without it, you have problems.
Pilot DAR