Ask HN: Power Raspberry Pi in car, after engine goes off?

Do you have some extra accessories, or just the RPi? RPi itself should be fine with just a standard mobile phone powerbank. If you get one that can connect to both the phone and power supply at the same time, it should be enough.

You may need to think of some way to shut down automatically though - otherwise you'll drain the whole battery after the engine turns off and a short 5min drive won't recharge it enough to hold for another 1min after shutdown.

Depending on the car, some have accessory power that stays on for five minutes or so after the ignition is killed, and most of not all have lines that power the security systems. I would tap into one of those.

there are UPS specifically for the pi, e.g. http://piups.net/ (just an example, there are others and I have no experience with any of them.)