Exercise 2. Discharge Characteristics EXERCISE OBJECTIVE DISCUSSION OUTLINE DISCUSSION. Cutoff voltage versus discharge rate

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Esther Eaton
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1 Exercise 2 Discharge Characteristics EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the discharge characteristics of lead-acid batteries. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Cutoff voltage versus discharge rate Effect of the discharge rate on the available capacity and discharge time Effect of the temperature on the available capacity Energy released during the discharge cycle Specific energy and energy density Sulfation DISCUSSION Cutoff voltage versus discharge rate As learned in the previous exercise, a battery can be damaged when discharged to a voltage below the cutoff voltage. As Figure 15 shows, the recommended cutoff voltage for cells in a lead-acid battery depends on the discharge rate. It is higher at low discharge rates because active materials work more effectively at low current. Voltage (V) Cutoff voltage 4 Minutes Hours Discharge Time Figure 15. Cutoff voltage versus discharge rate. Festo Didactic

2 Exercise 2 Discharge Characteristics Discussion Effect of the discharge rate on the available capacity and discharge time The curves shown in Figure 15 also show that the discharge time decreases more rapidly than the discharge rate is increased. To facilitate the visualization of this characteristic from the curves of Figure 15, the corresponding values have been reported in Table 4. Table 4. Discharge time and battery capacity versus discharge rate. Discharge rate Discharge time (h) Battery capacity (% of 20) The discharge time obtained when the battery is discharging at a rate is 20 hours. If the discharge rate was increased to, the expected discharge time would be 1 hour (20 times shorter than at the rate) if the relation was linear (if the battery capacity remained the same no matter what the discharge rate is). However, the actual discharge time at a discharge rate of is only 0.7 hour. The variations in discharge time are a consequence of the reduction in battery capacity observed when the discharge rate is increased. As Figure 16 shows, the available capacity of a lead-acid battery decreases rapidly as the discharge rate is increased. The available capacity is approximately 100% of the rated capacity when the discharge rate is (20 h test rated battery capacity) but it is only 60% when the discharge rate is as an example Percentage of available capacity (%) Trendline Discharge rate ( ) Figure 16. Percentage of available capacity versus discharge rate. 20 Festo Didactic

3 Exercise 2 Discharge Characteristics Discussion This characteristic means that the discharge time of a battery in great demand will decrease more rapidly than a battery powering a load at a low discharge rate. Effect of the temperature on the available capacity Figure 17 shows the effect of the temperature on the available capacity of a leadacid battery for various discharge rates. As shown in the figure, the battery can operate over a very wide temperature range. However, the available capacity decreases gradually as the temperature decreases from room temperature (about 20ûC). Percentage of available capacity (%) Temperature ( C) Figure 17. Effect of the temperature on the delivered capacity of a lead-acid battery. Energy released during the discharge cycle A watt-hour (Wh) is a unit of energy, equal to the work done by one watt acting for one hour. The amount of energy that a battery releases during a complete discharge cycle can be determined using the characteristic discharge curves. This is done by determining the average battery voltage during a complete discharge, multiply this voltage by the discharge current to find the average power, and multiply the average power by the discharge time. The released energy is expressed in watthours Wh. Example The energy released during a complete discharge cycle at by the 200 Ah lead-acid battery whose discharging characteristics are shown in Figure 10 is determined as follows: Average battery voltage First, the average battery voltage is determined by adding the voltage at the beginning of the discharge cycle and the voltage at the end of the discharge cycle, then by dividing the result by two: (12.75 V V) 2 = V Average power Then the average power is determined by multiplying the average battery voltage by the discharge current: V 10.0 A = W Festo Didactic

4 Exercise 2 Discharge Characteristics Discussion Energy released The energy released is determined by multiplying the average power by the discharge time: W 20 h = 2336 Wh Specific energy and energy density The energy contained in a battery is often expressed as a ratio related to battery weight and volume (size). The ratio of battery energy to battery weight, specific energy, is calculated by dividing the energy released by the battery during a complete discharge cycle by the battery weight. The specific energy of lead-acid batteries currently available does not exceed 40 Wh/kg, which is one of the lowest values among the various types of secondary batteries available. The ratio of battery energy to battery volume, energy density, is calculated by dividing the energy released by the battery during a complete discharge cycle by the battery volume. The energy density of lead-acid batteries currently available is about 70 Wh/L, which is also one of the lowest values among the various types of secondary batteries available. The specific energy and energy density depend on the discharge rate. Both decrease as the discharge rate increases because the capacity of lead-acid batteries decreases when the discharge rate is increased. Sulfation Sulfation refers to the process whereby a lead-acid battery loses its ability to hold a charge after it is kept in a discharged state too long due to the crystallization of lead sulfate. As learned in the previous exercise, lead-acid batteries generate electricity through a double sulfate chemical reaction. Lead and lead dioxide, which are the active materials on the battery's plates, react with sulfuric acid in the electrolyte to form lead sulfate. When formed, the lead sulfate is in a finely divided, amorphous form, which is easily converted back to lead, lead oxide, and sulfuric acid when the battery is recharged. Over time, lead sulfate converts to a more stable crystalline form, coating the battery's electrodes. Crystalline lead sulfate does not conduct electricity and cannot be converted back into lead and lead oxide under normal charging conditions. As batteries are "cycled" through numerous discharge and charge sequences, the lead sulfate that forms during normal discharge is slowly converted to a very stable crystalline form. This process is known as sulfation. Sulfation is a natural, normal process that occurs in all lead-acid batteries during normal operation. Sulfation clogs grids, impedes recharging, and ultimately can expand and crack the plates as it accumulates, destroying the battery. Crystalline lead sulfate is resistant to normal charging current, and does not re-dissolve completely. Thus, the amount of usable active material necessary for electrical generation declines over time. The process can often be at least partially prevented and/or reversed by devices known as desulfators, which repeatedly send short but powerful current surges through the damaged battery. Over time, this procedure tends to break down and dissolve the sulfate crystals, restoring some of the battery's capacity. 22 Festo Didactic

5 Exercise 2 Discharge Characteristics Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Battery voltage and energy supplied during a discharge at Battery voltage and energy supplied during a discharge at Battery capacity versus discharge rate Specific energy and energy density PROCEDURE Setup and connections In this part of this exercise, you will set up and connect the equipment. 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise. Install the equipment required in the Workstation. 2. Set the main power switch of the Four-Quadrant Dynamometer/Power Supply to O (off), then connect the Power Input to an ac power outlet. Set the Operating Mode switch of the Four-Quadrant Dynamometer/Power Supply to Power Supply. Connect the Four-Quadrant Dynamometer/Power Supply to a USB port of the host computer. Turn the Four-Quadrant Dynamometer/Power Supply on by setting the main power switch to I (on). 3. Turn the host computer on, then start the LVDAC-EMS software. In the LVDAC-EMS Start-Up window, make sure the Four-Quadrant Dynamometer/Power Supply is detected. Select the network voltage and frequency that correspond to the voltage and frequency of the local ac power network, then click the OK button to close the LVDAC-EMS Start-Up window. 4. Connect the left battery in the Lead-Acid Batteries module to the Four- Quadrant Dynamometer/Power Supply as shown in Figure 18. Festo Didactic

6 Exercise 2 Discharge Characteristics Procedure Four-Quadrant Dynamometer/Power Supply * * 12 V Lead-acid battery N (*) Meter in the Battery Discharger window of LVDAC-EMS Figure 18.Battery connected to the Four-Quadrant Dynamometer/Power Supply operating as a battery discharger. Battery voltage and energy supplied during a discharge at In this part of the exercise, you will measure the voltage and energy supplied by a lead-acid battery during a discharge at. 5. Before performing this part of the exercise, make sure that both batteries in the Lead-Acid Batteries module are fully-charged by performing the Battery state-of-charge (residual capacity) evaluation described in the Procedure of Exercise In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window and make the following settings: Set the Function parameter to Battery Discharger (Constant-Current Timed Discharge with Voltage Cutoff). Set the Discharge Current to 2.3 A ( ). Set the Discharge Duration to 45 min. a Set the Cutoff Voltage to 9.5 V. Reset the meter Energy. The setting of the cutoff voltage corresponds to the nominal cutoff voltage of the batteries in the Lead-Acid Batteries module for a discharge at. 24 Festo Didactic

7 Exercise 2 Discharge Characteristics Procedure 7. In LVDAC-EMS, open the Data Table window. In the Timer Settings window of the Options menu, set the timer to make 90 records with an interval of 30 seconds between each record. This setting corresponds to a 45-minute period of observation. In the Record Settings window of the Options menu, select Voltage, Energy, Current, and Time Data as parameters to record. 8. In the Four-Quadrant Dynamometer/Power Supply window, start the Battery Discharger then immediately start the timer in the Data Table window. Once the period of observation is completed, save your data. Note that the recorded data will be used later in this exercise. Battery voltage and energy supplied during a discharge at In this part of the exercise, you will measure the voltage and energy supplied by a lead-acid battery during a discharge at. 9. Replace the battery connected to the Four-Quadrant Dynamometer/Power Supply by the right battery (fully charged) of the Lead-Acid Batteries module. 10. Modify the parameters of the Battery Discharger as follows: Set the Discharge Current to 4.6 A ( ). Set the Discharge Duration to 20 min. a Set the Cutoff Voltage to 8.7 V. Reset the meter Energy. The setting of the cutoff voltage corresponds to the nominal cutoff voltage of the batteries in the Lead-Acid Batteries module for a discharge at. 11. Open a new data table in the Data Table window and set the timer to make 40 records with an interval of 30 seconds between each record. This setting corresponds to a 20-minute period of observation. In the Record Settings window of the Options menu, select Voltage, Energy, Current, and Time Data as parameters to record. In the Four-Quadrant Dynamometer/Power Supply window, start the Battery Discharger then immediately start the timer in the Data Table window. Once the period of observation is completed, save your data. Festo Didactic

