I've been running my NL battery box in the back of the bakkie for some time. Just with a power cable running from the engine compartment and then plugging in via the red BH plug on the box. So therefore using the built in solenoid system in the box.

I want to add another battery to the system to bump up my capacity to around 200ah or more. (Currently looking at the Omnipower 120ah)

Now that I have a drawer system fitted there is some space to fit a battery between the drawers and the side behind the wheel arch... but the NL box won't fit in there so that box has to stand on the drawer system behind my kitchen cupboard in front of the wheel arch.

What would the best way be to connect the two to have them both charging when driving and when I plug in the solar system?