PSpice Application Notes

APPLICATION NOTE 5 Model Differences for Different Battery Types Alkaline cells Alkaline cell resistance is not fixed throughout the discharge.The resistance model is developed by determining the relationship of the output's current and voltage, then linking this to the battery's state of charge. The cell has a small resistance increase from 100% to 20% cell capacity, then increases to twice its initial value at 0% capacity. When the discharge current approaches 100mA, the discharge capacity versus discharge rate produces a kink in the discharge curve. Below 100mA, the cell loses capacity gradually; above 100mA, the rate of lost capacity increases significantly. Although the 100mA discharge rate is the same for all of the cell sizes, the C rating is not. Because it is not possible to relate this kink to a specific C discharge rate, a separate E_Lost_Rate table must be developed for each cell size. A separate subcircuit model is used to model the 9V Alkaline cell. The cell resistance change versus discharge state is more pronounced in this type of battery, and is modified accordingly in the model. Nickel-cadmium cells These cells are often used at very high discharge rates up to 20C. Discharging a fully charged cell in 5 or 6 minutes (10C rate) releases significant amounts of heat. To account for this, a thermal model is included in the cell subcircuit. The thermal temperature rise of a cell per watt dissipated in free air is approximately Θcell = 13.41 * V- 0.61 where V is the cell volume in cubic inches, and Θcell is the thermal rise of the cell in °C per watt dissipated.The thermal time constant for material of the density used in making NICD batteries is approximately 20 minutes per pound or expressed in more convenient terms, 2.65 seconds per gram. These empirical relationships are used with the calculated cell power dissipation (Cell Discharge Current 2 * Cell Resistance) to get a temperature rise and time constant model for the cell temperature. The cell temperature rise above ambient temperature is available at node CELL_TEMP in the NICD model. The temperature information is also used to add or subtract from the cell discharge voltage to account for the cell temperature E_Temp in the NICD model. Another small modification is made to the NICD model to facilitate the direct entry of manufacturers' rated capacity data. Most NICD batteries are not rated at their maximum capacity for low discharge rates. The norm is to rate them at the C to C/5 rate, leaving 30% more than the rated capacity if the cell is used at low discharge rates. To account for this difference a Voltage- Controlled Current Source (VCCS), G_LowRate is used to add a small amount of current to C_CellCapacity during discharge at rates less than 1C. Nickel-Metal-Hydride cells These cells are modeled like the NICD cells, but without the fast discharge thermal effects. Lead-acid cells Since these cells are almost universally used in batteries composed of 3, 6, or more cells (6 or 12 Volt batteries), the model is changed slightly. The single cell voltage is multiplied by the number of cells to get the total battery voltage.The schematic in Figure 3 refers to the schematic shown in Figure 2 implemented for NICD battery with an additionally added cell temperature block.

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