Battery fuel gauge: factual or fallacy?

(Editor's note: this is a follow-on to "Risk management in batteries for medical (and other) applications"; to read it, click here).

   People often think of a battery as an energy-storage device that’s similar to a fuel tank dispensing liquid fuel. For simplicity reasons, this is somewhat accurate. However, measuring stored energy from an electrochemical device is far more complex. The battery fuel gauge is generally poorly understood, particularly in the medical field.

   While an ordinary fuel gauge measures liquid flow from a tank of known size, a battery fuel gauge has unconfirmed definitions and only reveals the open-circuit voltage (OCV), a reflection of state-of-charge (SoC). The specified ampere-hour (Ah) rating remains only true for the short time when the battery is new. In essence, a battery is a shrinking vessel that takes on less energy with each charge, and the marked Ah rating is no more than a reference of what the battery should hold. A battery can’t guarantee a quantified amount of energy because prevailing conditions restrict delivery.

   These are mostly unknown to the user and include battery capacity, load currents, and operating temperature. Considering these limitations, one can appreciate why battery fuel gauges can be inaccurate.

   Techniques for assessing state-of-charge include reading voltage as well as Coulomb counting. Each has sources or error and requires calibration, as well as compensation for battery use and age. This article investigates the methods and how to compensate for their characteristics, as well as real-world battery dynamics.

To read the article, which appeared at Medical Electronics Design, click here.

About the author

Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For three decades, Buchmann has studied the behavior of rechargeable batteries in practical, everyday applications, and has written articles and books, including “Batteries in a Portable World.”

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