PSpice Application Notes

APPLICATION NOTE 4 E_Rate is the sensed discharge current in Amps divided by the Amp-hour capacity of the cell. The node, RATE, is the instantaneous rate at which the cell is being discharged, as shown in Figure 2. This instantaneous rate information can almost be fed directly to E_Lost_Rate to determine the actual available capacity. But, when the discharge is a low duty cycle, high value pulsed load, the cell supplies a large initial current which decays in seconds to a lower value. For pulsed loads, the cell recovers between pulses and delivers a higher proportion of its capacity than a cell under constant discharge. The delayed rate is modeled by an RC lowpass filter, as shown Figure 2 as R1 and C1. The exact value of the RC time constant depends on the type and size of cell being simulated. E_Lost_Rate is built like the E_Cell table as follows. E_Lost_Rate 50 SOC TABLE { V(x) } = (0.0,0.0) (1.5,0.5) The table entries indicate the capacity unavailable from the cell at high discharge rates. The table entry shows that at a discharge rate of 0, the cell loses 0% of its capacity (first entry). If the discharge rate is 1.5 times the rated capacity of the cell (1.5 C), the cell loses 50% of its capacity (second entry in the table). In Figure 2, the State-Of-Charge (SOC) node is the subtraction of the voltage on the capacitor C_CellCapacity and E_Lost_Rate. The SOC node represents the capacity in the cell for a given discharge rate during the simulation. G_Discharge discharges C_CellCapacity at the cell rate. The voltage on node 50 relates to the capacity remaining in the cell if the discharge rate is low enough to actually run the cell dry. At low discharge rates, these two nodes are the same; at high discharge rates, node SOC is at a lower potential than node 50. If, at the end of a high discharge rate the cell reverts to a low discharge, nearly the entire rated capacity can be recovered from the cell. At the high discharge rate, approximately 60% of the cell's rated capacity can be used. Linking SOC with Cell Voltage to Determine Output You can now determine the output by linking the state of charge with the cell voltage to determine an output. The state of charge is 1 Volt for 100%, while the cell voltage table is just the opposite. To make the cell voltage correct, the state-of-charge voltage must be inverted as shown in Figure 2. The schematic in Figure 2 represents the subcircuit model common to all batteries without temperature effect. Figure 2:Functional schematic developed for all of the modeled cell types; only minor changes are required to complete each detailed model type

• Cover