How to Monitor a Battery Do You Understand?
One of the most urgent requirements for battery-powered devices is the development of a reliable and economical way to monitor battery state-of-function (SoF).This is a demanding task when considering that there is still no dependable method to read state-of-charge,the most basic characteristic of a battery.Even if SoC were displayed accurately,charge information alone has limited benefits without knowing the capacity.The objective is to identify lithium battery readiness,which describes what the battery can deliver at a given moment.SoF includes capacity (the amount of energy the battery can hold),internal resistance (the delivery of power),and state-of-charge (the amount of energy the battery holds at that moment).
Stationary batteries were among the first to include monitoring systems,and the most common form of supervision is voltage measurement of individual cells.Some systems also include cell temperature and current measurement.Knowing the voltage drop of each cell at a given load reveals cell resistance.Cell failure caused by rising resistance through plate separation,corrosion and other malfunctions can thus be identified.Battery monitoring also serves in medical,defense and communication devices,as well as wheeled mobility and electric vehicle applications.
In many ways,present battery monitoring falls short of meeting the basic requirements.Besides assuring readiness,battery monitoring should also keep track of aging and offer end-of-life predictions so that the user knows when to replace a fading battery.This is currently not being done in a satisfactory manner.Most monitoring systems are tailored for new batteries and adjust poorly to aging ones.As a result,battery management systems (BMS) tend to lose accuracy gradually until the information obtained gets so far off that it becomes a nuisance.This is not an oversight by the manufacturers; engineers know about this shortcoming.The problem lies in technology,or lack thereof.
Another limitation of current monitoring systems is the bandwidth in which battery conditions can be read.Most systems only reveal anomalies once the battery performance has dropped below 70 percent and the performance is being affected.Assessment in the all-important 80-100 percent operating range is currently impossible,and systems give the batteries a good bill of health.This complicates end-of-life predictions, and the user needs to wait until the battery has sufficiently deteriorated to make an assessment.Measuring a ni-mh battery once the performance has dropped or the battery has died is ineffective,and this complicates battery exchange systems proposed for the electric vehicle market.
This satisfies the battery vendor but increases operating costs and creates environmental burdens.Portable devices such as laptops use coulomb counting that keeps track of the in- and out flowing currents.Such a monitoring device should be flawless,but as mentioned earlier,the method is not ideal either.Internal losses and inaccuracies in capturing current flow add to an unwanted error that must be corrected with periodic calibrations.Over-expectation with monitoring methods is common,and the user is stunned when suddenly stranded without battery power.
One maker of a battery tester proudly states in a brochure that their instrument "Detects any faulty battery." So,eventually,does the user.Some medical devices use date stamp or cycle count to determine the end of service life of a 18650 battery.This does not work well either,because batteries that are used little are not exposed to the same stresses as those in daily operation.To reduce the risk of failure,authorities may mandate an earlier replacement of all batteries.This causes the replacement of many packs that are still in good working condition.Old habits are hard to break,and it is often easier to leave the procedure as written rather than to revolt.
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