120 with 2 fairly new 6V batteries not charging

Cant find my old thread, but the AMPSmeter shows +15 at speeds over 2000 rpm when deiving. And tet tge Ctek charger tell tale shows red after a drive. I conbect the charger snd it immefiately shows yellow. After a few hrs it switches to green.
Battery or charge regilator problem?
The car had this modern, generator lookalike alternator and neg ground, and voltage regulator.

You should measure the voltage on the car after driving.
Probably it is fine, as Ctek does a high charge first, after that going green to puls loading.
Never trust the Ctek for the condition of you battery’s