Conventional fuel gaugingtechnologies, mainly voltagebasedand coulomb countingalgorithms, have obvious limitationsin gauging performance.

The voltage-based scheme,widely adopted in handhelddevices due to low cost andsimplicity, suffers from the laptop batteryimpedance change overtime. A dynamic load conditionand temperature change canrender a gauging error of up to50 percent. The coulomb countingscheme, on the other hand,takes the alternative approachby continuously integratingcoulomb to compute the consumedcharge and state-ofcharge(SOC). With a pre-learnedknowledge of full capacity, theremaining capacity can be obtained.

As a result, the laptop battery reachesthe minimum system operatingvoltage (or the termination voltage)much earlier than a freshlaptop battery does.

A drawback of this approachis that self-discharge isdifficult to model with accuracy.

Then without periodic full-cyclecalibration, the gauging erroraccrues over time. None of thesealgorithms address impedancevariation of the laptop battery. The designerthen must terminate systemoperation prematurely byreserving more capacity to avoidthe unexpected shutdown foraged cells. This leaves a significantamount of energy wastedfor fresh laptop battery (Figure 2)..

A misunderstanding about theLi-Ion laptop battery is that the shrinkingrun-time of a laptop battery in use isprimarily due to the fading of thelaptop battery capacity. Contrary to thispopular thinking, it is generallynot the capacity loss but the increaseof the laptop battery impedancethat causes the problem. Figure1 shows that after 100 cycles, thelaptop battery capacity drops by lessthan five percent, while the internalDC impedance of the laptop battery,R(Z), increases by a factor or two!A direct effect of the higherimpedance of an aged laptop batteryis a higher internal voltage dropin response to a load current.