Just another point of view, but I would rather use a battery disconnect unit (like this
https://www.youtube.com/watch?v=lMT1RdnkxnY). Reason is, as mentioned above, is that what is important is not only to keep the battery from getting flat, but also to keep it from charge/discharge cycles. Car batteries hate being discharged past a certain level (unlike phone batteries, for example). A solar panel doesn't work at night, so that's when the battery discharges anyway, and this is what will wear the battery quickly. A quick disconnect switch (or simply disconnecting the battery each time after driving the car) is what will preserve the battery life. Of course, disconnecting every time is not handy, but otherwise sooner or later the battery will need replaced.