Characterizing the battery life of an IoT device using the Otii Arc and Nutaq PicoLTE
The article was written by Altus Technologies, a Canadian manufacturer of cold chain monitoring and other IoT solutions.
Are you designing a battery powered IoT device? You should know the challenge with accurately estimating the battery life. The power consumption of devices relying on low power radios to communicate (eg : NB-IoT, ĿTE-M) varies greatly according to the network conditions. While the wireless network coverage is poor or temporarily out of reach, the device will consume more power. Not to mention the numerous configurable features of the radio itself (eg: PSM, e-DRX) which are not supported the same way on all networks but have a great impact on power consumption when they are. With all the factors impacting power consumption, how can the designer meet a battery life specification? One needs a testbed where network and device settings as well as network conditions are controllable and accurate power measurements can be conducted.
This article describes how Altus Technologies uses the Otii Arc and Nutaq PicoLTE to characterize the battery life of IoT devices. To characterize the battery life of an IoT device, we need two key measurements: the current consumption profile of the device over time, and the amount of charge available in the device’s battery. We can then combine these two measurements to calculate how long it will take for our battery to become discharged to the point where our device no longer works. The Otii Arc is instrumental for us in implementing both halves of our method. To characterize the current profile and voltage requirement of our IoT device, we use the Arc as a controllable power supply with integrated current consumption profiling. Then, we use the Arc software’s battery profiling toolbox to measure our battery’s discharge capacity specific to the device’s electrical characteristics.
Part 1: Setting up the PicoLTE LTE-M/NB-IoT network in a box
The PicoLTE is used to provide NB-IoT connectivity (both eNodeB and EPC) while Sierra Wireless’ HL78xx CAT-M is used in to exchange data with our remote application servers. HL78xx is connected to PicoLTE over-the-air (Figure 1).
Figure 1: PicoLTE and HL78xx setup to test PSM and eDRX
PicoLTE is running multiple LTE-M and NB-IoT cells concurrently. The eNodeB is connected to the EPC and supports different PSM cycle settings and eDRX paging window lengths.
Figure 2: Spectrum of an NB-IoT cell at 801 MHz (LTE Band 20).
Part 2: Characterizing the peak current profile and min voltage
To characterize the current profile and voltage requirement, we set up the Otii Arc as a power supply. We remove the battery from the IoT device, and instead connect its power input to the Otii Arc’s output. Meanwhile, in the Otii software, we select “Power box” as Supply and set the main voltage to the nominal voltage of the device’s battery. Still within the software, we start plotting the Main Current and turn on the Arc’s power.
While the power supply is running at nominal voltage, we inspect the Main Current plot to identify 1 duty cycle of the IoT device (Figure 3). We find the largest current peak within a duty cycle and measure its duration and peak current. The Arc software lets us hone in on any small section of the current plot which helps us take these measurements with precision (Figure 4). Then, we measure the duration and average current of the rest of the duty cycle (the rest of the duty cycle should be primarily idle time).
After measuring the current profile, we decrease the Arc’s output voltage by a small step (e.g. 0.2V) and monitor the IoT device’s key metrics (sensor accuracy, wireless signal strength, etc.) to determine whether the device is still operating adequately. If it is, we measure the current profile again, decrease the Arc’s output voltage one step further, and rinse and repeat until finding the voltage where the device stops operating correctly. At that point, we define the previous step’s voltage as the IoT device’s minimum voltage requirement.
Figure 3: An example of the key portion of an IoT device’s duty cycle
Figure 4: Focusing on the measurements of the primary current peak