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An effective cell-balancing algorithm during both charge and discharge phases is presented. This charger can be used either as a standalone application to charge a battery pack with two serial connected Li-Ion/Li-Pol batteries or embedded in residential, office, and industrial applications. Introduction A modern portable system requires more operating voltage than a single-cell Lithium-ion (Li-Ion) or Lithium-polymer (Li- Pol) battery can provide. A serial connection results in a pack voltage equal to the sum of the cell voltages. To increase the battery pack capacity, the cells are connected in parallel. For many applications, two cells in series are sufficient, with one or more cells in parallel. This combination gives nominal voltage and the necessary power for laptop computers and medical and industrial applications. Problems can occur when the cells have different capacities or charge levels. During charging or discharging, the cells in the battery pack do not have matched voltage every cell. Therefore, the battery pack is not balanced. The unbalanced charge between cells causes the following problems: . Reduced overall battery pack capacity to the value of the cell with the least capacity. During the charge process, this cell reaches the maximum charge level before the other cells, and during the discharge process this cell is depleted before the other cells in the pack. . Reduced overall battery pack life. The charge or discharge of cells at different values increases pack imbalance. . Cell damage, which occurs if the charger monitors only the summary voltage. For example, if the lower cell has a capacity deficiency of at least 10 percent, its cell voltage begins to rise into the dangerous area above 4.3 volts. This can result in additional degradation of the cell or a safety system response that greatly reduces pack capacity. This application note describes a two-cell Li-Ion/Li-Pol battery charger. An effective cell-balancing algorithm is designed. It avoids the issues that appear in battery packs with two cells in series. Through modification of the configuration parameters, the cell-balancing algorithm can easily be adapted for various applications and selected batteries. The unique architecture of the PSoC® device provides an integrated hardware solution for a two-cell battery charger and a flexible .C-based, cell-balancing algorithm with minimal external components at a very affordable price. The CY8C24x23A PSoC device family used in this implementation reduces the total device cost even further. When you want to use algorithms for the latest charging or cell-balancing technologies, only the firmware needs to be modified. PSoC Designer’s in-circuit and self-programming capabilities make these operations simple. Specifications for a two-cell Li-Ion/Li-Pol battery charger with cell-balancing support are listed in Table 1 on page 2. Table 1. Specifications for Two-Cell Li-Ion/Li-Pol Battery Charger with Cell-Balancing Support Item Item Value Battery Charger Parameters Built-In Battery Charger Type Two-cell Li-Ion/Li-Pol battery charger Power Supply Voltage 10…14V Power Consumption 35 mA Battery Current Measurement Error (Not Calibrated) 5 percent Battery Voltage Measurement Error (After Calibration) 0.5 percent Battery Thermistor Resistance Measurement Error 5 percent User Interface 2 LEDs PC Communication Interface RS232 PC Communication Speed 115200 Cell-Balancing Parameters Cell-Balancing Algorithms 1. During charge phase 2. During discharge phase Cell-Balancing Configuration Parameters . Cell-balance circuit resistors nominal . Cell-balance interval parameter . Minimum cell-balance parameter for charge phase . Minimum cell-balance parameter for discharge phase . Minimum charge current value when cell balancing is allowed . VMID value for discharge phase (voltage of middle charged state) Minimum Cell Balancing During Charge Phase Equal to the voltage measurement error value (15 mV-30 mV) Minimum Cell Balancing During Discharge Phase Equal to the voltage measurement error value (15 mV-30 mV) plus the internal impedance error (10 mV-30 mV) Cell-Balancing Foundation This section describes the fundamentals of cell-balancing techniques. Cells are considered balanced when: Equation 1 The value is the charge of cell N. The equation for the charge is: 12QQcellcell cellNQ Equation 2 QItCV Therefore, Equation 1 can be transformed into the following equation: Equation 3 The value is the electrochemical potential of the fully charged cell. The potential is fixed for a given set of electrodes is fixed and does not change from cell to cell. When two cells are unbalanced, the following is true: 1122CVCVcellcellcellcell VcellN VcellN Equation 4 12QQcellcell Equation 5 1122CVCVcellcellcellcell However, does not change from cell to cell. Therefore, the cells are unbalanced if: Vcell Equation 6 12CCcellcell Equation 6 shows two cells that have different capacities, which is one cause of cell imbalance. A differenc...
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