Splitting up the batteries and charging
I want to install a second battery dedicated to running the forced air furnace. I know there are going to be a lot of suggested alternatives like generators, catalytic heaters, rent a space with hook...
Forum Discussion
Why have separate battery banks? Why not just add a couple of additional 6V batteries to your main bank? Having a separate bank just seems to me to be increasing the complexity and reducing the flexibility of the system...you can't use the primary bank to power the furnace should the secondary bank be run down (at least without some additional switching), you can't use the additional capacity for other things in mild weather, etc.
That aside, charging both banks together if they're at quite different states of charge can't be ideal, though I don't think it would cause any great problems either. As shown, you have the possibility to accidentally have the solar controller connected to no battery; many (though not all) solar controllers will be damaged if this is done, so check the documentation for yours carefully. It would be possible to eliminate this problem with (the right choice of a) single switch to choose between battery banks for charging.
You would need some fuses that aren't shown in your diagram. In particular, if both switches are closed and one battery bank is rather discharged compared to the other, a quite high current could flow between the two banks depending on the overall impedance of the connecting wire.
That aside, charging both banks together if they're at quite different states of charge can't be ideal, though I don't think it would cause any great problems either. As shown, you have the possibility to accidentally have the solar controller connected to no battery; many (though not all) solar controllers will be damaged if this is done, so check the documentation for yours carefully. It would be possible to eliminate this problem with (the right choice of a) single switch to choose between battery banks for charging.
You would need some fuses that aren't shown in your diagram. In particular, if both switches are closed and one battery bank is rather discharged compared to the other, a quite high current could flow between the two banks depending on the overall impedance of the connecting wire.
