Referring to the following diagram, the configuration shows four batteries with their negatives connected together to form a common negative rail.
The positives are all terminated individually to the poles of four discretely attached SPDT switches.
The one of the two changeover contacts of the SPDT switches are appropriately connected with the output load while the others with the battery charger positive.
All the above terminations are made via separate rectifier diodes, each for output and input positives of the batteries.
The discussed parallel battery charger with changeover circuit using SPDT switches allows the user with options to connect as many number of batteries as desired in the array, and also to select which battery or how many batteries need be integrated with the charging system, or with the output, or both.
The diodes in the system make sure that the batteries don't interact with each other producing a cross discharge across each other, and ensures a step wise charging and discharging for the same.