Ammeters are always hooked in series with the load. In this case, it's
normal to hook them between the battery and the rest of the system. The
full current passes through the ammeter with a couple of coils internal to
the meter which creates a slight magnetic field and the needle moves
accordingly. It only works in series. Since many meters have very fine
windings inside, you can imagine a 60 amp current running through them
would result in so much molten slag. Instead they have a shunt or heavy
resistor in place that will let 90% of the current bypass the ammeter, but
enough passes through so that you get a representation of the true
load. These are usually physically bolted to the two terminals of an
ammeter and look like a metal plate with some epoxy or lump in the middle.
That's the way it used to be done.
If you hook it between the alternator and the battery, then you are only
measuring the load (battery and the rest of the system) you are putting on
the alternator/generator alone. It won't recognize if you have the engine
off and the lights on. It only senses a load passing through from the
alternator.
You could also hook the meter between the battery and the main power feed
to your electrical system. That way you get a chance to see discharging
as well as charging aspects. This only works with center-zero types of
meters though.
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