I have a situation that my altenator is 24 VDC and I have two 12 VDC Batteries. I also have equipment that needs 24 VDC power and some that requires 12 VDC. I understand to series the batteries off the altenator for charging 24VDC. I want to use a common DC GND and between the two batteries use the 12 VDC for the devices needing 12 VDC. Others say I need a DC to DC converter. Why?