I have a 3d printer in my car powered by a bluetti eb3a. The bluetti is charged by the 12v outlet in my car while driving. I have a 24v solar panel i want to mount to the roof of the car. My bluetti has only one dc 7909 input. So got a a Y splitter to combine power from the car and panel. But when plug it into the car outlet that turns off when i turn off the car, the solar panel turns on the stereo. Thats not the case with the outlets that are always on. I am worried that i will danage the car electrical system from a 24v solar panel flowing electricity back into the car?
A relay switch could work. You would put the car 12V and ground across the coil connections, the 12V on the NO connection, the panel on the NC connection, and the power pack on the C connection. Make sure you get a 12VDC relay.
Are you going to have the panel installed permanently on your car?
Definitely don’t connect a 24V source directly to your cars 12V system. There’s a good chance of frying something important. A fuse would not protect from overvoltage so don’t rely on those to protect components.