https://pilotworkshop.com/tips/instant-voltmeter/

extraits

The digital voltmeter makes an excellent troubleshooting tool—and it shouldn’t read only 12 or 24 volts.

A healthy 12-volt charging system reads about 14 volts in flight. That’s needed to replenish the battery. That value of about 14 volts should stay the same no matter how many electrical items you turn on. Seeing much less than 14 volts could mean your generator or alternator isn’t keeping up with demand, even though your ammeter reads about normal. Seeing less than 12 volts probably means you’re running on battery power. You should see a discharge on your ammeter at that point.

For a 24-volt system, you should see about 28 volts in flight.