Lithium Battery State of Charge Determination


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Paradigm shift In lithium battery SoC measurements.

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Lithium Battery State of Charge Determination

  1. 1. 31-4: Diamagnetic Measurements in Lead Acid Batteries to Estimate State of Charge Jörn A. Tinnemeyer Vice President – Research & Development Cadex Electronics Inc. 22000 Fraserwood Way, Richmond BC, Canada V6W 1J6 or Abstract: With the development of start/stop technology to entertainment systems and start-stop technologies (micro improve fuel economy and a larger prevalence of electronic hybridization) to name two [1]. Recent technological loads in standard automobiles, the accurate determination developments promise to use battery power for critical of State of Charge (SoC) is highly desirable. The market is safety features of the vehicle - such as, electrical steering already experiencing a paradigm shift away from handheld (drive by wire) and braking systems. In such instances diagnostic equipment to onboard sensors in high end when critical operations of the automobile depend on automobiles. Concurrently, the adoption has also battery power, accurate knowledge of the state of the stagnated in the last few years due to inaccuracies of these battery is not a nicety, it is paramount. sensors. Specifically, all sensors on the market today employ voltage, current and temperature sensing to estimate the SoC. Although these methods are reasonable As our demands for battery sensor technologies have (defined as having a +/- 15% accuracy to relative state of increased, so too have the types of sensor technologies that charge) when the battery is new, the errors increase are available. Initial automotive battery sensors consisted dramatically with age. The reason for this is that different of a primitive shunt style system that assessed the cranking failure modes resulting from varying operating conditions capacity of the battery. Now, battery sensor systems have will have independent effects on battery health. In order to been placed in mid-level and luxury automobiles that make a more direct assessment, Cadex Electronics Inc. is monitor the current of the battery from a few milliamps to developing a sensor based on measuring the change in hundreds of amperes. These modern battery sensors rely, magnetic susceptibility of lead / lead sulphate as the almost exclusively, on mathematical models. The sensors battery changes SoC. Measurements of the diamagnetic take in a number of parameters from the battery - voltage, field changes have demonstrated improved predictive current, and temperature - and then an algorithm estimates capability (+/- 7%) within a challenging automotive the battery’s SoC. The computational models are dynamic environment. and are based on the parameters of a new battery plus an aging model. Not surprisingly, these models work reasonably well (+/-15% SoC) when the battery is new, Keywords: state of charge; lead acid; diamagnetic; charged, and left standing for a prolonged period of time magnetic susceptibility (allows voltage to be directly correlated to SoC). However, if the battery is not new, is or has been recently polarized, then these estimation techniques become less accurate Introduction (+/-30% SoC or worse). The greatest hurdle that these In our portable world, we use batteries to keep our estimation techniques face is the ability to obtain an electronic devices functioning and we monitor the state of accurate measure the battery’s capacity. The capacity of the battery to assure that the equipment will operate as we the battery decreases as the battery ages. Importantly, the expect. Realtime battery monitoring is commonplace in speed of this aging processes is modified by a host of devices, which use the battery as the primary source of different environmental conditions (e.g., outside power - such as, cellular phones, laptop computers, etc. By temperature, usage rate, degree of charge-discharge cycles) contrast, battery monitoring is less frequent in electronic [2], which thwart the meaningful development of a systems that place fewer demands on the battery. computational estimation system. Automobiles and other vehicles provide a good example of this later trend. Traditionally, automobiles used the onboard battery system to start the engine; thus, realtime Unfortunately, this roadblock to effective battery battery monitoring was not required or implemented. monitoring occurs at the same time as the demand for a reliable, realtime measure of SoC grows: the ADAC reports that 40% of roadside automotive failures are battery-related This trend is changing. The last two decades have brought [3]. More importantly there is an increasing trend towards many advances in automobile technologies, which place these failures (36% in 2007 for example) [4]. It is time to more demands on the automobile’s battery: onboard consider a novel approach.
