This is dc, so an SCR as a switch is a bit problematic. Once switched
on, they can be quite difficult to switch back off again. It can be
done, but cheaper power rectifiers are all that is needed in this
application.
All power rectifiers are diodes - not all diodes are power rectifiers.
You want diodes able to carry the current and with low forward voltage
drop - power rectifiers are the type of diode designed to be used in
such roles. They often need to have the chip bonded onto a big chunk of
metal, in order to be able to transfer waste heat into a heatsink.
Typically, you take a wire from the battery to a fusible link - or
resettable current trip - anything to protect the battery from a fault.
I usually use a battery connector that includes a manual
disconnect/isolator.
From the protection device, the wire goes on to the contactor contact.
From the other contactor contact, a wire goes to one of the power
rectifiers. The other power rectifier, with a wire from the other
battery, via its own protection device and contactor, can usually be
bolted to the same heatsink. This heatsink is then the common output
terminal.
Obviously the heatsink has both batteries connected to it and so it is
"live". There would be a big bang if this heatsink gets connected to the
battery return. But, hopefully, the protection device(s) will limit the
damage. You can get "semiconductor" fuses that should protect the power
rectifiers - if there is going to be a possibility that this could
happen. They are specially designed to blow faster than the rectifier
diode, under fault conditions.
