Each pair of relays handles one battery: relay 1+ handles the positive terminal of battery 1, while 1- is connected to the negative terminal of battery 1.

With no power to the switching circuit, all of the relays are in their default position of 87a, and there is no open path to any of the batteries.

When the switching circuit turns on, relays 1+ and 1- are switched on. This allows battery 1 to charge. After a couple of hours, the switching circuit turns off relays 1+ and 1-, and turns on 2+ and 2-. This means the charging current flows through relays 1+ and 1- through to 2+ and 2-, and into battery 2.

One reason for the terminal block is to cut down on the number of wires from the car to the relays, as the relays are not kept on board.

Although not shown in the diagram I used Anderson Powerpole connectors to connect the wires in between the batteries and the terminal block, as the charger is not kept in the car.