APPLICATION NOTE
2
Figure 1: AA cell discharge rate
As shown in Figure 2 and Figure 3, the batteries are modeled using the following functional blocks:
Capacitor representing the A-H capacity of the cell.
Discharge rate normalizer to determine the lost capacity at high discharge rates.
A circuit to discharge the A-H capacity of the cell.
Cell voltage versus state-of-charge lookup table.
Cell resistance.
The thermal effects of the cell under high discharge rates for NICD batteries.
To model a cell, perform the following steps:
Generate a parameterized PSpice lookup table
Model discharge current sense and cell resistance
Modeling the State-of-Charge (SOC)
Link SOC with cell voltage to determine output
Generating a Parameterized PSpice Lookup Table
To model a cell, start by generating a parameterized PSpice lookup table Voltage Controlled Voltage Source (VCVS) using the
following steps:
1. Measure several actual discharge curves on a computerized constant-current load analyzer [1] at a low
rate (20 to 200 hours) to get an actual voltage versus capacity curve.
2. Make a single curve either by averaging several curves or by picking a typical curve from the data.
3. Convert the data into a parameterized PSpice lookup table VCVS. This models the cell's output voltage
versus the state-of-charge at low discharge rates. A simplified VCVS definition is
E_Cell +OUT -OUT TABLE {V(x)} = (0,1.5) (0.5,1.3) (1.0,0.0)