When you have completed this exercise, you will be familiar with the charging characteristics of lead-acid batteries.
The Discussion of this exercise covers the following points:
Charging fundamentals
Valve regulated lead-acid battery (VRLA)
Gassing voltage
Rules for proper charging
Charge efficiency
Charging fundamentals
A secondary battery can be recharged by connecting a source of dc electric power to the battery. The battery charging process uses the dc electric power supplied by the source to convert the active chemicals in the battery to their original state – the high-energy state the chemicals have when the battery is fully charged.
The number of charge-discharge cycles that can be performed during the life of a battery (cycle life) depends on the discharge conditions, notably the depth of discharge, as well as the charge conditions. As will be seen later, a proper charging control method is important to prevent harmful effects to the battery that will ultimately shorten its life. Lead-acid batteries can provide up to more than 1000 charge-discharge cycles under optimal charge-discharge conditions.
For the conversion of the active chemicals to their original high-energy state to be safe and cause no harm to the battery, the voltage and current of the dc power source must be carefully controlled during battery charge. In particular, the lead-acid battery should not be allowed to overheat or to produce too much oxygen and hydrogen gases (this is commonly referred to as gassing) due to electrolysis of the water contained in the electrolyte. When too much gassing occurs, water is lost from the battery electrolyte, thereby reducing the amount of electrolyte and modifying its chemical composition – two harmful consequences leading to a deterioration in performance of any lead-acid battery and a shortened life.
Valve regulated lead-acid battery (VRLA)
Lead-acid batteries designed for many small portable and some larger fixed applications have often been referred to as sealed and/or maintenance free batteries. The newer designation for these designs is Valve Regulated Lead-Acid Battery or VRLA. These batteries include a pressure release valve.
When gas is generated in a battery under overcharge conditions due to erroneous charging, charger malfunctions, or other abnormalities, the valve opens to release the excessive battery pressure and maintain the gas pressure within a specific range. During ordinary use of the battery, the valve is closed to shut out outside air and prevent oxygen in the air from reacting with the active materials.
Gassing voltage
Gassing occurs in a lead-acid battery when the charging voltage measured across the battery exceeds a certain value, referred to as the gassing voltage. The value of the gassing voltage is about 2.4 V per cell (about 14.4 V for a 12 V battery). Gassing during battery charging can thus be detected by monitoring the voltage across the battery.
Figure 20 shows the relationship between the charging voltage measured across a battery cell versus the charge current rate for various battery states-of-charge. It should be noted that the higher the battery state-of-charge, the lower the maximum charging current that can be used without producing gassing (i.e., without the charging voltage exceeding the gassing voltage).
Figure 20. Charging voltage of lead-acid battery at various state-of-charge.
Proper charging is important in order to obtain optimum cycle life from any lead-acid battery under any conditions of use. The followings rules must be observed for proper charging:
The charge current at the beginning of the charge cycle can be any value
that does not produce an average cell voltage in the battery greater than
the gassing voltage.
During the charge and until 100% of the previous discharge capacity has
been returned, the current should be controlled to maintain a voltage
lower than the gassing voltage. To minimize the charge time, this voltage
can be just below the gassing voltage.
When 100% of the discharged capacity has been returned under this
voltage control, the charge rate will have normally decayed to the charge
finishing rate. The charge should be finished at a constant current no
higher than this rate, normally about 20.
The first two rules ensure that no excessive gassing occurs during battery charging to preserve the battery’s chemical properties, and thus, extend the battery life. To minimize the charge time, the initial current can be set to the maximum charging current recommended by the battery manufacturer as long as the charging voltage across the battery does not exceed the gassing voltage. As the battery charges, however, the charging current must be decreased gradually to avoid the charging voltage developed across the battery from exceeding the gassing voltage.
Charge efficiency
Charge efficiency is the ratio of the current that is actually used for the electrochemical conversion of the active materials (from lead sulfate to lead and lead dioxide) to the total current supplied to the cell being recharged. The current which is not used for charging is consumed in destructive reactions within the cell such as corrosion and gas production.
Charging lead-acid batteries is a process that is highly efficient; it can even be close to 100% under optimal charge conditions. As Figure 21 shows, the charge-current efficiency is a direct function of the state-of-charge. It is high until it approaches full charge, at which time the overcharge reactions begin and the charge efficiency rapidly decreases.
Figure 21. Charge efficiency versus state-of-charge.
Figure 22 is a curve of the charge efficiency versus charge voltage. As shown in this figure, increasing the charge voltage above the gassing voltage value (2.4 V) decreases the efficiency significantly because of increased destructive currents. The efficiency also shows a marked decrease below the open-circuit voltage, typically 2.15 V to 2.18 V for a fully-charged lead-acid battery, because the charge voltage is not high enough to support the charging reaction.
Figure 22. Charge voltage versus charge efficiency.
Figure 23 is a curve of the charge efficiency versus the charge rate. At charge
rates up to , the charge efficiency approaches 100%. At higher rates, there is a decrease in efficiency because, as the cell approaches the fully-charged state, the surfaces of the plates become fully charged. This increases the charging reaction rates and results in increased voltages and gassing. At very low charge rates, the efficiency drops because the charge current is equivalent to the destructive currents and the battery voltage approaches the open-circuit value.
The Procedure is divided into the following sections:
Setup and connections
Battery discharge to 20% of residual capacity
Battery charge at (0.92 A)
Battery charge at (0.69 A)
Battery charge at (0.46 A)
Battery charge at (0.23 A)
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 battery to the Four-Quadrant Dynamometer/Power Supply as shown in Figure 24.
Figure 24. Battery connected to the Four-Quadrant Dynamometer/Power Supply operating as a battery discharger.
Battery discharge to 20% of residual capacity
In this part of the exercise, you will discharge one battery of the Lead-Acid Batteries module to approximately 20% of residual capacity. During the discharge cycle, you will observe the battery voltage and current as well as the energy released by the battery. This discharge cycle is necessary to observe the behavior of the battery during the charge cycles in the next parts of the exercise.
5. Before performing this part of the exercise, make sure that the battery that you will use is fully charged by performing the “Battery state-of-charge (residual capacity) evaluation” described in the Procedure of Exercise 1.
6. 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 parameter to 30 min.
Set the Cutoff Voltage to 9.45 V.
Reset the meter Energy.
a The setting of the discharge duration corresponds to the time required to remove approximately 80% of the energy contained in a fully-charged battery when discharging at a rate of .
Four-Quadrant Dynamometer/Power Supply
*
* 12 V Lead-acid battery
(*) Meter in the Battery Discharger window of LVDAC-EMS
7. In LVDAC-EMS, open the Data Table window. In the Timer Settings window of the Options menu, set the timer to make 300 records with an interval of 30 seconds between each record. This setting corresponds to a 150 minute period of observation, which includes the time required to recharge the battery. The actual period of observation should be shorter.
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.
Depending on the state-of-charge of the battery at the beginning of the discharge, the discharge cycle may end before the discharge duration (30 min) has elapsed if the cutoff voltage is attained. Stop the timer as soon as the discharge duration has elapsed or the cutoff voltage is attained.
9. Record the energy released by the battery (indicated by meter Energy in the Four-Quadrant Dynamometer/Power Supply window) during the discharge cycle.
Energy released during discharge: Wh
Battery charge at (0.92 A)
In this part of the exercise, you will charge the battery whose residual capacity is now approximately 20%, at a rate of (0.92 A) until its voltage attains the gassing voltage (14.4 V). During the charge, you will observe the battery voltage and current as well as the energy returned to the battery.
10. Wait at least 30 minutes for the battery’s chemical reaction to stabilize before proceeding with the next step.
11. In the Four-Quadrant Dynamometer/Power Supply window, modify the settings as follows:
Set the Function parameter to Lead-Acid Battery Charger (Fast). When the lead-acid battery charger (fast) function is selected, the Four-Quadrant Dynamometer/Power Supply operates as a lead-acid battery charger whose operating parameters are controlled by the battery characteristics: maximum charge current, gassing voltage,
current, and float voltage.
Set the Maximum Charge Current parameter to 0.92 A. This sets the maximum charge current of the Lead-Acid Battery Charger (Fast) to 0.92 A.
12. In the Four-Quadrant Dynamometer/Power Supply window, start the Lead-Acid Battery Charger (Fast) then immediately start the timer in the Data Table window.
Let the battery charge until the voltage at its terminals attains the gassing voltage 14.4 V. At this moment, stop the Lead-Acid Battery Charger (Fast), then immediately stop the timer in the Data Table window.
13. Does the energy value displayed by the meter Energy in the Four-Quadrant Dynamometer/Power Supply window show that the energy returned to the
battery during the charge at equals or exceeds the energy released by the battery during the discharge?
Yes No
14. Determine which proportion (expressed in percentage) of the energy released by the battery during the discharge has been returned to the battery during the charge at before the battery voltage attains the gassing voltage.
Proportion of the energy released by the battery during the discharge that
has been returned to the battery during the charge at : %
15. What can be done to return more energy to the battery without exceeding the gassing voltage?
Battery charge at (0.69 A)
In this part of the exercise, you will reduce the charge current to continue the battery charge until the gassing voltage is attained. During this charge you will observe the battery voltage and current as well as the energy returned to the battery. You will observe that it is possible to continue to return energy to the battery without exceeding the gassing voltage by reducing the charge current.
