Hi Jean,
Just had a better read of this thread, it appears that both your batteries hold the same charge, is this all the time or just when you first installed the monitors?
If so it would appear the CTEK is more or less just balancing the batteries, though the main battery gets the charge and also powers everything, the second battery is either charging at the same rate or discharging the same way. That being the case you may as well just have a bigger single battery or the 2 batteries connected in parallel with a basic balancer between them, if at all. And it would also support the idea that the second battery is trickle charging the main battery...
A post on page 21 showing the Ineos quick-start guide appears to show that Ineos is aware the battery system can be fully drained (missed this while giving it a speed read on when I took the vehicle home!) suggesting CTEKs claims of battery guard are either not true or the system doesn't work as they advertise.
Cheers,
Paul