Andyroo wrote: ↑
Sat Oct 06, 2018 10:25 pm
Would something like this do http://www.allspectrum.com/mopower/
You would need to sort power surges, voltage differences and check if the Pi does not draw power when off (hate to find it flat Monday going to work) but it may be a start point.
That is my design linked above. It has an automotive grade TVS for voltage spikes and an operating voltage up to 30v DC.
It draws a super low 0.25mA when the Pi is shut down. You can omit the UPS battery and use it on a constant (not ignition switched) supply, or operate it on a switched ignition with an additional battery (or supercapacitor). I prefer the constant supply since you can also program the UPS to power up your Pi at certain intervals.
If wired to constant power you can also setup the UPS to detect when the car (alternator via voltage) is ON/OFF and turn on and off your Pi without running an additional switched input. Check the "UPS Input Control Settings" section for an example of this.