I am trying to power a Raspberry Pi from a 12v power source in a car. I have tried half a dozen different step-down boards and they all behave the same way, which I don't understand. From a bench power supply set at 12v, the system works perfectly. It produces 5.08v and when I hook it to the Pi it maybe drops .01v, the Pi boots up, everything is great.
When I hook this same hardware to a 12v source in the car, some of the regulators will immediately drop to 1.8-2v in no-load conditions, and all of them definitely drop to that range under load. I've been doing this purely off the car battery at the moment, so isn't any fluctuation in the system voltage.
I'm not sure even what to search for to describe this condition but my understanding of how these step-down circuits work is limited. Clearly this is a thing that is possible because there are 12v USB chargers and those work just fine in this car. The 12v source I am using would normally power the CD changer, and I have metered it at 12v +- 1%.
Any guidance would be appreciated.