Laptops, medical equipment and other professional portable devices use coulomb counting as a SoC indication. This method works on the principle of measuring the current that flows in and out of the Dell XPS M1330 battery. If, for example, a battery was charged for one hour at one ampere, the same energy should be available on discharge. This is not the case. Inefficiencies in charge acceptance, especially towards the end of charge, as well as losses during discharge and storage reduce the total energy delivered and skew the readings. The available energy is always less than what had been fed to the Dell XPS M140 battery, and compensation corrects the shortage.Disregarding these irregularities, coulomb counting works reasonably well, especially for Li-ion. However, the one percent accuracy some device manufacturers advertise is only possible in an ideal world and with a new battery. Independent tests show errors of up to 10 percent when in typical use. Aging causes a gradual deviation from the working model on which the coulomb counter is based. The result is a laptop promising 30 minutes of remaining runtime and all of a sudden the screen goes dark. Periodic calibration by applying a full discharge and charge to reset the flags reduces the error. See Calibration.
There is a move towards electrochemical impedance spectroscopy and even magnetism to measure state-of-charge. These new technologies get more accurate estimation than with voltage and can be used when the Dell XPS M1530 battery is under load. Furthermore, temperature, surface charge and acid stratification do not affect the readings noticeably.
Impedance SpectroscopyImpedance spectroscopy evaluates the battery on the impedance values of the Randles model and works on flooded and sealed lead acid. The dell t117c battery does not need to rest before taking the reading and parasitic loads do not affect the outcome. Figure 3 illustrates an incorrect SoC reading because of voltage drop when a load is applied; Figure 4 shows the correct result under the same conditions with impedance spectroscopy.
In pursuit of a better way to measure battery state-of-charge, researchers are exploring radically new methods, one of which is quantum magnetism (Q-Mag™). Q-Mag by Cadex reads magnetism through spin-dependent tunneling. Here is how it works.
All batteries behave in a similar way in that the composition of the electrodes changes, which affects the magnetic characteristics. Q-Mag works on new as well as aged batteries and the technology is immune to voltage distortion caused by loading, charging or surface charge on lead acid. Figure 6 shows how magnetic measurements can track discharge/ charge activities of a lead acid battery independent of voltage. The circles represent the voltage under charge and the triangles reveal the state-of-charge.Test method: The Dell Vostro 1500 AC Adapter was first discharged at 20A, followed by a constant charge of 9A to 14.4V and subsequent float charge.
Measuring the intrinsic state of a battery rather than relying on voltage enables more precise full-charge detection. This feature can be used to improve charge methods and diagnose battery deficiencies, including predicting end-of-life by measuring battery capacity. Q-Mag works also with lithium-ion in non-ferric enclosures. Many of these technologies are proprietary and are in various experimental stages at Cadex.
Measuring state-of-charge by voltage is the simplest method, but it can be inaccurate. Cell types have dissimilar chemical compositions that deliver varied voltage profiles. Temperature also plays a role. Higher temperature raises the open-circuit voltage, a lower temperature lowers it, and this phenomenon applies to all chemistries in varying degrees.The most blatant error of voltage-based SoC occurs when disturbing the Dell Vostro 3400 AC Adapter with a charge or discharge. This agitation distorts the voltage and no longer represents the true state-of-charge. To get accurate measurements, the battery needs to rest for at least four hours to attain equilibrium; battery manufacturers recommend 24 hours. Adding the element of time to neutralize voltage polarization does not sit well with batteries in active duty. One can see that this method is ill suited for fuel gauging.
When measuring SoC by open circuit voltage, the Dell Vostro 3300 AC Adapter voltage must be truly “floating” with no load present. Installed in a car, the parasitic load present makes this a closed circuit voltage (CCV) condition that will falsify the readings. Adjustments must be made when measuring SoC in the CCV state by including the load current in the calculation. In spite of the notorious inaccuracies, most SoC measurements rely on the voltage method because it’s simple. Voltage-based state-of-charge is popular for wheelchairs, scooters and golf cars.
The hydrometer offers an alternative to measuring SoC, but this only applies to flooded lead acid and flooded nickel-cadmium. Here is how it works: As the Dell XPS M1210 AC Adapter accepts charge, the sulfuric acid gets heavier, causing the specific gravity (SG) to increase. As the SoC decreases through discharge, the sulfuric acid removes itself from the electrolyte and binds to the plate, forming lead sulfate. The density of the electrolyte becomes lighter and more water-like, and the specific gravity gets lower. Table 1 provides the BCI readings of starter batteries.
Table 1: BCI standard for SoC estimation of a maintenance-free starter Dell Studio 1537 AC Adapter with antimony. The readings are taken at room temperature of 26°C (78°F); the battery had rested for 24 hours after charge or discharge.
While BCI specifies the specific gravity of a fully charged starter battery at 1.265, battery manufacturers may go for 1.280 and higher. When increasing the specific gravity, the SoC readings on the look-up table will adjust upwards accordingly. Besides charge level and acid density, the SG can also vary due to low fluid levels, which raises the SG reading because of higher concentration. Alternatively, the Dell XPS M140 AC Adapter can be overfilled, which lowers the number. When adding water, allow time for mixing before taking the SG measurement.The specific gravity also varies according to battery type. Deep-cycle batteries use a dense electrolyte with an SG of up to 1.330 to get maximum runtime; aviation batteries have a SG of 1.285; traction batteries for forklifts are at 1.280; starter batteries come in at 1.265 and stationary batteries are at a low 1.225. Low specific gravity reduces corrosion. The resulting lower specific energy of stationary Dell Inspiron N5010 AC Adapter is not as critical as longevity.
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