CTEK D250SA Dual and the old D250S

How to fix the charging between batteries the best way, and also be able to use a solar panel.

There are however a pretty new D250SA modell out, in which it is possible to charge even your starter battery from the solar panel.

The new Ctek D250SA, the "old" one D250S is still available. Also a new Smartpass 120 have been added, the old one was just called "Smartpass"

Perhaps one of the best methods if you have start and service batteries:
CTEK D250S Dual