Yes, you’d damage the car’s electrical system. First of all it’s not designed to feed in energy through that outlet. It’s made to output energy.

And most importantly: 24V is way too much. 2 times the intended voltage would fry most electronics. Your stereo, the power steering, airbags, … There is a good margin and car electronics are designed to be pretty robust, but you’re pushing it.

I think they’re still fine because what happens is your car battery absorbs that extra voltage. But it’s really dangerous. On a sunny day you’ll charge your car battery beyond the 14V or so the chemistry can handle. And at that point it’ll degrade fast. The acid in there is going to start to boil, producing hydrogen, so in addition to a destroyed battery, you’re in for a small explosion if you’re very unlucky. And once the battery is gone it’ll start frying the cars electronics because now there isn’t anything keeping the voltage down.

Get a switch that exclusively connects either the car or the solar panel to the bluetti. One switch that switches between two things, not an On/Off switch. And make sure it’s rated for the current.

Edit: Or a relais that toggles between both. It can switch if there’s power on the 12V rail, and connect the bluetti to either or.

I might be a bit spoiled by the ESP32 and all its supported protocols.

maybe RS-485 or CAN