The 'smart' battery has some notable downsides,one of which is price.An SMBus battery costs about 25% more than the 'dumb' equivalent.In addition,the smart battery was intended to simplify the charger but a full-fledged Level 3 charger costs substantially more than a regular model.A more serious drawback is the requirements for periodic calibration or capacity re-learning.The Engineering Manager of Moli Energy,a manufacturer of lithium-ion cell commented,"With lithium-ion we have eliminated the memory effect;but is the SMBus battery introducing digital memory?"
How often is calibration needed?The answer lies in the battery application.For practical purposes,a calibration is recommended once every three months or after every 40 short cycles.Many batteries undergo periodic full discharges as part of regular use.If the portable device allows a deep enough discharge to reset the battery and this is done regularly,no additional calibration is needed.However,if no discharge reset has occurred for a few months,a deliberate full discharge is needed.This can be done on a charger with discharge function or a battery analyzer.
Why is calibration needed?The calibration corrects the tracking errors that occur between the battery and the digital sensing circuit while charging and discharging.The most ideal battery application,as far as fuel-gauge accuracy is concerned,would be a full charge followed by a full discharge at a constant current.In such a case,the tracking error would be less than 1% per cycle.In real life,however,a battery may be discharged for only a few minutes and the load pulses may be very short.Long storage also contributes to errors because the circuit cannot accurately compensate for self-discharge.Eventually,the true capacity of the battery no longer synchronizes with the fuel gauge and a full charge and discharge is needed to 're-learn' the battery.
What happens if the battery is not calibrated regularly?Can such a battery be used in confidence? Most 'smart' battery chargers obey the dictates of the chemical cells rather than the electronic circuit.In this case,the battery will fully charge regardless of the fuel gauge setting and function normally,but the digital readout will become inaccurate.If not corrected,the fuel gauge simply becomes a nuisance.
An addition problem with the SMBus battery is non-compliance.Unlike other tightly regulated standards,the SMBus protocol allows some variations.This may cause problems with existing chargers and the SMBus battery should be checked for compatibility before use.The need to test and approve the marriage between a specific battery and charger is unfortunate,given the assurance that the SMBus battery is intended to be universal.Ironically,the more features offered on the SMBus charger and the battery,the higher the likelihood of incompatibilities.
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