Technical Article

Fuel-gauging offers more than convenience for the consumer.In some mission-critical applications in, for instance, theindustrial and medical markets, data integrity – or evenhuman health – can be compromised by unexpectedshutdowns that occur when the battery pack runs out ofpower. Reliable, accurate fuel-gauging enables the devicedesigner to implement safe shutdown processes. It alsoallows the user to switch to a power-saving mode toextend operation until the battery can be recharged.

Older implementations of fuel-gauging in consumerdevices with rechargeable lithium-ion (Li-ion) or lithium-polymer (Li-Po) batteries prioritised low cost over accuracyand robustness, using cell voltage measurement toestimate the state of charge. This method, however, isboth imprecise and inaccurate. Across 70% of thedischarge cycle of a lithium battery the cell voltage isbetween 3.9V and 3.6V (see Figure 1). This means that alarge rise in the Depth of Discharge (DoD) value isassociated with a minute fall in the monitored voltage.

The cell voltage then falls steeply from 3.6V as the DoDvalue rises from around 80% to 100%. Voltage monitoringschemes therefore typically sacrifice accuracy across themiddle portion of the discharge cycle. In theory, it would bepossible to provide very high-resolution sampling of the

Users today expect to find a battery status indicator in the display screen of sophisticatedportable electronic devices. The implementation of battery ‘fuel-gauging’ has beenpopularised through its use in mobile phones and laptop computers.

cell voltage between 3.9V and 3.6V, but this would beexpensive, and is not merited since the voltage-monitoringtechnique suffers from another fundamental drawback: thecorrelation between cell voltage and DoD changes overtime and over temperature (see Figures 2 and 3). So evenif a precise value for cell voltage could be measured, theDoD value it produced would still not be reliable.Voltage monitoring, then, is cheap and simple toimplement, but also highly inaccurate and unreliable.

Understanding the latest techniques forimplementing high value-added battery fuel-gauging

Power Pack Solutions

Fig. 1: typical discharge curves of a Li-ion (Prismatic) cell

The typical accuracy of a coulomb-counting circuit in anew battery is 2-3%, but over time, as the cell’s chargecapacity shrinks, this can rise as high as 25% after 500cycles. To counter this effect, fuel-gauging ICmanufacturers such as Texas Instruments and Maxim havesought to implement algorithms that compensate for theeffects of ageing. Devices such as the bq27200/bq27210from TI, for instance, have the ability to ‘relearn’ the chargecapacity of a cell, based on the amount of charge drawnover a full discharge cycle (ie between the end of a fullcharging event and the point at which the cell voltage fallsto a pre-programmed minimum level).

This should reduce the extent to which a coulomb-counting implementation loses accuracy over time. But theeffect of this workaround can be limited. First, thisrelearning process can only take place if the user allowsthe battery to become fully discharged. A mobile phoneuser who always recharges the battery with two or morebars showing on the fuel gauge will therefore never allowthe system to relearn the battery’s capacity.

Second, even if the battery does go through a fullcharge/discharge cycle, a wide range of conditions –including cold temperatures, light load, a fast voltage drop,excessive charging and excessive self-discharge –invalidate the relearning process.

In summary, then, a coulomb-counting circuit’s accuracy isonly as good as its most recent charge capacity reading –and in many cases this might be the first reading takenwhen the battery was new.

A new technique to maintain accuracy over thelifetime of the batteryWhile coulomb-counting can in certain circumstancesachieve high accuracy over the lifetime of a battery, itcannot guarantee this lifetime performance. Forapplications in which accurate SoC data is mission-critical– such as certain portable medical devices – a morerobust technique is required.

A new approach called Impedance Tracking™, usingtechnology patented by TI, promises to offer a guarantee

Fig. 2: typical temperature profile of a Li-ion cell, showing that thedischarge curve is strongly dependent on temperature, and that capacityis different at different temperatures

Accurate fuel-gauging requires a different approachThis article examines the options for system developerswho wish to implement a fuel-gauging circuit that remainsaccurate for the lifetime of the battery pack.

A technique widely adopted in mobile phones, known as‘coulomb counting’, goes some way towards addressingthe problem of maintaining accuracy as the physicalcharacteristics of a cell change over time. A coulomb-counting circuit requires a current sense resistor and afuel-gauging IC. (Such devices are readily available on themerchant IC market.) Over an initial full charge/dischargecycle, the IC ‘learns’ the actual charge capacity of thebattery. (The nominal capacity for common battery types isstored in the device’s memory.) By monitoring the voltageacross the current sense resistor, the device can thenmeasure the amount of charge added to or drawn fromthe battery.

Since the fuel-gauging IC knows the amount of charge thebattery holds when fully charged, and the amount ofcharge that has been drawn at any given time, it can easilyderive dynamic DoD, remaining capacity and remainingtime values.

