Get better battery fuel gauging

September 25, 2014

Get better battery fuel gauging

When we talk about the frustration of a mobile phone powering down, we're talking about an emotion felt by somewhere around seven billion subscribers...

When we talk about the frustration of a mobile phone powering down, we’re talking about an emotion felt by somewhere around seven billion subscribers worldwide, or the equivalent of over 90 percent of the global population. That’s a lot of sighs, tuts, and calls and text messages that didn’t quite get made or sent.

As a population we have, over the course of little more than a decade, become somewhat dependent on battery-powered portable electronics to allow us to communicate with one another or connect to the Internet. Today’s smartphones, for example, provide numerous functions people need multiple times every day: email, text, GPS, camera, calendar, notepad, media player, and even the occasional telephone call.

There seems to be a reluctant acceptance that until we see a quantum leap in battery technology, the only gains in how long our portable devices stay charged is going to come down to improved efficiency and reduced power demands achieved by manufacturers of the components that combine to make devices work and support the functionality we all enjoy. However, the accuracy and dependability of the fuel gauges on smartphones, tablets, laptop PCs, and more recently phablets is something that can now be addressed to at least make us a little less surprised when the screen goes black.

Coulomb-counting is the technique traditionally used in portable devices. However, it’s not only inaccurate, leading to a high risk of unexpected shutdown, but is also subject to temperature-related errors and consumes precious battery energy that may be better utilized to power other functional circuitry. The approach uses an expensive precision current-sensing resistor to continuously monitor the battery’s output current. The current is integrated over time, and the result is compared to the known maximum battery charge to calculate the remaining charge available.

This method is inherently inaccurate because battery self-discharge events can’t be detected, since the self-discharge current doesn’t pass through the coulomb counter’s sense resistor. In addition, self-discharge events tend to increase the ambient temperature, which changes the resistance of the sense resistor, thereby further skewing accuracy. Finally, the battery must be fully charged every time for an accurate calibration. In short, it’s not the greatest technological solution, and is out of step with some of the truly wonderful technology in portable electronics.

A better approach that’s emerging, for lots of reasons, is based on measuring the voltage across the battery using a precision analog-to-digital converter (ADC). The measured voltage is compared to values held in a lookup table stored in the device. In developments to-date, it’s been determined that just four tables, one for each common battery technology, are sufficient for all applications, and that new device-specific lookup tables aren’t needed.

In tests, this new method has achieved an accuracy of better than 3 percent compared to around 8 percent for coulomb counting. That’s a big difference and means a lot, especially in those critical times when the battery is getting close to empty. In addition, unlike coulomb counting, measuring battery voltage using an ADC isn’t affected by ambient temperature.

Significantly smaller size and integration, plus in-operational power savings complete the positive picture, and hopefully mean that future portable devices will come to market incorporating the new technology.

Steve Sheard is a program development manager at ON Semiconductor, supporting the System Solutions Group. Mr. Sheard has over 35 years of experience in the electronics industry. He holds an MBA from the University of Phoenix and a BSEE from the City & Guilds of London.

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Steve Sheard, ON Semiconductor