I attached a sketch about my project. There are two battery banks which are connect in series and parallel each. The fully charged bank consists of two 12 V batteries should always connected in series. That means that the parallel connection should be disconnected. The battery bank which is discharged should be in parallel (both 12 V batteries in parallel). That mean that the serial connection should be connected. The load (which is the same) should be connected always to the fully charged battery bank. I hope it is clear. :) Which type of relays would you engage?
To detect the SOC of both banks I want to use an Arudino controller. But the first question is how to solve this problem?