If you connect a solar panel directly to a car battery, it will eventually overcharge the battery and damage it. That’s why you need to use a solar charge controller. This is my own design using the latest microcontroller and MOSFET circuitry. It consumes less than 1mA (that’s very little) and has a neat little blue LED that tells you the battery voltage. It can also be connected in various different configurations so you can do clever things like charge two batteries from one solar panel.