The unit is Millivolt [mV] and the usable charge for iMetos stations is between 6000 and 6600 mV. The battery values can also go above 6600 mV when there is more power coming from the solar panel. Below 6000 the device will stop transmitting and at 5800 the station cannot collect data anymore.

For LoRa and NBIoT devices the usable range is between 2800 and 3200 mV.