How to avoid wasting 135 batteries a year in IoT applications with displays

December 16, 2016 // By EDN Europe
Scott Soong, CEO, Pervasive Displays
E-paper displays can operate for up to 20 years on a single coin cell. Here's why.

Designers of battery-powered IoT applications are often focused on finding the lowest power sensors, microcontrollers and wireless connectivity. Code optimization is also a key consideration to keep energy consumption to a minimum. However, as in so many applications, the power consumption of the display can be the dominant factor in determining battery life – particularly if it’s a dot matrix type, which needs to be refreshed perhaps 50 times per second. E-paper is a great alternative here.

Assuming a CR2032 coin cell as the power source, its theoretical capacity will usually be around 220 mAh but in practice applying a factor of 0.88 will give us a more realistic figure – 193.6 mAh. Many IoT applications, for example metering, may only need the display to be updated and read every few days, weeks or months. However, for this example, let’s assume it needs to be refreshed six times per day.

TFT LCD displays typically need to be refreshed every 20 ms to keep the image on the screen. A 2-inch display of this type will consume around 30 mA when it’s bright enough to read, dropping to 3 mA in standby mode. Even if the display is permanently in standby mode, this equates to a daily power consumption of 72 mAh, meaning the battery will need to be changed every 193.6/72=2.69 days. That’s almost 136 batteries consumed each year, which with CR2032 cells weighing 10 g each, equates to 1.36 kg of potentially damaging environmental waste. If only 1% of the 21 billion IoT devices predicted to be deployed by 2021 (source: Ericsson) operate this way, the annual battery waste would be 285,600,000 kg., or 285,600 metric tonnes, in that year alone.

Many development boards and kits, like these AdapTag wireless versions developed with TI are now available from distributors such as Digi-Key. In short, e-paper gives designers an option to add environmentally-friendly, energy friendly displays to IoT applications without burning their power budget within a few days. A number of development kits based on popular microcontrollers accelerate design work. A detailed white paper on e-paper technology and its applications can be downloaded here.