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CELL BALANCING IN A MULTICELL CAPACITOR THROUGH VOLTAGE REGULATION USING DIODES IN PARALLEL TO EACH CELL

IP.com Disclosure Number: IPCOM000008827D
Original Publication Date: 1998-Sep-01
Included in the Prior Art Database: 2002-Jul-17
Document File: 4 page(s) / 251K

Publishing Venue

Motorola

Related People

Kirk Watson: AUTHOR [+4]

Abstract

Multi-celled, electrochemical capacitors have a tendency to become unstable if operated on or near their maximum rated voltage (i.e. 1.2V per cell for aqueous electrolytes) for a long time. The long time instability arises due to variations in the leakage resis- tance between cells in the bipolar stack. These capaci- tors are composed of individual cells of approximately 1-4 mils thick stacked on top of each other. To over- come the voltage instability, we show that connecting diodes in parallel to each cell alleviates this problem. This method could be achieved by either connecting diodes externally to each cell in a multicell device or by integrating the diode elements at the rim of the sub- strate during the manufacturing process. The diodes required in this application are of very small size and could also be embedded in the sealant epoxy used at the border of the electrodes.

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m MOZOROLA Technical Developments

CELL BALANCING IN A MULTICELL CAPACITOR THROUGH VOLTAGE REGULATION USING DIODES IN PARALLEL TO EACH CELL

by Kirk Watson, Hitendra Pate& John Nerz and Sanjay Goel

ABSTRACT

   Multi-celled, electrochemical capacitors have a tendency to become unstable if operated on or near their maximum rated voltage (i.e. 1.2V per cell for aqueous electrolytes) for a long time. The long time instability arises due to variations in the leakage resis- tance between cells in the bipolar stack. These capaci- tors are composed of individual cells of approximately 1-4 mils thick stacked on top of each other. To over- come the voltage instability, we show that connecting diodes in parallel to each cell alleviates this problem. This method could be achieved by either connecting diodes externally to each cell in a multicell device or by integrating the diode elements at the rim of the sub- strate during the manufacturing process. The diodes required in this application are of very small size and could also be embedded in the sealant epoxy used at the border of the electrodes.

PROBLEM DESCRIPTION

   Multi-celled, electrochemical capacitors have a tendency to become unstable if operated on or near their maximum rated voltage (i.e. 1.2V per cell for aqueous electrolytes) for a long time. The long time instability arises due to variations in the leakage resis- tance between cells in the bipolar stack. (see Figure I for approximation of model of an individual cell)

  On applying a voltage to a capacitor stack, current surges into the capacitor charging each cell. The volt- age uniformly distributes over each cell if the capaci- tance for each cell is similar. After a relatively long time (5Rleakage x Ccell ), the current supplied by the

Fig. 1 First order approximation of capacitor

power source to the capacitor will decrease to a finite value defined as the leakage current of the device. Since DC current (i.e. infinitely low frequencies and long time constants) cannot go through a capacitor, a resistive path in parallel to each cell exists for the leak- age current. The product of the leakage current and the leakage resistance of each cell determines the voltage over the resistor, hence the cell. Consequently for long times, higher potentials exist across cells with high leakage resistance and lower potentials exist across

cells with low leakage resistance. If the distribution in values for the leakage resistance is not within some tol- erance limit, then the potenfal over some of the high leakage resistance cells may approach the electrolyte decomposition voltage and cause the cell to fail [l].

  This voltage balancing phenomenon is also observed in electrical engineering designs. In some applications, a number of equivalent sized capacitors are connected in series to achieve a higher voltage than that for a single capacitor. Voltage variation over the series connected capacitors is observed and attributed to the imbalance between each...