Car battery/ aux battery wiring question

[enorman66]

I have a diesel van that has a dual battery starting system. Each battery has 850CCA.
I have 75w of roof mounted solar panels that are running through a 100amp controller and charging an aux battery that has 55amp hours. All batteries are 12v.

I would like to be able to charge my aux battery when the vehicle is running without drawing down my starting batteries when the vehicle is not running. I do know the difference between series and parallel. I will be wiring 12v parallel.

Several questions:
Can I use a simple disconnect switch instead of a controller or do I need to have a controller becuase the aux is smaller and less amps than the starting batteries? (still 12v)

Can I run the positive for the aux battery through a controller or switch to the closest positive terminal on the #2 starting battery (which is about 10ft closer than the other stARTING battery) or do I have to run it up to the alternator?
With this setup can I just connect the aux negative to the frame or does it have to connect to the charging battery?

I don't see why I couldn't just tie into the closest battery and use an isolator in between but the RV dealer and my personal mechanic are telling me I have two run all my wiring all the way back up to the front of the vehicle to the alternator. There is about a 10ft differece and a real pain compared to jumping to the battery that is 2ft away. But I am not sure if it will damage the system in some way... If it will improperly drain the one starting battery and cause some sort of charge loop problem.

Also, I am concerned that the aux battery will get damaged becuase it is not the same amp rating as the starting battery... does it make a difference?

Since I have the solar panels on its own controller will it damage the panels when charging from the alternator? I don't thing it would but am not sure.

 

[Okefenokee]

A big capacity battery can charge a smaller battery but not the other way around. The big battery can possible draw more current during a charge than the smaller battery can safely supply. If they're similar in size then it's probably ok but don't quote me on that.

Other than that, yes you can connect the batteries through a switch. Actually, connecting the aux battery to the alternator would be the same exact thing as connecting it to the main battery. I'm pretty sure that the alternator is wired to the battery. If you have an Ohmeter then test between the Pos of the battery and the Pos of the alternator. I bet it reads 0 which means that it's wired directly to it.

Yes, you can use the chasis as a ground.

It's probably safe to connect the output of your solar panel controller to another source. Think about this. In the dark, the panels don't generate anything so the controller has to block the charged 12V battery from sending current backwards through the panels.

Remember that the current through the switch between the batteries could be high if one of them is dead. Don't undersize it. Don't undersize the wire either.