Mauro,
Do you see the red light only when the engine is idling? If so, then I personally wouldn't worry too much about it. If you continue to see the light when you increase the engine RPMs, then you can ignore the rest of my post.
Warning: I am not an automotive electrician, so the following is my opinion:
When I was young and driving (American) cars from the 50s and 60s, it was very common for the charging light to come on (usually dimly), at idle with the headlights and radio on. It just meant that at idle, the cars electrical accessories were pulling more amperage than the alternator (or generator) was capable of supplying (at idle). So the difference was made up by the battery, and once you increased the engine RPMs the battery would begin charging again.
The 14.6 volts at idle with no accessories on means that your alternator is charging the battery under light load, even when idling. The 13.3 volts even when the light is on means that your battery is healthy and is making up for the small usage deficit. The draw on your battery is probably very slight, and it would take a long time for the battery voltage to drop significantly.
Again, just my humble opinion.
-David