The goal of most designers of battery-powered IoT applications is finding the lowest power microcontrollers, sensors, and wireless connectivity. Of course, code optimisation is necessary to keep power consumption to an absolute minimum, too.
For many applications, the power consumption of the display can also be a significant factor in determining battery life, especially as dot matrix type displays typically require refreshing approximately 50 times per second. As Scott Soong, CEO, Pervasive Displays says, to appreciate this let’s consider a typical use case and look at the figures.
If you have selected a CR2032 coin cell as your power source then, in theory, its capacity will be about 220 mAh. In reality, applying a factor of 0.88 should give a more realistic figure of 193.6 mAh. Applications for the IoT such as metering may require the display to be refreshed and read every other day, once a week or in some cases only monthly. However, let’s look at an example where the display needs to be updated six times a day.
If your application has a TFT LCD display, the screen will need refreshing every 20 ms or so. The 2-inch display will consume roughly 30 mA of power when the display is readable and this drops to 3 mA when in standby mode. You should estimate a daily power consumption of 72 mAh even if the screen happens to be permanently in standby mode.
Let’s consider a scenario to calculate battery usage. Every 193.6/72=2.69 days the battery will need changing, which works out to needing 136 batteries over the course of a year. If just 1% of the 21 billion IoT devices forecast to be deployed by 2021 use this type of battery, that equates to an annual consumption of 28,560 million batteries. With CR2032 batteries weighing 10 g each, this translates to an eye-watering environmental waste of 285,600 metric tonnes of batteries in that one year alone!
If you consider e-paper as an alternative, a 2-inch display with an eTC (external timing controller) will draw 2.33 mA during each refresh, which takes 2.32 seconds. The display is bistable, which means that it retains the displayed image even when all power sources are removed.
In practice, this means that the display is consuming power only when something is changing. And, as it is reflective, it is readable in daylight without needing a backlight so for applications requiring a refresh six times per day, the power consumption is just 32.43 mAs. Over a period of five years, the display will consume 59,185 mAs which is a mere 8.5% of the capacity of a coin cell battery.
This means that, in theory, it would be possible to use the same coin cell battery for almost 59 years! Of course, you have to add in current leakage as a limiting factor but it is reasonable to calculate an operating life of 20 years depending on the environment.
Even if you assume a more conservative estimate of ten years’ operation, you’re still talking about reducing battery waste by a factor of 1358 compared to a TFT LCD display using the same application. Some power-thrifty e-paper IoT applications use NCF energy harvesting and therefore waste is negligible.
In summary, designers now have the option to make their IoT application displays even more environmentally-friendly without burning their power budget overnight.
The author of this blog is Scott Soong, CEO, Pervasive Displays.
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