PSpice Application Notes

APPLICATION NOTE 11 terminal voltages however, they do not show this voltage recovery. The battery chemistry tries to make a voltage potential difference even if only a few molecules of unused material remain. In this state of discharge, the internal resistance of the cell can be an order of magnitude, or more, than its initial value. If any load is reconnected, the terminal voltage will quickly collapse again to zero. Usage beyond standard cutoff voltages not accounted for: The models were designed to be used with the standard cutoff voltages as specified by the battery makers. For NICD batteries, this is 0.8 to 1.1 Volts per cell. For Alkaline cells, the cutoff is 0.8 to 1.2 Volts per cell. For Lead-Acid cells, the cutoff voltage is typically 1.5 to 1.7 Volts per cell. Usage beyond these limits should be studied carefully because they are not specifically accounted for in the modeling process. Models not designed for parallel connection: These models are not designed to be connected in parallel.This is not acceptable in consumer design anyway, because there is no way to guard against the end user putting an Alkaline cell in parallel with a NICD or Carbon cell. If these different types of cells are connected together, the charge most likely will not equalize. This results in overcharging and leakage of the weak cell, causing damage. Simulation Speed The goal of simulation is to obtain results faster than can be achieved with the hardware, or to measure behaviors which cannot easily be accessed in the hardware. The following notes should help when making speed/accuracy trade-offs. Be careful with the models that you attach to these batteries during a simulation. Simulate the power drain from your circuit, not the transistor-level circuit itself. These models have been tried and verified with many different discharge regimens, and are believed to be accurate enough to allow finding maximum or minimum battery life. The trends these models simulate are believed to be basically accurate, even though the absolute capacity simulated may be 20% or so off. Do not simulate pulsed current loads with cycle times less than 5 seconds. Using short cycle time pulsed currents may make the simulation run slower than real time. To speed up the simulation with fast pulsed loads, use the RMS average of the pulsed current. This will provide you with a ballpark answer. Use a minimum of semiconductor models hooked up to these models. Semiconductors contain many internal nonlinear equations that must be solved for each time point, thus slowing simulation time. If you experience convergence problems, use the .IC directive to set the initial voltages on critical nodes in your circuit. Annexure The following are the five different Cell discharge models.These models are presented in the library file battery.lib and its associated symbols are presented in the battery.olb file.These .lib and .olb files can be used to perform discharge test of other batteries. ** PSpice Alkaline battery discharge model ** .SUBCKT ALKALINE +OUTPUT -OUTPUT SOC RATE + PARAMS: CAPACITY=1, RESISTANCE=1 C_CellCapacity 50 0 { 3600 * CAPACITY * 1.01 } R1 50 0 1G E_Resistance 20 10 VALUE = {I(V_Sense) * RESISTANCE * V(Cell_Res)} E_Cell_R Cell_Res 0 TABLE { V(50) } = (0,2) (0.2,1) (1,1) R3 Cell_Res 0 1G V_Sense -OUTPUT 20 0 E_Invert Invert 0 TABLE { V(SOC) } = (0,1) (1,0)

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