He wants to use one 12V (or 7.2V) battery to power electronics and other appendages and one 24V battery to power the motors. But he wants only one charger, the 24V battery charger. For the 12V charger (mounted on the robot) he wants to get power from the 24V charger and use the DE switching regulator to step down to 12V. The reason for all this trouble is to have the robot up and alive during charging and at least the electronics to be able to survive a motor battery low power if the robot doesn't find the charger station in time. It's like having a backup battery. I would have a relay controlled from the MCU to switch 12V to motors to get the robot to the charging station in case of primary battery fails.