VW Fox 2010 1.2 not enough current for battery
So I have recently bought VW Fox 2010, 1.2 engine. After few days, the engine wasnt able to start so we have replaced old acumulator / battery for the new one and we was informed that alternator is not working correctly - it was making 13.7V instead of 14.4V
So we have drove to mechanic and he tried everything, he replaced used parts of alternator, tried other alternators and still same thing.
On cold engine, everything turned on, I got 14.4V but when engine gets warm it drops to 13.7V and it stays there. So does anyone have faced similar problems?
One more thing, computer doenst give us any errors.