Battery overcharge

HI
Very unlikely that the Dc Dc charger is overcharging
But you may be getting false readings ,check ALL connections are clean & tight Especially battery terminals
PeterQ

I had to relocate mine from under the bonnet to inside the canopy, as I had heat faults coming up. All good since Ctek 250D

Being a 5 odd stage charger, the ctec will probably spike to 'overcharge' levels during desulphation which is quite normal and would result in your other monitor going berserk. Check your user guide for how they work.

Where is the Ctek installed? Where is the battery installed and where is the temp probe positioned?
Yes you can overcharge with a D250S. Usually happens if the temp probe has not been placed next to the battery so temp compensation doesn't occur.