Henk Jan Bergveld

State-of-the-art of battery state-of-charge determination

From the early days of its discovery, humanity has depended on electricity, a phenomenon without which our technological advancements would not have been possible. With the increased need for mobility, people moved to portable power storage—first for wheeled applications, then for portable and finally nowadays wearable use. Several types of rechargeable battery systems, including those of lead–acid, nickel–cadmium, nickel–metal hydride, lithium ion and lithium-ion polymer exist in the market. The most important of them will be discussed in this review. Almost as long as rechargeable batteries have existed, systems able to give an indication about the state-of-charge (SoC) of a battery have been around. Several methods, including those of direct measurements, book-keeping and adaptive systems (Bergveld et al 2002 Battery Management Systems, Design by Modelling (Philips Research Book Series) vol 1 (Boston: Kluwer)) are known in the art for determining the SoC of a cell or battery of cells. An accurate SoC determination method and an understandable and reliable SoC display to the user will improve the performance and reliability, and will ultimately lengthen the lifetime of the battery. However, many examples of poor accuracy and reliability can be found in practice (Bergveld et al 2002, cited above). This review presents an overview on battery technology and the state-of-the-art of SoC methods. The goal of all the presented SoC indication methods is to design an SoC indication system capable of providing an accurate SoC indication under all realistic user conditions, including those of spread—in both battery and user behaviour, a large temperature and current range and ageing of the battery.

Modeling Battery Behavior for Accurate State-of-Charge Indication

Li-ion is the most commonly used battery chemistry in portable applications nowadays. Accurate state-of-charge (SOC) and remaining run-time indication for portable devices is important for the users convenience and to prolong the lifetime of batteries. A new SOC indication system, combining the electromotive force (EMF) measurement during equilibrium and current measurement and integration during charge and discharge, has been developed and implemented in a laboratory setup. During discharge, apart from simple Coulomb counting, the effect of the overpotential is also considered. Mathematical models describing the EMF and the overpotential functions for a Li-ion battery have been developed. These models include a variety of parameters whose values depend on the determination method and experimental conditions. In this paper the battery measurement and modeling efforts are described. The method of implementing the battery model in an SOC indication system is also described. The aim is an SOC determination within 1% inaccuracy or better under all realistic user conditions, including a wide variety of load currents and a wide temperature range. The achieved results show the effectiveness of our novel approach for improving the accuracy of the SOC indication.

Battery management systems

This chapter gives general information on Battery Management Systems (BMS) required as a background in later chapters. Section 2.1 starts with the factors that determine the complexity of a BMS and shows a general block diagram. The function of each part in a BMS is discussed in more detail in section 2.2 and examples of adding BMS intelligence are given. The BMS aspects of two types of portable devices are discussed in section 2.3. This serves to illustrate the theory presented in sections 2.1 and 2.2.

Survey and Benchmark of Fully Integrated Switching Power Converters: Switched-Capacitor Versus Inductive Approach

This paper surveys and discusses the state-of-the-art of integrated switched-capacitor and inductive power converters. After introducing applications that drive the need for integrated switching power converters, implementation issues to be addressed for integrated switched-capacitor and inductive converters are given, as well as design examples. At the end of this paper, a comprehensive set of integrated power converters are compared in terms of the main specifications and performance metrics, thereby allowing a categorization and providing application-oriented design guidelines.

Module-Level DC/DC Conversion for Photovoltaic Systems: The Delta-Conversion Concept

Photovoltaic (PV) systems are increasingly used to generate electrical energy from solar irradiation incident on PV modules. PV modules are formed by placing many PV cells in series. The PV system is then formed by placing a number of PV modules in series in a string. In practical cases, differences will exist between output powers of the PV cells in the various PV modules, e.g., due to (part of) the modules being temporarily shaded or pollution on one or more PV cells. Due to the current-source-type behavior of PV cells and their series connection, these differences will lead to a relatively large drop in PV-system output power. This paper addresses this problem by adding dc/dc converters on PV-module level. The so-called delta-conversion concept is introduced that aims at averaging out differences in output power between groups of PV cells within modules and between modules inside the PV system. All groups of PV cells can then output their maximum available power, such that a drop in output power of the total system is prevented. This paper describes implementation details, compares the delta-conversion concept with other state-of-the-art module-level power-conversion concepts, and presents first measurement results obtained with a demonstrator system.

