Battery Power - May/June 2012 - (Page 6)

Feature Battery Management Considerations for Large Cell Count Systems Karthik Kadirvel, Design Engineer Texas Instruments Over the last few years there has been an increased use of lithium-ion (Li-Ion) batteries in high-power applications such as power tools, yard tools, uninterruptible power supplies (UPS) and hybrid electric vehicles (HEV), which traditionally have been served by nickel metal hydride (NiMH) batteries. Reasons for this trend include higher power density, reduced cost and improved electronics that are needed to manage Li-Ion batteries. In these high-power applications, high voltage and high current is achieved by connecting multiple cells in series and parallel. These high-voltage and high-current Li-Ion battery packs present new challenges for battery management electronics. Traditional single-cell and two-cell Li-Ion battery packs such as those used in cell phones and digital cameras do not face these issues. In this article, key considerations in battery management design for large cell count systems are discussed. These include cell balancing, open sense line detection, communication between the monitoring integrated circuits (ICs) and random cell connection. In Li-Ion battery packs containing multiple series and parallel connected cells, there exists small differences in cell capacity, cell impedance and state-of-charge between the individual cells. The reasons for these differences include manufacturing tolerances, uneven heating of the different cells in the pack and aging. These small differences manifest themselves as different open circuit voltages for each of the cells in the battery pack. For the battery pack end user, cell voltage imbalance leads to reduced battery capacity per charge-discharge cycle and reduced overall battery lifetime. To address this problem, battery management ICs must include circuitry to balance the individual cells in the battery pack. Two commonly used algorithms for balancing the cells are voltage balancing and capacity balancing. In voltage balancing, the simpler of the two algorithms, charge is removed from or added to the various cells in the battery pack until the voltages of individual cells match. In capacity-based balancing, the state-of-charge for each cell is measured using various gas gauging techniques. The charge is then removed or added to match the individual cells’ state-of-charge. Two main techniques for implementing these algorithms are passive balancing and active balancing. In passive balancing, a known load, implemented using a resistor and a MOFET switch, is connected across the individual cells to reduce the voltage or capacity to match that of the other cells. The switch and resistor can be internal or external to the IC. TI’s bq77pl910 implements passive bleed balancing using internal switches.[1] Figure 1 shows a picture of a four-cell system using external resistors and switches to perform passive bleed balancing. In this system, the IC periodically measures the cell voltages. Once the cell voltages are measured, the IC determines the cell with the lowest voltage and turns on the switches across the other cells to reduce their voltage to match the cell with the lowest voltage. The external resistor and the MOSFET ON resistance determine the peak-balancing current. Different average balancing currents can be obtained by using pulse-width modulation (PWM) of the MOSFET gate signal. The main advantage of passive balancing is ease of circuit and algorithm implementation. The main disadvantage of this scheme is that system energy is wasted as heat in the resistor. Furthermore, the heat produced by the bleed resistors can affect cell voltage measurement and potentially can accelerate cell-aging. Figure 1. Schematic of four-series Li-Ion cells showing external resistors and switches (dashed boxes) for passive (dissipative) balancing. In active balancing, the excess capacity in a cell is transferred to a cell with lower capacity using external inductors or capacitors. When external capacitors are used, the principle of operation is similar to that of charge pumps. The excess capacity is first transferred to an external transfer capacitor. Then the capacitor is connected to the cell with lower capacity to transfer the energy. This technique also can be used to balance the voltage as opposed to balancing capacity. When external inductors[2] are used for active balancing, the principle of operation is similar to that of a traditional switching (buck/boost) converter where energy is temporarily stored in the magnetic field of an inductor, then transferred to the cell of interest. The main advantage of active balancing is that the excess capacity (or voltage) in a cell is not wasted as heat. It is efficiently transferred to a cell with lower capacity (or voltage). The main disadvantage of this technique is the increased component count as one inductor and two switches are needed www.BatteryPowerOnline.com 6 Battery Power • May/June 2012 http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - May/June 2012

Battery Power - May/June 2012
New Battery Interface Specification to Address Key Consumer and Manufacturers’ Issues
Demand Spike for Backup Power Systems Stokes Growth in the Global Stationary Lead Acid Battery Market
Battery Management Considerations for Large Cell Count Systems
Flywheel Energy Storage – A UPS Battery’s Best Friend
Zero-Volt: Medical and Satellite Battery Technology Can Help Improve Safety of Electric Vehicles
Second Edition of IEC 62133: The Standard for Secondary Cells and Batteries Containing Alkaline or Other Non-Acid Electrolytes is in its Final Review Cycle
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Battery Power - May/June 2012