Tech Papers and Guides
Example: Select a battery
This example shows you how to select a battery for a hypothetical sensor application.
This is a Tabs door and window sensor, TBDW100-915.
Key parameters required:
- Physical size: 50mm x 20mm x 50mm
- Temperature: -20°C to +50°C
- Minimum current/sleep current: 5uA
- Maximum current/Tx current: 135mA
- Times the device will be active per day: 36
- Expected battery life in years: 5 years
LoRa® is the technology. Based on the sensor function, calculate the current consumption-per-hour when the device is active.
LoRa device’s parameters can be conservatively estimated as: Payload = 8 bytes, SF = 10 and BW 125.
Based on the SX12xx IC spec
- Maximum current/Tx current: 135 mA
- Maximum supply voltage: 3.9V
- Nominal voltage: 3.6V
Tx time/ToA: 250 ms (Calculated using LoRa calculator)
Based on the sensor IC spec
- Measurement current: ~ 10mA
- Measurement time: 10ms
- Length of time device is active (per hour) (TX time + RX Time + Measurement time): 391 ms
- Current when device is active (TX current +RX current +Measurement current): 148mA
- Usable Capacity (%) (self-discharge limits battery life): 85%
Plugging these values into the battery calculator above, we get:
Required Battery Capacity = 1091 mAh
Based on this example, these are the key battery requirements for the door and window sensor:
- A primary battery
- A button cell or ½ AA size battery, based on the physical dimensions of the sensor
- A battery chemistry that best supports a temperature range of -20°C to 50°C
- A battery with the nominal voltage of 3.6
- A battery with a peak load current greater than 135mA
- Note: Check the maximum discharge rate in the battery specifications. For example, if the 1200 mAh capacity battery has maximum C discharge rate, the supported peak current is 1200mA.
- A battery with a capacity of greater than 1091mAh
Compare the characteristics of batteries available with the battery requirements for the application and then use these requirements to choose a battery.
Based on this example, the 3.6 V 1/2AA Li-SOCI2 1200mAh battery meets all the requirements.