You have a car? That is a huge upgrade since you started with only a car battery a few days ago.Goodbye.
So we started with a battery and a generator.Then a solar cell was added.Now a car.It really makes me wonder what more is missing from the whole picture.You know, it works a lot better if you give the whole picture from the start, because now there will always be the question "what other important info are you keeping from us?"
Cars and their batteries seem to co exist in millions of examples so I'm lost as to what the problem is about . If you want to charge a second battery in your car look at split charging relays .