Except… the capacity of a lithium battery shrinks withevery charge/discharge cycle, so over time the system’sstored value for charge capacity, and the actual chargecapacity, steadily move further apart. This in turn makesthe fuel gauge steadily less accurate.

Take as an example a new battery with a 1,000mAhcapacity. Starting from a fully-charged state, the hostdevice draws charge equivalent to 700mAh; the fuel-gauge IC therefore calculates that the SoC is 30%(300mAh/1,000mAh).

After a given number of charge/discharge cycles, assumethe battery’s charge capacity falls to 900mAh. Now thesame device usage, drawing 700mAh of charge, producesan actual SoC of 22.2% (200mAh/900mAh). A basiccoulomb-counting fuel-gauge, however, will still registerthe SoC as 30%, because the charge capacity valuestored in its memory is 1,000mAh.

Fig. 3: voltage-monitoring as a fuel-gauging technique becomes lessaccurate as the cell ages. Source: Texas Instruments

of accurate fuel-gauging over time. Impedance Trackinguses coulomb-counting in its operation, but alsoimplements other techniques to negate the limitations ofcoulomb-counting.

Impedance Tracking is based on the fact that a cell’s OpenCircuit Voltage (OCV – when the battery is being neithercharged nor discharged) can be correlated to the DoD.But this correlation changes over time: the morecharge/discharge cycles a cell undergoes, the lower itsOCV for any given value of DoD.

To counter this effect, the Impedance Tracking techniquetracks changes in the cell’s internal resistance over time,because the change in the ratio OCV:DoD itself correlatesto the change in this internal battery resistance. At thesame time, an Impedance Tracking implementation willconstantly track the reduction in the cell’s charge capacity(Qmax) over time.

In normal operation, an Impedance Tracking system willmeasure the OCV whenever possible. It will then offset thisvoltage value to take account of the voltage drop causedby the battery’s internal resistance. It can then read off aDoD value from data stored in memory.

When the device is being charged or discharged, it is notin an OCV condition, so to track the DoD it then countscoulombs in or out until the next OCV event, when it resetsthe DoD value using an updated reading for the internalbattery resistance.

Because the system is also dynamically monitoring Qmax,it is also able to derive from the accurate DoD value anaccurate value for remaining charge and remaining time.

Since it is based on impedance data that track changes inthe battery’s physical condition over time, this technique isable to achieve constant accuracy over the whole lifetimeof a battery of better than 1%.

Fuel-gauging ICs from Texas Instruments that implementImpedance Tracking, such as the bq20zxx family, providestraightforward outputs via SMBus, providing values forSoC, remaining charge, Qmax and so on using thestandard Smart Battery System (SBS) 1.1 protocol. Theyalso integrate: • protection features that eliminate the need for certain

external components• a data-logging function. This provides battery-condition

data that are useful to service and repair technicians

How to choose the appropriate fuel-gauging technologyIn the view of this author, the only justification forimplementing basic voltage-monitoring as a fuel-gaugingtechnique is when the actual readings are unimportant, andthe information is presented to the user for cosmetic purposes,rather than as an actual guide to SoC. Voltage monitoringis too inaccurate for users to rely on its SoC readings.

For applications in which high accuracy is required, or acertain level of accuracy must be guaranteed, developersshould implement Impedance Tracking. In suchapplications, the additional bill of materials cost is justified:Impedance Tracking ICs are available prices ranging fromaround $1.40 up to around $4.25; ICs that implementcoulomb-counting typically cost in the region $1.00 -$1.25.

When SoC information is not mission-critical, the coulomb-counting technique will sometimes be accurate enough,although the designer must bear in mind that the level ofaccuracy is not guaranteed, and might decline over time ifthe fuel-gauging IC is not able to ‘relearn’ a cell’smaximum charge capacity. And while nearly allmicrocontroller-based applications will support the I2Cinterface used by coulomb-counting systems, some mightnot have the SMBus interface required for ImpedanceTracking implementations.

Whichever technique is chosen, developers who do notwish to master the details of fuel-gauging can have VARTAMicrobattery design it for them, through its CellPac PLUSservice. VARTA Microbattery will both design a customisedbattery pack including Impedance Tracking or coulomb-counting technology, and manufacture it in volume on itscustomer’s behalf.

If you are interested in learning more about the latesttechniques for implementing high value-added batteryfuel-gauging, please contact your nearest VARTAMicrobattery sales office. Details can be found atwww.varta-microbattery.com.

Press Contact:VARTA Microbattery GmbHSonja Peitl-Steinert – Corporate CommunicationsDaimlerstrasse 174379 EllwangenGermanyTelephone +49 7961 921-526E-mail: Sonja.peitl-steinert@varta-microbattery.com