Owners of Inventists Solowheel Xtreme can use this free app to connect their phone and Xtreme to display the battery charge and speed.
Bluetooth LE (LE = Low Energy. "BLE" for short.) is not the same Bluetooth used for headsets and speakers. BLE is designed for remote sensors (bicycle speedometers, heart rate sensors, etc.) and is very battery efficient. The radios transfer data slowly and only over short distances.
The Solowheel Xtreme only announces itself for BLE pairing for the first 30 seconds after power up. The app attempts to reconnect, but if the connection is completely lost, the Xtreme may need to be power cycled again in order to announce itself to the app.