Originally Posted by toshu
you raise an interesting point....do deep cycle batteries need a "different" or separate alternator/regulator for charging them as opposed to regular batteries??
That obviously needs to be defined before anything else can be decided....
I do want the system to be fully run by the car engine, as I see this as an emergency set up, where I may not have my generator with me.
You CAN charge them both off the same alternator/regulator. That's what happens with standard camper wiring - the camper battery(s) is/are put in parallel with the tow vehicle battery. The charging regimen is optimized for the tow vehicle battery, however sensing will be thrown off by the charge state of the camper battery if it is significantly different.
Bottom line is that recharging of a deep cycle camper battery by the standard tow vehicle setup will be much slower than if an optimized charge regimen was used (assumes a fairly discharged deep cycle battery in the camper).
Some truck tow packages have a cutoff relay that prevents the camper from taking power from the tow vehicle battery when the ignition is off - but certainly not most tow vehicles (mine don't have that relay). Without that relay, the camper can run the tow vehicle battery down to nothing during an overnight stop if the fridge is running on DC.
But the above is not what you originally asked. You specifically wanted the extra battery to be isolated and not be able to drain the starting battery when in use.
The simple way to power your ham radios is to wire the extra battery in parallel with an ignition-driven relay installed to disconnect the extra battery when the ignition is not on. The ham radio is wired only to the extra battery. You accept the slower charge rate (takes much longer to fully charge) of the extra battery as the cost of simplicity and automation. If the extra battery is drained, your ham radio is inop. If you leave the ignition switch on with the engine off, you will drain one battery into the other if they are at different charge states.
just my thoughts, yours may differ