Learn why calibration is needed and how often it is required.
A common error in fuel gauge design is assuming that the battery will always stay young and that the setting will remain unchanged as the battery ages. Such an oversight creates a false sense of security by clinging to displayed data that may increasingly become inaccurate. For the casual user of a mobile phone or a laptop, fuel gauge errors will only be mild irritants but the problem is more acute with medical and military devices, as well as the electric drivetrain that depends on precise predictions to reach the destination.|
A “smart” battery can be viewed as consisting of two parts: the chemical battery and the digital battery. The chemical battery represents the energy storage vessel while the digital battery serves as peripheral support system gathering data, establishing communications and relaying the information to the user.
A “smart” battery should be self-calibrating by taking advantage of occasional full discharges and recharges, but in real life this does not always happen. The discharge is often in form of sharp random pulses that are difficult to capture. The partially discharge pack may then be partially recharged and stored at a high temperature, causing elevated self-discharge that cannot be tracked.
These anomalies contribute to errors that are inherent to all smart batteries and in time manifest themselves in false state-of-charge estimations and other discrepancies. Figure 1 shows a digital battery that is drifting away from the chemical battery; calibration periodically corrects the tracking discrepancy. The values are assumed and accentuated.
Figure 1: Tracking of Electrochemical and digital battery as a function of time
Note: The accumulating error is application related; the values on the chart are accentuated.
To correct the tracking error that occurs, a “smart battery” should periodically be calibrated. Calibration occurs by running the battery down in the equipment until “Low Battery” appears. This sets the discharge flag and the subsequent recharge establishes the charge flag. The distances between the flags enable state-of-charge (SoC) estimations for a time. After a while, the reference lines become 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.
A better calibration method is using a battery analyzer. In addition to calibration a controlled discharge also provide the all-important capacity readings. Capacity is the leading health indicator that governs battery runtime and predicts replacement when low.
How often does a battery need calibrating? The answer depends on the application. For a battery in continued use, a calibration should be done once every three months or after 40 partial cycles. If the portable device applies a periodic full deep discharge cycles on its own, no additional calibration is 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 increasingly more skewed.
The SMBus battery relies exclusively on the information obtained from charge and discharge cycles. The chemical battery is the master while the digital battery is the slave providing peripheral support. Although the digital battery enables stunning and believable readouts, the data are estimations only.
Some smart batteries come with impedance tracking that are self-learning and reduce or eliminate the need to calibrate. Calibration is still recommended and smart batteries with impedance tracking may require several discharge/charge cycles to correct the tracking error. The iPad Instruction reads: “For proper reporting of SoC, be sure to go through at least one full charge/discharge cycle per month.”
The traction between the chemical and digital battery is measured by the Max Error. A low reading indicates good traction, and as the battery is being cycled the number tends to move up. The Max Error serves as policeman and if the number rises too high, the SMBus battery prompts for calibration. Many host devices and SMBus chargers call for calibration when the Max Error reaches 10.
Some manufacturers recommend calibration at a Max Error of 8 percent; readings above 12 percent may trigger an alarm, and higher levels could render the battery unserviceable. Although the SMBus follows an established protocol, no unified standard exist as to what Max Error level requires service or constitutes an error. Every battery manufacturer applies its own formula. Figure 3 illustrates a screenshot of the data stored in an SMBus battery.
Figure 3: Screenshot of a typical SMBus battery
Courtesy: Texas Instrument
Last updated 2015-05-06
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