Refer to figure.
The figure depicts the centre-zero ammeter, which shows the current flow to and from the battery. Indications to the right show charge flowing into the battery, and indications to the left show the battery discharging (current flowing into the electrical system from the battery).

Before starting the engine, the battery master switch will be turned on which connects the electrical system to the main battery. This will of course, show a small drain in current from the battery as it powers the electrical system. Battery power is used to run the starter motor to start the engine. Once the engine is started, the alternator can be turned on - this generates electrical power from the rotation of the engine. The alternator will then charge the battery (showing a deflection of the ammeter to the right) and power the electrical system for the rest of the flight. During flight, the ammeter should display zero current flow - showing the battery is fully charged, and is not being drained.

Therefore the ammeter can be used to also indicate if the alternator is working correctly, even though it only shows the current flow to and from the battery.