I really didn't want to start cutting into wires on my new car and I couldn't find any points in the under-dash fusebox to tap that had ACC switched power, so I built a circuit to sense the voltage and activate a relay when it was higher than the resting voltage of the battery. When you start the engine, the voltage will go up to > 13 volts, when you turn it off, it should go down to < 12.5 or so.
I started with a circuit found online intended to cut off power when the input voltage dropped too low. Here's the circuit (with final values in it)
I breadboarded it up and tested it by attaching it to my variable bench power supply and running the voltage up and down to make it trigger. I originally tried a 12V zener, but it triggered a bit too high, because the transistor has a saturation voltage of around 1 volt which needs to be added to the zener voltage to get the trigger voltage. An 11 volt zener turned out to be about perfect. The original circuit also used a few hundred μF capacitor, but that took tens of seconds to trigger both ways, so I changed to a
2.2μF 680μF capacitor. (NOTE changed later to 680μF - my car drops voltage to < 12.6V occasionally when the battery is topped off, so in field testing the output actually switched off and on frequently - the larger capacitor keeps it running for a few minutes until the voltage comes back up again)
I also tested its current draw. When in the off mode, this circuit draws less than my meter can measure, so it's less than 0.1 microamps. This is completely irrelevant to a car battery and won't run anything dead. When in the on mode it draws about 40 milliamps to drive the relay, but the car is running then and that's a perfectly acceptable draw.
Once I was happy with the trigger voltage and speed, I took it out to the garage to test that it turned on and off as expected:
Once satisfied, I wired up a permanent version on a variboard. Most of the parts are stuff I just keep around for throwing together random ideas. The 12v relay was salvaged from my old lightning destroyed garage door opener before trashing it (I got 4 relays from that).
I designed and 3D printed a box to put it in complete with some lugs on the side to zip-tie it up under my dashboard. The box design is up on YouMagine at this link.
I clipped the female socket off of a 12V Y adapter (the cheapest way to get one in the shop) and wired it to the ground and output, and used a standard fuse tap to get power. I'll find a ground under the dash somewhere and wire that to the ground, and lash the whole thing up there with zip ties.
Here's the box ready to go into the car: