Feature Article
Lead-Acid Battery Model
Powertechnic has developed a secondary battery model that is accurate enough to replace battery manufacturer's data sheets. The model is capable of predicting temperature-compensated discharge time information.
The model estimates voltage, current and time during discharge or re-charge. Table 1 shows a simple power system consisting of a 60 amp rectifier, 100 amp-hour battery and a constant power load having a current drain of 50 amp at 54 volts. The three states of a battery are illustrated, namely, float charge, discharge and re-charge. The power schematics were taken directly from Power Designer.
| Battery on Float Charge mains1 on |
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| Battery Discharging mains1 off |
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| Battery Charging mains1 on |
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Table 1: Battery model (bat1) undergoing float, discharge & re-charge
Figures 1 and 2 show a comparison between a UXL 550-2 and the software model for both discharge and re-charge times.
Figure 1 - Model/data sheet comparison, battery discharge*
Figure 2 - model/data sheet comparison, battery re-charge*
*data sheets courtesy of Century Yuasa
In both graphs the battery model produces close correlation to the manufacturer supplied data. The model is readily constructed from manufacturers data sheets. The battery model can be used to estimate discharge behaviour into complex non-linear and time varying loads.
To define a battery model, users have a choice of the short-form or long-form method. In the short-form, users describe the battery by entering its amp-hour capacity only. To more accurately define a battery the long-form method is used by providing the following information:
6 discharge points - constant current, time and end cell voltage
battery temperature coefficient
dc resistance
no-load voltage at full charge and end of discharge
three charge parameters
This information is obtainable from battery manufacturers data sheets.
Once the information is entered, the battery model will give reliable discharge information from 5 minutes to more than 24 hours.