Battery Fuel Gauge Technology

The lifespan of a battery cannot be defined by the number of cycles or by age alone but to a large extent by its usage (or misusage). As the capacity fades, the runtime gets shorter.

The lifespan of a battery cannot be defined by the number of cycles or by age alone but to a large extent by its usage (or misusage). As the capacity fades, the runtime gets shorter. The smart battery captures this capacity fade by reading the previous energy delivered, but these vital health statistics remain mostly hidden from the user. The battery continues to be a “black box” that conceals vital performance records and masks when the battery should be replaced.

One of the main tasks of the smart battery is to establish communication between the battery, charger and user. A fuel gauge indicating state-of-charge (SoC) fulfills this in part. When pressing the TEST button on a fully charged SMBus battery, all signal lights illuminate. On a partially discharged battery, half the lights illuminate, and on an empty battery all lights remain dark or a red light appears. Figure 1 shows a fuel gauge of a battery that is 75 percent charged with three lights glowing.

State-of-Charge Readout

While the SoC information displayed on a battery or a display screen is helpful to the user, the readout does not guarantee the runtime. The fuel gauge resets to 100 percent with a full recharge regardless of how much capacity the battery can store. A serious breach of trust occurs if an aged battery shows 100 percent SoC while the battery’s ability to hold charge has dropped to 50 percent or less. We ask, “100 percent of what?” If, for example, 100 percent of a good battery results in a 4-hour runtime, a battery holding half the capacity would run for only 2 hours. Many users are unaware that the fuel gauge only shows SoC; capacity, the leading health indicator, remains unknown.

Other than applying a controlled discharge, there is no reliable method to measure the capacity of the “chemical battery” but there is a way to read the “digital battery.” The term chemical battery refers to the actual capacity derived by discharging a fully charged pack, whereas the digital battery is a peripheral monitoring circuit that stores the estimated capacity derived by coulomb counting when charging and discharging a battery as part of field use.

The SMBus battery stores the factory-set design capacity in Ah or 100 percent by default. With each full charge, the battery resets the full-charge flag and during discharge, the coulomb counter measures the energy consumed. The in-and-out-flowing coulombs can be used to estimate battery state-of-health known as full charge capacity (FCC). As the battery fades with usage and time, so also does the delivered energy decrease, and the FCC number will decline. The FCC accuracy of a battery that is being deep cycled is about +/-5 percent compared to capacity readings taken by discharging. Periodic calibration will improve the FCC accuracy.

Capacity can also be estimated by coulomb counting during charging. This works best with an empty battery. A battery with a 100 percent capacity will receive the full coulomb count; one with only 50 percent capacity will only accept half before the battery reaches full charge. Not knowing the residual SoC when the coulomb count begins will affect the accuracy. SoC can be estimated by measuring the battery’s open circuit voltage (OCV), but this only gives a rough approximation as agitation after charge or discharge, as well as temperature, affects the OCV.

The SoC and capacity information can be shown on a linear display (Figure 2) using colored LEDs. The green lights indicate the usable capacity; the empty part of the battery is marked with un-lit LEDs; and the unusable part is shown with red LEDs. The results can also be a shown on a digital display.

Tristate fuel gauge

The tri-state fuel gauge provides state-of-function (SoF), the ultimate in battery diagnostics. Some device manufacturers are hesitant to offer this feature to consumers because this could lead to elevated warranty claims. A replacement only becomes mandatory if the battery capacity drops below 80 percent; keeping the evidence hidden is seen as the least disruptive method. SoF can always be accessed by a service code. SoF works best for industrial applications.

Vehicles with electric propulsion do not show the charge but only the remaining driving range, thus hiding the capacity. To accommodate capacity fade that would shorten the driving range, the EV battery is being oversized and does not fully charge and discharge when new. Only as the battery ages and the capacity fades does the charge range increase. Shorter driving ranges only become apparent once this reserve capacity has been consumed.


When designing a fuel gauge, engineers commonly make a misjudgment by assuming that a battery will always stay young. As with people, batteries age and the changing characteristics must be taken into account to maintain accuracy. Fancy fuel gauges can provide a false sense of security when users believe that the displayed battery readings are correct. For the casual user of a mobile phone or laptop, a fuel gauge error is only a mild irritant, but the problem escalates with medical and military devices, as well as drones and electric drivetrains that depend on precise range predictions.

The chemical battery representing the actual energy storage remains the master while the digital battery provides peripheral support by relying on the information obtained from charge and discharge cycles. But like all fine machines, precise settings begin to shift and need adjustment. The same happens with an SMBus battery that also requires periodic calibration. The instructions for an Apple iPad reads: “For proper reporting of SoC, be sure to go through at least one full charge/discharge cycle per month.”

Figure 3 demonstrates a digital battery that is drifting away from the chemical battery; calibration corrects the tracking error. The accumulating error is application related and the drift on the chart is accentuated for effect.

Tracking of the electrochemical and digital battery as a function of time.

