The app reads data from the car's CAN network such as speed and electricity use and updates the distance to the next charging station, the difference between the remaining range and the distance to the station, and suggests a speed to the station. At the start of a journey, the driver enters the distance to the next station in the first screen shown here.
The app uses Bluetooth and can save data on the phone's SDcard. The app was developed on an older phone running Android 4.3 with an INTEY OBDII and Vgate Scan both inexpensive Bluetooth dongles. It has been test on a newer phone running Android 6.0. The app does not exchange data with the Internet and it does not use GPS.
There is no user manual yet. However, I will try to answer questions from owners of iMiev, CZero and iOn cars. The app still has bugs so please be patient with it and I take no responsibility for any consequences of its use.

Acknowledgements and references:
Much of the code for OBDZero comes from Blueterm by pymasde.es.
The commands to the Bluetooth dongle were found in ELM327DSH.pdf from www.elmelectronics.com
The interpretations of the CAN PIDs for speed, voltage and current etc. were found on http://myimiev.com/forum/ posted by jjlink, garygid, priusfan, plaes, dax, cristi, and kiev.
Special thanks to Ander Fanøe and Allan Korup for their advice on electric car and CAN technology.