Calculating Battery Life
Batteries can only store a certain amount of energy, indicated as milliamp-hours and abbreviated as mAh. The more mAh a battery has, the longer it can last, which generally correlates with the size of the battery itself. If you know how much power an end device uses, and you know the capacity of the battery, you can calculate the battery’s lifetime with this simple formula:
Let’s look at examples of a device operating in each of the four possible different energy modes:
| Mode | Description |
|---|---|
| Deep Sleep | The
device is powered on, but is not active |
| Awake | The device is awake and sensing its environment |
| Sending Data (TX) | The device is transmitting data |
| Receiving Data (RX) | The time during which the device listens for downlink packets |
When in Class A mode, an end device is able to receive a downlink message shortly after transmitting a message. After a transmission, the LoRa transceiver opens two receive (RX) windows, so it can listen for any incoming messages. When the device receives a downlink, the radio stays active until the full LoRaWAN packet is received.
Receiving data from multiple gateways costs a significant amount of energy, although not as much as sending data. The transmission power of a Microchip LoRa transceiver module is about 40mA. In comparison, receiving data consumes 14.2mA
Class A end devices are the most battery efficient. Class B end devices consume more energy, because the end device needs to listen to incoming messages periodically. Class C end devices consume the most energy because the radio needs to be active all the time. Having an end device in Class C mode for a long time is nearly impossible to achieve using batteries.