PSpice Application Notes

APPLICATION NOTE 1 As the use of battery-operated electronic devices become more widespread, so does the need for simulation models used to analyze the operating characteristics of batteries. The most common batteries in use today are: non-rechargeable Alkaline cells, rechargeable Nickel-Cadmium (NICD) cells, Nickel-Metal-Hydride (NIMH) cells, and sealed Lead-Acid cells. This article presents PSpice behavioral models for simulating the four battery types mentioned above. Battery Variables All of the battery types modeled here share some common characteristics and deviations from ideal during discharge. Only parameters that vary by 15% or more are considered in these models. Parameters that change less than 15% are not considered because the capacity of cells may vary from +/- 20% to +/- 50% when shelf time, number of recharge cycles, and manufacturing variances are taken into account. A reduced state of charge is specified for the start because the capacity of a completely charged cell decays with time. For Alkaline cells, this decay takes years to affect the usable capacity. For NICD and Lead-Acid batteries, the decay is 10% to 30% per month. Note: The term cell is used to indicate a single energy source. The term battery is used to indicate a power source composed of a single cell or several cells. The usable capacity of a cell varies depending on the discharge rate. At very low discharge rates (<) 100 hours, all batteries are very efficient. At very fast discharge rates (< 10 hours), the batteries are not as efficient and the usable capacity is lost. For pulsed loads with cycle times greater than 10 seconds, a cell gives more total capacity than under a constant load. The rest portion of the pulsed load allows the battery chemistry to recover some of the lost capacity. But, as the pulsed load cycle time becomes less than 1 second, the cell does not have enough time to recover and usable capacity is not increased. In these cases, the RMS value of the pulsed discharge current should be used in the simulation. The affect of cell temperature on cell resistance is not modeled because, for the models, the change in resistance versus temperature falls below the 15% change threshold. The cell temperature changes may be accounted for by adjusting the parameters passed to the various cell subcircuits. Note: Cell temperature affects both the cell resistance and usable capacity. Low cell temperatures reduce the usable capacity; only a slight decrease is noted at high temperatures. Cell resistance is a function of the cell's state of charge. Although there is a negligible effect on Lead-Acid and NICD types, Alkaline cells show a 2:1 to 4:1 increase in cell resistance from full charge to full discharge. Still, cell resistance is fairly flat and constant until 80% discharged, when the resistance increases sharply. The sharp fall in cell voltage during discharge can be looked upon as a large increase in cell resistance. Cell discharge voltage versus temperature is modeled only for NICD subcircuit because the open circuit cell voltage variation with temperature even for a 0°C to 60°C range is much less than the difference in actual cell discharge voltage for the other types. Behavioral Modeling Figure 1 shows the results of discharging seven identically rated NICD cells to see how well their capacity track. These are old cells, , used every week for 1 to 2 years, and exhibit a 2:1 spread in measured capacity. Alkaline and Lead-Acid batteries have similar variations even between new cells. The results indicate that there is little practical value in overly accurate models. Therefore, only those battery characteristics that show a 10 to 15% or greater change during discharge are modeled.

• Cover