A smart battery self-calibrates by taking advantage of occasional full discharges, but in real life this seldom happens. Most discharges are intermittent and go to random depth. In addition, the load signatures often consist of high frequency pulses that are difficult to capture. The partially discharged battery may be partly recharged and then stored in a warm room, causing elevated self-discharge that cannot be tracked. These anomalies add to the display error that amplifies with use and time.

To maintain accuracy, a smart battery should periodically be calibrated by running the pack down in the device until “Low Battery” appears and then applying a recharge. The full discharge sets the discharge flag and the full charge establishes the charge flag. A linear line forms between these two anchor points that allow SoC estimation. In time, this line gets blurred again and the battery requires recalibration. Figure 4 illustrates the full-discharge and full-charge flags.

Full-discharge and full-charge flags.

A battery charger with discharge function or a battery analyzer enables calibration of a smart battery. The analyzer fully charges the battery and then applies a controlled discharge that provides the all-important capacity readings of the chemical battery. This discharge measurement is a truer reading than what coulomb counting provides by capturing past discharge events of the digital battery.

How often should a battery be calibrated? The answer depends on the application. For a battery that is in continued use, a calibration should be done once every 3 months or after 40 partial cycles. If the portable device applies a periodic full deep discharge on its own accord, then no additional calibration should be needed.

What happens if the battery is not calibrated regularly? Can such a battery be used with confidence? Most smart battery chargers obey the dictates of the chemical battery rather than the digital battery and there are no safety concerns. The battery should function normally, but the digital readout may become unreliable.

Some smart batteries feature impedance tracking. This is a self-learning algorithm that reduces or eliminates the need to calibrate. If calibration is required, however, several cycles instead of only one may be needed to achieve the same result as with a standard system.

The accuracy between the chemical and digital battery is measured by the Max Error. Max Error stands for “maximum error” and is presented in percentage. A low reading indicates good accuracy, and as the precision diminishes with partial cycles, the Max Error number increases steadily. This supervisory watchdog can be compared to a medical doctor who measures a medical condition by a number.

Some manufacturers recommend calibration at a Max Error of 8 percent; readings above 12 percent may trigger an alarm and 16 could render the battery unserviceable. No unified standard exists to determine what Max Error level requires service or what constitutes an error; every battery manufacturer follows its own recommendation.

Processing Data from a Smart Battery

The SMBus system provides a wealth of information that includes battery manufacturing date, battery model and serial number, capacity, temperature and estimated runtime, as well as voltages down to the cell levels. It is an engineer’s delight to have all this data in a table, but the fine print may confuse the user more than providing help. A busy nurse in a hospital, the policeman on duty and the solider in combat has only one question: “Will the battery last for my mission?” Table 5 illustrates a screenshot of the data stored in an SMBus battery.

SMBus Battery

Of special interest in terms of battery state-of-health (SoH) is full charge capacity (FCC), the coulomb count that is hidden in the table among tons of other information. FCC can be used with reasonable accuracy to estimate battery SoH without applying a full discharge cycle to measure capacity. Best accuracies are achieved if the battery is being cycled with a full charge and an occasional deep discharge. If used sporadically, a deliberate calibration involving a full discharge/charge cycle will be needed from time-to-time to maintain accuracy.

Even though smart batteries have been in service since the mid-1990s, they still do not communicate well with the outside world. Device manufacturers continue to mandate that the battery be replaced on a date stamp rather than refer to the more relevant FCC information contained in the battery. Expensive packs are thus discarded every 2–3 years instead of utilizing the typical full 5-year life expectancy of Li-ion.

A new frontier is opening that provides easy access to battery information. The Battery Parser (by Cadex) does this by establishing communications between the user and the battery by fetching intrinsic battery data to reveal state-of-function (SoF). The Fishbowl icon as shown in Figure 6 consists of the Charge Ring indicating state-of-charge and the Status Dome with PASS, CHARGE, CHECK, and FAIL messages. The Status Dome also illustrates the energy-storage capability together with Battery Fade that moves towards the Pass/Fail line with usage and age.


Battery status indicators must separate state-of-charge and capacity and treat them as unrelated entities. While a battery with low SoC can be recharged, capacity loss is permanent and predicts end-of-life. This condition is demonstrated with the encroaching black ceiling bar on top of the Fishbowl. Pressing the Status Dome on a device featuring a touchscreen reveals possible deficiencies, as well as information relating to the battery model, specifications, serial number and manufacturing date. The Fishbowl settings can be updated by the user.

Storing the battery test results in the cloud enables an overview of the entire battery fleet in terms of location, application, performance and service requirements. This is made possible with the availability of the serial number and manufacturing date in a smart battery. To check batteries needing replacement, the operator simply calls up packs that have dropped below the 80 percent capacity or are older than, say, 5 years. The operator can also verify SoC by listing all batteries with less than 10 percent reserve before charging. Tight reserves can lead to failure during heavy traffic or in an emergency.

Last updated 2016-07-29

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, has written award-winning articles including the best-selling book “Batteries in a Portable World,” now in its fourth edition. Cadex specializes in the design and manufacturing of battery chargers, analyzers and monitoring devices. For more information on batteries, visit; product information is on