Is the ammeter not supposed to return to the center position after the check?
Ok, I think 'Charging systems 101" is in order here.
Note: This is a general description that pretty much applies to most aircraft and cars - actual numbers and some details may vary.
Lets define some terms:
Voltage - think of voltage like pressure - high voltage is high pressure - measured in volts.
Current - think of current like a flow rate - measured in amps (this is what your ammeter reads).
Power - this is the voltage times the current - measured in watts. Simce the voltage in your 150 is nearly constant, we can use the words "power" and "current" somewhat interchangably to indicate which direction or how much stuff is flowing (in or out of a device).
A pressure washer like you use to clean your parents deck is a high pressure, low flow device - high voltage, low current.
If you dump out a 5 gallon bucket, you get a lot of flow (current) but there isn't much pressure.
Ok, on to the electrical system.
You have a few different kinds of things in the electrical system
The battery stores electricity in the form of potential chemical energy and can either supply current/power (discharge) or absorb current/power (charging). When the battery is just sitting there and not supplying or absorbing current, it will be sitting at about 12 or so volts. The actual voltage will depend on temperature and how well the battery is charged. (typically - there are also 24 volt systems in a lot of aircraft)
The alternator converts mechanical energy into electrical current / power and is controlled by a regulator that tries to keep the system voltage at about 14.5 to 14.7 volts
And you have electrical loads that use the current / power to do things like let you talk over the radio, light things up, start the engine, etc.
The ammeter measures how much current is flowing into or out of the battery.
Lets walk through what should happen with a "good" system.
You get in, flip on the master and perhaps one or two other things (strobe, beacon, whatever). At this point, the battery is supplying all of the current that is being used by the loads and the ammeter should show a slight discharge. It might be hard to see because the current draw of things like modern radios / transponders is very small. The system voltage will be about 12 or so volts.
When you start to crank the engine, the starter motor takes a LOT of current out of the battery to start the engine. The starter motor has it's own wire from the battery so the current doesn't flow through the ammeter (it is too high and would burn the meter out) so you don't see much action on the meter. But as you probably know, it doesn't take long for a starter to suck all of the power out of the battery if the engine doesn't start. During the crank the system voltage will drop quite a bit.
Now, the engine starts. As it spins up, it turns the alternator which can now start generating electricity. The system voltage should jump up to about 14.5 volts and current will start to flow back into the battery to make up for what the starter had pulled out. Right after the engine starts, you look down at the panel to check that the oil pressure is coming up and glance at the ammeter as well. The ammeter should show a high charging current for a few seconds then back off to about zero. How high and how long it takes to back off will depend on how much cranking you did (or if you left the master on for a couple hours with the engine off...)
Now, at this point, the alternator has topped off the battery - so current is neither flowing in or out and the ammeter sits in the center. (The alternator is supplying any current necessary to keep the elecrical loads happy.) And the system voltage will sit at about 14.5
Suppose you turn on a load that takes a lot of current - say a landing light. Right when you turn it on, it will draw more current than the alternator is supplying so the battery will take up the slack and you will possibly see the ammeter needle jump. But, after a fraction of a second, the regulator will crank up the alternator enough to supply the necessary current and the ammeter will go back to zero. When you turn the light off, you might see the ammeter flick again.
Note: If you turn on everything electrical (lights, beacon, pitot heat, landing light, Super-Homer, etc.), and the engine is just idling, it is possible that you will be consuming more current than the alternator can supply and you will see some discharge, but if you speed up the engine a bit (say 1500 - 2000 rpm) the needle should (on a modern aircraft or automobile) move back to the center.
Now, lets think about what happens if the alternator fails.
First of all, once the engine starts, the alternator is not putting the cranking current back into the battery so you don't see the ammeter show any charging for a few seconds like you would expect (that's one reason to look at it right away). Instead, the ammeter will continue to show about the same discharge that it was before you started the engine (but depending on what is turned on, it may be hard to see on the gauge). Your system voltage will be about 12 volts and gradually decline...
Now, if you turn on the landing light, all of the current has to come out of the battery all of the time since the alternator is not supplying what is required - right? So, the ammeter will show a noticable discharge for as long as the landing light is on. And, speeding the engine up to 1500-2000 won't make any difference. Turn the landing light off and the current draw from the battery drops to what it was before and the needle moves back to about the middle (depending on what else is on). That's why turning the landing light on is a good check - it typically uses enough current to make the needle move a noticable ammount.
What happens with time? If the alterntor has failed, the more you have turned on, the faster you pull current out of the battery, the more likely you are to notice the discharge on the ammeter, and the faster it will go dead. But if you only have strobes and radios on, you are pulling very little current so you might not notice it on the ammeter and it will take a while to suck the battery dry.