So I've been toying with this idea for a while, I use a 2000W Hub Drive kit on my bike, and I never really got on well with the supplied display.  
  
I always have my phone mounted on my handlebars when I ride so the primary aim was to ditch the built in display and buttons altogether, and put together a simple smart phone app which will display Speed, Battery % and PAS Mode.

It's still early days but I have a working prototype/beta.   
I use an ESP32 which plugs directly into the display connector on the e-bike controller (the controller is nothing special, just one of those generic Chinese ones), the iPhone app communicates with the esp32 via BLE.

The battery level is read through a potential divider and into one of the esp32 adc inputs.  
All of the original display's functionality is implemented. PAS selection, Cruise control, Wheel size, Magnets, Low Voltage Cutoff, Only thing I didn't get round to yet is to trigger lights on/off (that won't be hard to implement I just never use lights so haven't got round to it)

Rides can be 'recorded' and exported as KML files so they can easily be imported into Google Earth etc.

It basically costs next to nothing to implement, an ESP32, a 5v buck converter to power the esp32 from the bike battery, and a couple of resistors for the divider....so around 10 £/$/euros all in.  
  
It really is cool to have just one device to look at for everything, bike data, maps/directions, and obviously all the usual notifications that modern life likes to throw at me...with the added bonus of de-cluttering my handlebars...the throttle is the only cable from the controller to my handlebars.

Just wondering if people have any suggestions or improvements in mind?

BTW: The debug mode is just generating random sweeps for speed and battery values.



Ideas for Smart Phone based Ebike Computer