16. In the Four-Quadrant Dynamometer/Power Supply window, set the maximum charge current of the Lead-Acid Battery Charger (Fast) to 0.69 A ( ). Do not change the setting of the other parameters.
17. In the Four-Quadrant Dynamometer/Power Supply window, start the Lead-Acid Battery Charger (Fast) then immediately start the timer in the Data Table window.
Let the battery charge until the voltage at its terminals attains the gassing voltage 14.4 V. At this moment, stop the Lead-Acid Battery Charger (Fast), then immediately stop the timer in the Data Table window.
18. Does the energy value displayed by the meter Energy in the Four-Quadrant Dynamometer/Power Supply window shows that the energy returned to the battery during the charges at and equals or exceeds the energy released by the battery during the discharge?
Yes No
19. Determine which proportion (expressed in percentage) of the energy released by the battery during the discharge has been returned to the battery
during the charges at and .
Proportion of the energy released by the battery during the discharge that
has been returned to the battery during the charges at and : %
Battery charge at (0.46 A)
In this part of the exercise, once again you will reduce the charge current to continue the battery charge until the gassing voltage is attained. During this charge cycle you will observe the battery voltage and current as well as the energy returned to the battery.
20. In the Four-Quadrant Dynamometer/Power Supply window, set the Maximum Charge Current parameter of the Lead-Acid Battery Charger (Fast) to 0.46 A
(at ). Do not change the setting of the other parameters.
a Do not reset the meter Energy.
21. In the Four-Quadrant Dynamometer/Power Supply window, start the Lead-Acid Battery Charger (Fast) then immediately start the timer in the Data Table window.
Let the battery charge until the voltage at its terminals attains the gassing voltage 14.4 V. At this moment, stop the Lead-Acid Battery Charger (Fast), then immediately stop the timer in the Data Table window.
22. Does the energy returned to the battery during the charge at , ,
and equal or exceed the energy released by the battery during the discharge?
23. Determine which proportion (expressed in percentage) of the energy released by the battery during the discharge has been returned to the battery during the charges at , , and .
Proportion of the energy released by the battery during the discharge that has been returned to the battery during the charges at and
: %
Battery charge at (0.23 A)
In this part of the exercise, once again you will reduce the charge current to continue the battery charge until the gassing voltage is attained. During this charge cycle you will observe the battery voltage and current as well as the energy returned to the battery.
24. In the Four-Quadrant Dynamometer/Power Supply window, set the Maximum Charge Current parameter of the Lead-Acid Battery Charger (Fast) to 0.23 A ( ). Do not change the setting of the other parameters.
a Do not reset the meter Energy.
25. In the Four-Quadrant Dynamometer/Power Supply window, start the Lead-Acid Battery Charger (Fast) then immediately start the timer in the Data Table window.
Let the battery charge until the voltage at its terminals attains the gassing voltage 14.4 V. At this moment, stop the Lead-Acid Battery Charger (Fast), then immediately stop the timer of the Data Table window.
26. Does the energy returned to the battery during the charge at , , , and equal or exceed the energy released by the battery during
the discharge?
Yes No
27. Save your data, then export it to a spreadsheet application. Plot the graph of the battery voltage, current, and energy versus time during the discharge and charge of the battery.
a It is suggested that you include the data table and graph plotted in this exercise in your lab report.
28. Do your observations confirm that to charge a battery rapidly, the current is set to a high level at the beginning of the charge cycle and is reduced in order not to exceed the gassing voltage as the charging cycle progresses?
29. If time allows, wait 30 min after the end of the charge cycle, then determine the current state-of-charge of the battery (expressed in percentage) by measuring the open-circuit voltage.
State-of-charge of the battery: %
30. Close LVDAC-EMS, then turn off all equipment. Remove all leads and cables.
In this exercise, you learned that lead-acid batteries can be recharged by connecting a source of dc electric power to the battery, and that the charging process is highly efficient. You saw that the number of charge-discharge cycles that can be performed during the life of a battery depends on the discharge conditions as well as the charge conditions.
You also learned that the charging voltage measured across the battery must never exceed the gassing voltage. You observed that to charge a battery rapidly, the current is set to a high level at the beginning of the charge cycle but it must be reduced in order not to exceed the gassing voltage as the charging cycle progresses.
1. Explain why the life of a lead-acid battery can be shortened if it overheats during charging.
2. What is the use of the pressure release valve in a valve regulated lead-acid battery (VRLA)?
3. At which charge rates is the charge efficiency near 100%?
