How can we determine the battery lifetime for our LoRaWAN devices.

When we designed our following product, the  zTemp LoRaWAN temperature and humidity sensor one of the key figure was the long battery lifetime. Before we started designing the circuit we selected all components specially to achieve this goal.

ztemp_insideAfter  the first prototype was made we performed a precise current measurement using digital oscilloscope.

On this diagram you can see what happening during a complete measurement cycle. The processor wake up in a specified time. The processor perform the measurements, calculations, then transmit the data by LoRa module. After this wait for some seconds if the gateway has confirmation or downlink command. Then all of them go back to sleep.n

To determine what is the energy needs of one LoRaWAN message we have to integrate the area below the curve. It takes approx 100mA*sec (a bit less, let’s use for simple figures this: 0.1As)

current_ztemp

The good quality AA battery has at least 2500mAh (2.5Ah) capacity. It is in seconds: 2.5*3600=9000As

One message needs 0.1As as specified above, so we can send theoritically 90.000 messages by one battery set (2xAA regular 1.5V batteries (our circuit needs 3V)).

We have to count with standby current also. Summarised all currents we can calculate as our zTemp temperature and humidity sensor can send messages in every 10 minutes for a year.

If we setup less measurement frequency, we can increase the battery lifetime for more than one year.

The LoRa communication with this small energy can transmit the message even up to 10km. This is the efficiency of new IOT technology!

ZANE Temp LoRaWAN temperature and humidity sensor z01349