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# 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.

Technology: LoRa

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.