  2. 2. Concept Description we can determine the magnetic field absorption. The The modern lead acid battery primarily consists of the degree of penetration into the metal, or skin depth, is given following reactions: by δ. The permeability of the material is represented by µ and the conductivity by σ. The frequency, f, reflects the depth of the material being sampled. Since equation 1 is inversely proportional, we know that deeper penetration of the material occurs at lower frequencies. The skin depth of pure lead is 29 mm at 60 Hz to 0.73 mm at 10kHz [6]. The magnetic field produced by a coil follows Biot- Savart’s Law, Thus, the voltage of the battery is a direct response of the ∧ ..2 µ IdI × r materials used at the cathode and anode poles. Changes in dB = 0 4π r 2 the SoC of the battery are accompanied by physical changes in the material properties of the cathode and anode poles that we can measure. in which... dB represents the vector quantity that describes the magnetic field at the desired point; I is the current; Table 1 - Magnetic susceptibilities of electrode species [5] dl is a vector quantity of an infinitesimal current element in the direction of the field potential; As evidenced in Table 1, the magnetic susceptibility of negative electrode changes from -23 xm/10-6cm3mol-1 when µ0 is the magnetic susceptibility dependent on the material; the battery is fully charged to -69.7 xm/10-6cm3mol-1 when ∧ the battery is discharged. To measure this three fold change r is the unit vector in the direction to where the magnetic in magnetic susceptibility, an excitation field is needed to field is to be calculated; and stimulate the metals and a sensor is needed that is capable of registering these minute changes in the magnetic field. r is the distance to the calculation point. To create an excitation field, a coil is used to generate eddy If we consider a current loop with a radius of R, and we currents. These eddy currents produce magnetic fields that wish to measure the field at a particular point x, the are countered by the diamagnetic response of the to-be- equation can be simplified to: measured material. The extent of this reduction depends on the strength of the diamagnetic response - more reduction is registered when the diamagnetic response is greater. In µ0 nIR 2 ..3 the case of lead acid batteries, the greater the diamagnetic B(x) = 3 response reveals a greater contribution of lead sulphate or, 2(R + x ) 2 2 2 in layman’s terms, the battery is more fully charged. The field may be a regular DC field or an AC field. which allows us to easily assess the material properties of the negative electrode. By using the definition, A sensor is then used to measure these changes in the magnetic field. Magnetic field sensor technology has changed significantly over the last decade, driven mainly by hard drive read head development. Magnetic tunneling 1 ..1 junction sensors are, currently, the state of the art. The δ= sensors are built by separating two metals, CoFeB, by an πµϑ f insulator of MgO that is only a few atoms thick. A biasing voltage is created between the metals, by allowing current
  3. 3. to flow across the insulator. The likelihood of quantum obtained at the same time, as evidenced in Figure 2. tunneling is directly related to electron spin alignment, Typical errors between actual SoC and estimated SoC are which can be manipulated and controlled by introducing on the order of ± 7%. external magnetic fields, with the following consequence: as the strength of the magnetic field increases, the electron spin alignment increases, and more electrons may tunnel across the insulator. As more electrons tunnel across the insulator, the resistance of the device falls [7]. Accordingly, the magnetoresistance of the sensor is the first indication of its performance: for example, anisotropic sensors have 2-3% magnetoresistance, whereas giant sensors have 15-20% magnetoresistance. By contrast, sensors that implement magnetic tunnel junctions have a magnetoresistance of 200% [7]. Finally, a fuzzy logic algorithm is applied to the outputs from the sensor to provide an estimate of the state of charge of the battery. Figure 2 - Model estimation compared to actual SoC Results within vehicle. Figure 1 highlights how these diamagnetic measurements can be used to track a typical discharge/charge profile of a lead acid battery. As evidenced in this figure, a systematic increase in Estimated SoC (blue triangles) is measured from the