If manually switching your device on/off is an inconvenient process, then there is an application to automate this. AutoSleeper is a free app that can on/off your device automatically when you turn your car on and off. That is, the app automatically wakes your device when you start car’s engine and put into sleep mode when you stop the engine – no looking for the Power Button, whatsoever, any more!
AutoSleeper makes use of external power on and off state to detect car on/off condition, which means you need to connect your car’s power supply to your device for proper working of the app. This technique works on most cars, because majority of cars disable 12V power when the engine is turned off, and only turn it back on when you start the engine or put the car into ACC mode. “Autosleeper is an absolutely useful freeware that makes life easier and adds great value to your Tablet!” The app features the followings: AutoSleeper requires Android 4.0 and up. It is designed to work with both Android Tablets and Smartphones. You can download the app free from Play Store (source)