Learn why calibration is needed and how often it is required.
When designing a fuel gauge, engineers commonly make the mistake of 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.
There is often a false sense of security in believing that the displayed battery readings are correct. Fuel gauge displays can be made fancy and believable but the truth is only as good as the information provided. 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 relying on the information obtained from charge and discharge cycles. But like all fine machines, precise settings begin to shift and they need adjustment. The same happens with an SMBus battery that also needs 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 1 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.
Figure 1: Tracking of Electrochemical and digital battery as a function of time. With use and time the electro-chemical and digital battery drift apart; calibration corrects the error.
Note: The accumulating error is application related; the values on the chart are accentuated.
A smart battery can be self-calibrated 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 at a high temperature, 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 apply 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 2 illustrates the full-discharge and full-charge flags.
Figure 2: Full-discharge and full-charge flags. Calibration occurs by applying a full charge, discharge and charge. This is done in the equipment or with a battery analyzer as part of battery maintenance.
Battery analyzers serve as a valuable tool to calibrate a smart battery. An 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 advises a patient to change a routine to correct a condition.
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 formula.
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?” Figure 3 illustrates a screenshot of the data stored in an SMBus battery.
Figure 3: Uinversal screenshot of SMBus battery. Data is organized in tables to assist analysis, a format that is less suited for the everyday battery user. Access is by a software tool.
Source: Texas Instrument
Of special interest in terms of battery state-of-health (SoH) is full charge capacity (FCC), which 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 full charge and occasional deep discharges. If used sporadically, a deliberate calibration involving a full discharge/charge cycle will be needed from time-to-time to maintain accuracy.
Last updated 2016-05-19
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