The Big IoT Challenge: Calculating Battery Life

The Internet of Things (IoT) is creating a new breed of measurement challenges for engineers trying to calculate the battery life of their products, an expert will tell attendees at the upcoming Embedded Systems Conference (ESC) in Boston.

“The oscilloscope you have in your lab today may not have low enough noise to make the kind of measurements you need,” Mike Borsch, an applications engineer for Rohde & Schwarz, told Design News. “When you’re measuring micro-amps or micro-volts, noise can be a real problem. Many of today’s scopes haven’t been built to make these measurements. A lot of things can go wrong.”

The problem can be a tricky one, not only for the engineers who design the IoT devices, but for the creators of electronic components that get used in those IoT devices. Device developers need to know the current draw of their products, and to do that, they need accurate characterizations from manufacturers who create the processors, ASICs, WiFi chips and other components that go into them. Only then can they make accurate assessments of device battery life.

The act of measuring has become more complex in recent years, however, because IoT devices burn so little current. “It’s really difficult measuring low currents and voltages on scopes,” Borsch said. “The noise in the instrument can cover up very small voltages and currents. So instead of measuring just the signal, you end up measuring the noise. It becomes difficult to assess – am I seeing the actual signal or am I also getting scope noise in the measurement?”

To make accurate assessments, modern instruments first and foremost need to have low noise. But engineers also need to know how to mitigate those low levels of noise through such techniques as averaging, signal conditioning and filtering of the signal, Borsch said. So-called “dynamic re-referencing” techniques can help stabilize the signal while engineers make measurements, he added.

Borsch also recommended that IoT device engineers use instruments that incorporate multiple capabilities in a single box, rather than employing separate spectrum analyzers, power meters and oscilloscopes. Doing so, he said, allows engineers to view simultaneous events in one place.

“A scope has to be able to do a lot more these days than just measure voltage versus time,” Borsch said. “Your instrument has to measure current and voltage and things like signal content in a frequency domain.”

Adding to the complexity is the fact that components may behave differently in lab tests then they will after integration into a product with other components, Borsch said. “You may know how much current a device is supposed to use in a certain state of operation, but when you integrate all the components together, and it goes out into the field, the conditions change.”

For IoT products, the biggest area of concern is sleep states, Borsch said. There, the measurement challenges are greatest, because the current consumptions are so miniscule. Moreover, end products may have multiple different sleeping modes. “Typical IoT devices have software algorithms built in to control how the sleep states operate,” Borsch said. “Some of these devices have 15 different sleep states. Depending on what the device is doing, it may have certain components energized or not energized.”

The bottom line is that product and component designers need to be aware of the tools and instruments that are available to them, especially when dealing with IoT products, Borsch said. What’s more, they need to know the best practices for using those tools. “Scopes have a lot of tools available,” Borsch told us. “But you have to know how to set up the instrument correctly to give you the most accurate and precise numbers.”

Senior technical editor Chuck Murray has been writing about technology for 33 years. He joined Design News in 1987, and has covered electronics, automation, fluid power, and autos.

 Mike Borsch will discuss test and measurement in a session titled, Measuring Battery Life in IoT Devices at the Embedded Systems Conference’s on May 3, 2017. Register today.