The fuel gauge has the inherent drawback that it needs periodic calibration, also known as capacity re-learning. This is unfortunate, but is required to correct the tracking error that develops between the chemical and digital battery over many charge and discharge cycles.
Calibration could be omitted if the battery received a full charge followed by a full discharge at a constant current. This way, the battery would reset with each full cycle and the tracking error would be kept at less than one percent per cycle. In real life, however, a battery may be discharged for a few minutes with a load signature that is difficult to capture, then is partially recharged and stored with varying levels of self-discharge depending on temperature. These anomalies contribute to an unavoidable error. The true capacity of the battery begins to deviate from the fuel gauge readout and the battery needs to re-learn. Battery engineers say jokingly that “Li-ion got rid of memory and SMBus adds digital memory.”
Calibration occurs naturally by occasionally running the equipment down until the battery is fully depleted and “Low Battery” appears. The full discharge sets the discharge flag, and the subsequent recharge sets the charge flag. By establishing these two markers, the battery can calculate the state-of-charge by knowing the distance between the flags. Figure 1 illustrates the full-discharge and full-charge flags.
Figure 1: 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.
How often does a battery need calibrating? The answer depends on the application. For practical purposes, a calibration should be done once every three months or after 40 partial cycles. If the portable device applies a periodic deep discharge on its own, no additional calibration will be needed. However, if the equipment had been in constant battery power with no full-discharge interval for a period of a few months, then a deliberate discharge is recommended. This can be done in the equipment, with a charger featuring a discharge function or a battery analyzer. Avoid doing too many deep discharges for the sake of calibration because this would wear the battery down.
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 electronic circuit, and there are no safety issues. In spite of being out of calibration, the battery charges fully and functions normally; however, the digital readout may become increasingly more inaccurate and will eventually become a nuisance.
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