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?
Put a 12V to 24V inverter just inside the firewall. This will isolate the “noisy” vehicle power system from the more fragile solar panel and power bank. It won’t be nearly as expensive as an inverter that can survive the engine bay.
The 24V output of the inverter should be wired in parallel with one or several 24V solar panels, then into the power bank input. In this configuration the vehicle power system and solar system can charge the power bank simultaneously. No action is needed to change between power modes. However, you should have a main cutoff switch, disconnecting all power from the power bank.
I highly recommend a Victron brand inverter.
Warning: Direct current (DC) systems present an extreme fire danger. Every wire must be the correct gauge, every fuse the right size, every crimp correct. No piece of the puzzle is difficult. But, one must have all of them to build a safe and reliable system.