Electronic-network modelling of rechargeable NiCd cells and its application to the design of battery management systems

In the first part of this paper, the development of a simulation model for a sealed rechargeable NiCd cell is described. Based on the concept of this cell type, a mathematical description of the various physical and electrochemical processes occurring inside the cell can be given. Subsequently, these equations are introduced in the form of electronic components into an electronic-circuit simulator. This enables the user to simulate the most important cell characteristics like voltage, temperature and internal gas pressure simultaneously and coherently under a wide variety of charging, discharging and open-circuit conditions. The construction of the model enables the user to investigate the course of each of the various reactions taking place inside the cell. Moreover, the electrical and thermal interaction with the surrounding electronics attached to the cell and with other cells, e.g., in a battery pack, can also be simulated. In the second part of this paper, some examples of simulations of cell characteristics are presented. The results of the simulated phenomena show good qualitative agreement with measured cell characteristics. An understanding of phenomena such as charge efficiency, self-discharge and overdischarge is presented using the model. Simulation of battery behaviour in an electronic system enables a system designer to design the optimal Battery Management System around the battery. In the third part of this paper, an example of applying the model in an electronic system is given, i.e., a shaver. Also, simulations of several cells connected in series forming a battery or battery pack are described.

A 50MHz bandwidth multi-mode PA supply modulator for GSM, EDGE and UMTS application

This paper describes the design and measurement results of a supply modulator for a PA for GSM, EDGE and UMTS application. The modulator combines a high-bandwidth class-AB linear regulator with an efficient DC/DC converter in a master-slave configuration. The DC/DC converter is current-mode controlled and has been designed to operate at switching frequencies between 1 MHz and 25 MHz. A damped dual-inductor LCR filter has been inserted in the output branch of the DC/DC converter for ripple suppression. The chip has been fabricated in a 0.25 mum CMOS process with an additional gate oxide for 6 V transistors and has an active area of 1.5 mm2. It achieves a bandwidth of 50 MHz (for UMTS) with a peak output power of 3.2 W (for GSM) and output rms ripple of less than 4 mV.

An inductive down converter system-in-package for integrated power management in battery-powered applications

With the increasing number of voltage conversions that have to be efficiently implemented in a mobile device, the PCB space occupied by switched-mode DC-DC converters with external passive components will become unacceptably high. Therefore, a clear need exists for small-form-factor high-efficiency DC-DC converters having the necessary passive components integrated within one package. This will enable the integration of a DC-DC converter with the load and consequently the system integration of power management. This paper describes the measurement results of an integrated inductive down converter, where the active electronics (power stage and driver circuitry) has been implemented in 0.18-mum CMOS technology and the passive components (output LC filter and decoupling capacitor) have been implemented in a state- of-the-art proprietary passive-integration process technology using high-density trench-MOS capacitors (80 nF/mm2 ) and an 8-mum thick copper top metallization layer. The active die of the converter has been flip-chipped on top of the passive die to reduce parasitic component values. This yields a System-in-Package (SiP) that achieves a step-down DC-DC conversion without any external components. Due to the limited inductance achievable with the used planar air coil in the acceptable area, the switching frequency of the DC-DC converter has been increased. At the same time, Zero-Voltage-Switching (ZVS) measures have been implemented to reduce the switching losses at this increased frequency. A maximum efficiency of 65% at 80 MHz has been achieved for an input voltage of 1.8 V, an output voltage of 1.1 V and an output current of 100 mA. After explaining the motivation behind integrated power management and the choice for an integrated inductive converter, this paper describes the main design aspects of the realized integrated inductive DC-DC down converter. Next, it presents some details of the used passive-integration process, the design of the passive die including the LC filter and the construction of the SiP. Finally, the measurement results of the converter are discussed and conclusions are drawn.

Battery aging and its influence on the electromotive force

Li-ion is currently the most commonly used battery chemistry in portable applications. Accurate state-of-charge (SOC) and remaining run-time indication for portable devices is important for user convenience and to prolong the lifetime of batteries. The actual SOC algorithms, which the main companies use in practice, make use of the so-called electromotive force (emf). In these SOC systems it is assumed that the emf of an Li-ion battery only depends on aging to a limited extent. In this paper, novel emf measurement and modeling efforts are presented as a function of battery aging. As will be shown, a better understanding of this dependence is useful for improving SOC accuracy.

A 65-nm-CMOS 100-MHz 87%-efficient DC-DC down converter based on dual-die system-in-package integration

The increasing number of efficient voltage conversions realized in small volumes in many applications has introduced a trend towards small-form-factor DC-DC converters with integrated passives. Preferably, the DC-DC converter is integrated with the load, often in nm-CMOS, allowing for local supply optimization yielding increased power efficiency. However, energy-storage densities in nm-CMOS are low and silicon area is expensive. Therefore, to limit cost of monolithically integrated systems, passive components have low values, leading to very high switching frequencies, which compromises efficiency. This paper follows an alternative approach, where the active converter part is realized in 65-nm CMOS and the passive part in a low-cost high-density passive-integration process. With the active die flip-chipped on the passive die a small System-in-Package (SiP) is obtained with a peak efficiency of 87.5% at 100 MHz switching frequency and 85 mW output power. This performance is mainly caused by the high quality of the integrated passives.