sensor that accompanies charging the battery. Two important points about the functioning of this diamagnetic sensor technique require explicit mention. First, the data shown in Figures 1 and 2 were collected within the electromagnetically noisy environment of the automobile; thus, represent data that one can expect from the field. Second, this technique has been tested on different batteries (flooded and AGM) of different ages, and has yielded identical results (within ± 7% of actual SoC); therefore, this diamagnetic method provides a very accurate measure of SoC for batteries of all ages. It is often possible to create technologies that function well, but are impractical by virtue of price and ease of implementation (or use) - if the price is too high or the device is too large or cumbersome, then it will not be adopted by industry. Figure 1 - Discharge/charge profile of a standard SLI Current battery sensors, which depend on mathematical (starting lighting ignition) battery modeling, cost automotive manufacturers approximately $15 per unit. Our diamagnetic sensor technology can be In this example, the battery was subjected to a 20A built a similar price level. discharge, followed by a constant current (9A) constant voltage (14.4V) charge. The data set contains both Moreover, our diamagnetic sensor easily adjusts to all polarized and float data; however, the voltage was makes and models of lead-acid batteries, as illustrated in subjected to float levels after 5 minutes of rest. Figure 3. Thus, it is easy to use. Importantly, this pattern is independent of voltage changes that accompany the charging of the battery, as evidenced by comparing the Estimated SOC (blue triangles) with the voltage (green circles). More important, this diamagnetic measure of SoC corresponds closely to actual measures of SoC that were
  4. 4. the automotive industry to implement drive-by-wire technologies, which will improve the efficiency of automobiles at the same time as reducing manufacturing costs. Taken together, at a time when consumers, governments, and industry ubiquitously demand more of the automotive battery, and at the time when 40% of roadside problems originate from battery-related failures, we have created a simple, accurate, and effective realtime battery monitoring system. Figure 3 - Simple overview of sensor and required References electronics 1. Blanke, H., Bohlen, O., Buller, S., Doncker, R., Fricke, B., Hammouche, A., Linzen, D., Thele, M., Taken together, consider the benefits of our realtime battery Sauer, D.U., Impedance Measurements on lead- monitoring system, in lieu of the current realities of battery acid batteries for state-of-charge, state-of-health monitoring systems that rely on mathematical models and cranking capability prognosis in electric and (Table 2). hybrid vehicles, Journal of Power Sources, 144, 2005. 2. Ruetschi, P., Aging Mechanisms and service life of lead acid batteries, Journal of Power Sources, 127, 2004. Current sensors Diamagnetic sensor employing modeling technique 3. ADAC Pannenstatistik 2009 techniques 4. ADAC Pannenstatistik 2007 Accuracy: +/-15% SoC +/-7% SoC New battery 5. Landolt-Börnstein,Numerical Data and Functional Accuracy: +/-30% SoC +/-7% SoC Relationships in Science and Technology, Aged battery Diamagnetic Susceptibility, Springer-Verlag: Ease of Heidelberg,1986. = = implementation/use 6. Hill, I. R., & Andrukaitis, E. E., Non-intrusive Cost = = measurement of the state-of-charge of lead-acid batteries using wire-wound coils, Journal of Power Sources, 103, 2001. Table 2 - Comparison of realtime SoC technologies. 7. Schrag, B.D., Carter, M.J., Liu, X., Hoftun, J.S., & Xiao, G., Magnetic current imaging with magnetic tunnel junction sensors: case study and analysis, Proceedings of the 2006 International Symposium Conclusions for Testing and Failure Analysis,2006. Our diamagnetic sensor affords the benefits of substantially better accuracy that is largely independent of the age or condition of the battery, but is equally easy to implement and costs approximately the same amount of money per unit. Now consider the following issues: (a) 40 % of all roadside failures originate from the (lack of) functioning of the battery, (b) the ever increasing demands for electric loads placed on the automotive battery originating from consumer desires for entertainment devices, governmental requirements for the implementation of start-stop technologies to help protect the environment, and desires of