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 car output is going to be ~12-14Vdc and the solar panel at 24V. You do not want to connect those two together. Ideally you have some kind of switch that connects the input to the bluetti only to the solar panel or only to the car outlet, and to the car output only if it is present. You can probably use a contactor or a heavy duty relay that is 12V and connect the NC contact to the panels, NO to the car, and COM to the bluetti. Add a capacitor to dampen spikes whenever the system switches and connect the grounds together. A bjt jank, and also have to check the relay ratings / coil power usage