Papyrus

Low power consumption mobile Situational Awareness with e-paper.

June 2021

The Challenge

For the dismounted soldier, loss of power to critical equipment is not an option. Two obvious solutions to this problem are rechargeable batteries; or carrying supplementary batteries. However, both have potential issues. Recharging depends on physical infrastructure that may not be present; whilst the burden of carrying numerous spare batteries adds significant additional weight to an already heavily laden soldier.

One such important tool requiring battery power is the mobile phone - often used for Situational Awareness tools. However, mobile phone battery life, when the display is continuously in use, is likely only 4-6 hours - which is typically shorter than that of its required operation.

Journey & Design

Rather than focusing on the battery, we decided to explore ways of reducing the power usage of the device. To better understand the problem, we started by performing a series of tests on a typical Android device to capture battery usage. We then set ourselves a goal of a Situational Awareness device capable of continually running with the display on for 5 days - around 20x the normal Android device.

Our hypothesis was that the power required by the display to remain continuously lit was the main power draw. In order to achieve the goal, we needed to rethink the display. Our idea was to use an e-paper (also called e-ink) display, which only requires power when updating the screen - no power is required to maintain an image after it has been updated. If we could limit power consumption to discrete bursts, we hoped to significantly reduce battery usage.

Assuming use of a mobile phone battery with capacity of 4080mAh (as found in a Google Pixel 5), 5 days continuous use would require an average draw of around 33mA. Our chosen display used 176mA for 1s to perform a full refresh of the screen. At 1 refresh per minute, it would draw on average 3mA; or more practically (given the use case) at 20 second intervals, average draw would be 9mA.

We created a prototype device with a simple Situational Awareness application written specifically for an e-paper display. The application supports use of external mapping and imagery, with basic navigational and tracking capabilities. By making use of partial updates we minimise the power consumption of the display. This minimal feature set provides enough representative functionality and resource usage to measure and estimate power consumption.

To meet our goal, we needed the background power consumption of the compute element of the device to be 25-30mAh. Even the least power hungry single board computers average a current draw of around 150mA at idle meaning we would need to adopt a very aggressive power management strategy, likely in the form of a suspend to RAM function together with a Wake-on-LAN type capability from the connected radio (in tactical networks mobile devices are typically connected to a radio for networking).

Today

With no such power management, 20 second refresh intervals and a 4080mAh battery, our prototype achieved just over 27 hours of non-stop use - significantly better than the 6 hours of the Android phone, but well short of our 5 day goal. Even with an aggressive power management strategy (and ignoring any potential wear implications of such a strategy), it is improbable that we could achieve an 80% reduction in consumption.

Whilst we achieved a noticeable improvement, the results fell below our exponential improvement aim, so we decided to end our investigation - for now.

The project was far from wasted time and energy:

  • We learned a lot about creating software for e-paper displays;
  • We think the 180 degree viewing angle, the absence of a backlight (for better light discipline), reflective display properties, and potential for flexible materials make e-paper more suitable than other displays for many military use cases;
  • We’ve got some other ideas for creating extremely power efficient Situational Awareness tools ...

If you’d like to discuss Papyrus more, reach out to the team at labs@xewli.com.