The 5V is not picked from Arduino. It's converted and regulated from the 12V. The 5V is for powering Arduino. Uh! So a second question arises: is the adapter somehow protected from voltage spikes coming from the car power? I read in a lot of post many recommendations in doing so (e.g. via a voltage...
Please download the latest source code instead the one of last year. All resources (downloads, guides) are available on the [url=http://freematics.com/pages/products/arduino-telematics-kit-3/product page[/url].
A new API of getVoltage() has just been added in OBD-II library for getting car battery voltage. This feature is provided by OBD-II adapter and does not requires car engine started. The latest code is in the repository - https://github.com/stanleyhuangyc/ArduinoOBD