Battery Charge fault

[Stufq78]

Hi

So I got C4 grand picasso 2015 1.6 blue hdi 115 with stop and start.
As above are developed battery charge fault, replace battery of the same specs, still error, change lower belt Tensioner as got knackered still error.
The wierd thing is before I even start the car I get 15.50v from alternator and after start goes to 18.50 and quickly drop to about 18.30v and it sort of hover around 18.30v. But the strangers thing is even if I start a car for two minutes battery get charge to about 15v and then after I stop the car the voltage will very slowly raise to about 16-17v.
But by morning voltage will drop to about 14.4v.
Now is this faulty alternator or that bloody Capacitor from stop/ start leak power. Voltage on super capacitor messuring 5.2v.
Please advice is driving me crazy.

[chunky2778]

The alternators in our fleet vans which sound like it might be the same engine are very common for alternators overcharging, have done loads of them in a Partner 16hdi around same year as yours.