no charge signal beeps every 10 - 60 sec

[p435]

Volvo Penta MD2040
Alternator: Valeo, exact type cannot be indentified since it type plate is painted; image attached.


The beeping signal appears roughly every 10 - 60 sec in irregular periods. It is accompanied with the "no charge" indicator lamp signal at the engine control panel at the helm station.


At the beginning the beep appeared only once the engine was running for 1 - 3 h. Now the beeping start shortly after the engine starts.


The alternator appears to charges normal. Voltage measured is normal 13.9 V depending a bit on the RPMs. Never measured at high RPMs

Questions:

1 - What could be the cause of the beeping?
2 - What is a possible remedy?
3 - What could be the exact type number of the Valeo alternator?


Thanks in advance for helping comments.