Just got my latest problem of excess acid corrosion on my 1917 Maxwell's batteries fixed and now I returned to my previous problem. Electrical systems have always been my weak area so Murphy's Law dictates that the type problems my car is giving me! When I first got this car running it charged for about 2 minutes and then quit reading on the dash panel charge indicator and the custom ordered auxillary ammeter. I knew that was my next repair when I started it to show a neighbor. For some reason instead of turning it off by the key switch I tried hitting the twin battery disconnect switches, I was shocked to see the engine stay running. Doesn't that mean my generator IS working and the gauges are NOT?
If it continues to run for any length of time with all batteries disconnected, then, yes the alternator has to be putting out SOME current.
However if the alternator is putting out 12.0 volts and your battery is at 12.6 volts, the ammeter won't show charging, but that's probably enough to keep the ignition system running.
Simplest test ... hook a digital voltmeter across the battery before starting the car, note the voltage. Start the car and run at about 1500 rpm in neutral. If the voltmeter doesn't show an increase in voltage (over what you first measured), you've got some sort of generator problem. Either the generator itself or the voltage regulator.
Not knowing whether your 1917 maxwell has the original charging system or not, I cannot answer for sure what you are experiencing. However, if the system you are using has a voltage regulator, the ammeter would read a high rate of charge right after you start the engine. Then after a few minutes the battery would be fully charged and the voltage regulator would keep the voltage just slightly more than the battery voltage, so would show very little or no charge at all.
If it were an older type generator such as the original Model T equipment, it would show a constant flow of current (amperes) as long as the engine is running.
If you disconnect the battery and continue to run the engine, you will be putting a very high voltage into the ignition system and it will soon fail. Don't do that again!