If I have a main battery that is connected to solar through a Victron charge controller and another 12 volt battery that is in a vehicle and I want to connect the two batteries so that if the vehicle is running I am supplying 12 volts (basically taping into the ignition key so only charges when the vehicle is on) do I hook the battery in the vehicle and the solar panel both to the single charge controller that controls the main battery.
I may need a DC-DC charger instead of hooking the lead acid battery directly to the charge controller.
Thanks in advance.
You need a dc to dc charger.
Terminology-- it helps if you use standard battery terminology. So, not main and battery in my vehicle. There are starter battery, the normal flooded lead acid one all cars have. And there is a House battery, which is usually a LiFePO4 lithium battery these days, to run camper loads like lights, fans, diesel heater, etc. There are also 2nd starter batteries, just to increase starting power.
Alternators are smart now and vary the voltage as loads change. Old isolator setup doesn't work sufficiently now.
Every company makes 4 or 5. There are 1000 threads on charging A 2nd or house battery from your alternator using a dc dc charger.
The dc to dc charger isolates the two batteries so your start battery doesnt get drained. Protects alternator from high fast loads possible with lithium. Provide correct voltages in 4 stages to properly charge lithium. Limit amps from your alternator so you don't burn it out overcharging. All that is needed.
Alternator should be run at 50 to 60% max capacity, not more when extended, or you will overtemp it and ruin it. Without a large alternator, 25A to 30A is probably most you should pull. They have 50A ones, be sure your alternator can handle that. Know your car's electric load with all stuff running, and alternator size.
Victron makes good ones, the Orion line.
Redarc makes good one, called BCDC25, BCDC40.
Kisae makes a good one.
Renogy makes one that also has SCC controller, but they break a lot, generally get replaced later with victron.