z22se Astra G Bertone low charging


I know this has been discussed multiple times but this one is stranger than usual , for me at least.
I'm having a 'small' charging issue with my 2.2, and I'm not sure where to start. I was thinking that changing the alternator is a sure start but decided I should ask first

When I first start the car up (from cold), the alternator charge is 14.1 (at the battery), but slowly drops down over time even as low as 12.5, 12.2 (at the battery) having the headlights on and the fans (when reaching operating temperature). I know having stuff like the headlights / AC / Radio on loud will lower the voltage – it goes down to 13.6 when doing this right after the car starts from cold. But doing this with the engine worm (after you drive if for 30-40 minutes) it will drop to 12.5 or lower.

Has anyone encountered this before? Any idea on where to start?
Some other (I hope relevant) info: The car also had the engine swapped with the same engine type (z22se) engine after it had ridiculous oil consumption issues ([email protected]) and seized but it did cover around 10000km since and only had this issue once. Now you can’t even do a daily commute.

I know most Astra’s have a ground strap on the front of the engine bay (like the 1.6 for e.g.) but my 2.2 is missing that – not sure if this is relevant.

the earth cable is direct from the battery to the engine, usually on the bell housing. and another from battery to the body. check these are good and clean.
after that it ill be a new alternator. you should see over 13V at the battery with engine running and all loads on. this will go up a bit with increased engine rpm.