8 Exercise 2 Discharge Characteristics Procedure Battery capacity versus discharge rate In this part of the exercise, you will plot the discharge curves using the data recorded during the three discharge tests made so far (,, and ). You will also determine the battery capacity at various discharge rates and plot the graph of the battery capacity versus discharge rate. 12. Transfer your data recorded at,, and into a spreadsheet application. Using the data measured before the voltage cutoff is attained during the discharges, plot the battery discharge curves at,, and (Battery voltage versus time). 13. From the curves you plotted in the previous step, determine the discharge time to specified cutoff voltage expressed in hours for each discharge rate (,, and ). Record your results in the appropriate cells of Table Calculate the capacity of the battery at,, and from the curves you plotted in the previous step and using the following suggested equation: capacity = discharge current discharge time to specified cutoff voltage. Record your results expressed in Ah in the appropriate cells of Table Express your calculated capacities as a percentage of the nominal capacity of the battery. Record your results in the appropriate cells of Table 5. Table 5. Capacity versus discharge current. Discharge rate Discharge current (A) Discharge time (h) Capacity (Ah) Capacity (%) * 2.3* * 2.12* * 1.98* * Data supplied by the battery s manufacturer. 26 Festo Didactic

9 Exercise 2 Discharge Characteristics Procedure 16. Using the data in Table 5, plot a graph of the battery capacity expressed as a percentage of the nominal capacity versus discharge rate. Battery capacity (%) Discharge rate () Figure 19. Battery capacity versus discharge current. 17. How does the battery capacity vary when the discharge rate increases? Specific energy and energy density In this part of the exercise, you will determine the specific energy and energy density of the batteries in the Lead-Acid Batteries module. Then you will observe how the specific energy and energy density vary with the discharge rate. 18. Using the energy values measured at each discharge rate and assuming that the weight of each battery in the Lead-Acid Batteries module is 0.91 kg (2.0 lb), calculate the specific energy at,, and. Record your results in the Specific energy column of Table Using the energy values measured at each discharge rate and assuming that the volume of each battery in the Lead-Acid Batteries module is L (22.15 in 3 ), calculate the energy density at,, and. Record your results in the Energy density column of Table 6. Festo Didactic

10 Exercise 2 Discharge Characteristics Conclusion Table 6. Specific energy and energy density of the batteries at 20, 20, and 20. Discharge rate Specific energy [Wh/kg (Wh/lb)] Energy density [Wh/L (Wh/in 3 )] 20. How do the specific energy and energy density vary when the discharge rate increases? 21. Close LVDAC-EMS, then turn off all equipment. Remove all leads and cables. CONCLUSION In this exercise, you were introduced to the discharge characteristics of lead-acid batteries. You learned that batteries can be damaged when discharged to a voltage below the cutoff voltage. You saw that high discharge rates reduce the available capacity. You learned that lead-acid batteries can operate over a very wide temperature range, but that the available capacity decreases gradually as the temperature decreases from room temperature. You were also introduced to specific energy, which relates the available energy to the weight of the battery, and to energy density, which relates the available energy to the volume of the battery. Both the specific energy and energy density depend on the discharge rate. You saw that if a battery is stored for a long period when discharged, the lead sulfate on the electrodes will crystallize and the battery will lose its ability to hold a charge. REVIEW QUESTIONS 1. Why is it important not to exceed the suggested cutoff voltage when discharging a lead-acid battery? 2. Briefly explain why it could be harmful to store a discharged battery for a long period. 28 Festo Didactic

11 Exercise 2 Discharge Characteristics Review Questions 3. Suppose that a 12 V lead-acid battery releases during a full-discharge cycle a total of 12 Wh. Considering that it has a weight of 2.2 kg (4.85 lb) and a volume of 0.8 L (48.8 in 3 ), what are its specific energy and its energy density? 4. How does the discharge rate affect the available capacity of lead-acid batteries? 5. How does the temperature affect the available capacity of lead-acid batteries? Festo Didactic

Battery Capacity Versus Discharge Rate

Exercise 2 Battery Capacity Versus Discharge Rate EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the effects of the discharge rate and battery temperature on the capacity