The Effect of Packet Size and Spreading Factor

Of course, end devices aren’t always asleep. In the example below, we have an end device that sends a data packet every 15 minutes. Let’s have a look at the impact of the packet size and the spreading factor on battery consumption. We’ll look into the following transmissions:

  • 4 bytes, SF7
  • 4 bytes, SF9
  • 4 bytes, SF12
  • 51 bytes, SF7
  • 51 bytes, SF9
  • 51 bytes, SF12

As you saw in the LoRaWAN Scalability Spreading Factors and Airtime video, with each step in spreading factor, the time-on-air doubles, making the difference between SF7 and SF12 a factor of 32. 

Table 1 provides an example of the difference in power consumption for these different transmissions when using a typical board.

Spreading Factor Payload Size (bytes) Average Power Drain (mA) Duration (sec) 
Base deep-sleep power  - 0.015 -
7 4 19 2.4
7 51 21 2.5
9 4 21 2.6
9 51 24 2.8
12 4 33 3.7
12 51 40 5.2
Table 1: Power Consumption During Data Transmission

As shown in the table, the longer a device transmits data, the more power it uses. As we mentioned earlier, we can calculate the life of the battery when we know its capacity and the frequency of the transmissions the device will make.

As an example, let’s calculate the battery life with the following parameters:

  • Battery capacity: 500mAh
  • Frequency of transmissions: every 15 minutes

To calculate the battery life, divide the battery capacity (500) by the average current drain per hour. The current drain per hour consists of four times the current drain per transmission (see Table 2), plus the current drain while the device is in deep sleep mode. The results of the calculations are shown in Table 2.

Spreading Factor Payload Size (bytes) Battery Life (hours) Batter Life (days) 
7 4 7619 317
7 51 6822 284
9 4 6612 275
9 51 5579 232
12 4 3320 138
12 51 2032 85
Table 2: Effect of spreading factor and payload size on battery life

The data in Table 2 shows that the battery lasts almost four times as long when sending four bytes at SF7, compared to sending 51 bytes at SF12.

In short, to extend the life of the battery in an end device, keep the functionalities that consume energy to a minimum, use a low spreading factor, and reduce the packet size and the frequency of transmissions.


Last modified: Friday, February 3, 2023, 9:21 PM