How do I know when to change the battery?
RadioBrdige Sensors are designed to provide several years of useful life before the battery needs to be changed. The effective life of the battery is dependent on many factors such as message volume, ambient temperature, signal strength and Adaptive Data Rate mode. The lithium battery provided with the sensor will retain a consistent voltage for most of its life and then drop off quickly when depleted. Because of these factors, there are difficulties in monitoring battery health in a consistent and linear fashion, for example, a dashboard indicator that shows a predictable battery percentage meter.
Note that the LoRaWAN "End-Device Status Request" command can not be reliably used to assess battery level. This command queries the radio module itself for its provided voltage, however RadioBridge Sensors utilize a charge-pump to provide the radio with maximum voltage for best performance.
The proper method of monitoring the battery level is through the last byte of the Supervisory Message which shows a two-digit human-readable battery voltage, for example a readout of 30 represents 3.0V.
The Supervisory Message also includes an Error Codes Byte which includes a bit for "battery low". This bit is set when the battery voltage drops below 2.75V. After this point the sensor will be capable of transmitting several thousand (4000-8000) further messages, however once the voltage drops below 2.54V all messages will be disabled. While the battery will tend to stay at around 3V for most of its life, during this low battery period the battery's voltage will drop more rapidly and decreases in voltage reading will be more frequent.
The replacement battery must be above 2.8V in order for the "low battery" flag to be reset.