The Discussion of this exercise covers the following points:

Transcription

1 Exercise 1 Battery Fundamentals EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with various types of lead-acid batteries and their features. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Description Battery types Cell versus battery Operation during discharge and charge cycles Open-circuit voltage State-of-charge Voltage regulation and internal resistance Battery capacity Depth of discharge Cycle life DISCUSSION Description A battery is a device that converts chemical energy into electric energy by means of an electrochemical reaction. When a load such as a resistor or a small light bulb is connected to the terminals of a battery, a chemical reaction starts and electricity is produced and consumed by the load. The chemical reaction continues until the chemical energy of the active materials inside the battery is exhausted. When no load is connected to the battery terminals, no chemical reaction occurs and no electrical energy is produced. Battery types There are three major types of batteries: primary (single use), secondary (rechargeable), and reserve (for long storage periods). Primary batteries are discarded when discharged. They are commonly used as a power source for portable electronic and electrical devices, lighting, and a host of other applications. Secondary batteries can be electrically recharged. They can be used as an energy-storage device, which is usually electrically connected to and charged by an energy source that delivers energy to a load on demand, as in automotive applications, emergency systems, and stationary energy storage systems. They can also be used essentially as primary batteries, but can be recharged after use instead of discarded. Lead-acid batteries are secondary batteries. Festo Didactic

2 Exercise 1 Battery Fundamentals Discussion Reserve batteries are designed for long term storage. Their active materials are physically isolated when not in use. In this condition, chemical deterioration or self-discharge is eliminated. When activated, they can deliver high power for relatively short periods, in missiles, torpedoes, and other weapon systems for example. Cell versus battery Schematic symbol of a battery The basic electrochemical unit that produces electric energy is referred to as a cell. A battery consists of a series/parallel arrangement of several cells. For example, the 12 V lead-acid battery used in any internal-combustion engine (ICE) car for starting, lighting, and ignition (SLI battery) consists of six cells connected in series, each cell having a nominal output voltage (voltage under load) of about 2 V. See Figure 2. Figure 2. A 12 V lead-acid battery contains six cells. Operation during discharge and charge cycles Each battery cell consists of two electrodes submerged in an electrolyte. The positive electrode is called the cathode and the negative electrode is called the anode. In lead-acid batteries, the cathode is made of lead dioxide (PbO 2 ), while the anode is made of metallic lead (Pb). The electrolyte is sulfuric acid (H 2 SO 4 ). Figure 3 shows the status of the active materials in a lead-acid battery cell when it is fully charged. Cathode PbO 2 Anode Pb Electrolyte H 2SO 4 - Sulfuric acid 35% - Water 65% Figure 3. Status of the active materials in a fully-charged lead-acid battery cell. 4 Festo Didactic

3 Exercise 1 Battery Fundamentals Discussion Scientists of the 17 th century arbitrarily decided that current flows from the positive terminal to the negative terminal. This so-called conventional current direction is still used today, and is the accepted direction of current flow, but it is worth noting that the actual direction of electron flow is opposite to the conventional current direction. When a load is connected to the battery terminals, a chemical reaction takes place in which the lead (Pb) of the anode and the lead dioxide (PbO 2 ) of the cathode combines with the sulfate (SO 4 ) of the sulfuric acid to produce lead sulfate (PbSO 4 ). During this process, both electrodes are thus gradually converted to lead sulfate. The chemical reaction between the anode and the electrolyte forces electrons out of the electrolyte at the anode. These electrons flow through the load to reach the cathode, where the electrons are accepted. The electric circuit is completed in the electrolyte by the flow of anions (negative ions) and cations (positive ions) to the anode and cathode, respectively. See Figure 4. Conventional current direction Load Flow of electrons PbO 2 decreases PbSO 4 increases Flow of anions Flow of cations Electrolyte - Sulfuric acid proportion decreases - Water proportion increases Pb decreases PbSO 4 increases Figure 4. Status of the active materials in a lead-acid battery cell during the discharge cycle. During the discharge cycle, the acid proportion in the electrolyte decreases, and the positive and negative electrodes become more like one another. Once the electrodes are similar, the voltage between them becomes null and the current stops flowing. At this point, both electrodes are converted into lead sulfate and the sulfuric acid is primarily converted into water as shown in Figure 5. PbO 2 and PbSO 4 Pb and PbSO 4 Electrolyte - Sulfuric acid 15% - Water 85% Figure 5. Status of the active materials in a discharged lead-acid battery cell. During the charge cycle, a dc power source is connected to the battery terminals as shown in Figure 6. Notice that the positive terminal of the dc power source is connected to the positive terminal of the battery to reverse the direction of current flow and the chemical reaction. During this process, the lead sulfate (PbSO 4 ) is dissolved, the electrodes and the electrolyte return to their original charged state. Festo Didactic

4 Exercise 1 Battery Fundamentals Discussion DC power source PbO 2 increases PbSO 4 decreases Pb increases PbSO 4 decreases Electrolyte - Sulfuric acid proportion increases - Water proportion decreases Figure 6. Status of the active materials in a lead-acid battery during the charge cycle. Open-circuit voltage The nominal open-circuit voltage (also known as theoretical voltage) corresponds to the voltage measured at the battery terminals without load when the battery is fully charged (the battery is neither charging nor discharging); it is the maximum voltage a battery can produce. The value of the nominal opencircuit voltage depends on the chemical reaction, which is determined by the type of active material, the composition of the electrolyte, and the temperature. The nominal open-circuit voltage per cell for the most popular secondary batteries is shown in Table 1. From the values in Table 1, we notice that the maximum voltage a 6-cell lead-acid SLI battery can produce is 12.6 V. Table 1. Nominal open-circuit voltage of various cell types. Cell type Nominal open-circuit voltage (V) Lead-acid (Pb) 2.10 Nickel-cadmium (Ni-Cd) 1.24 Nickel-metal hydride (Ni-MH) 1.20 Lithium-ion (Li-ion) State-of-charge When a lead-acid battery is not fully charged, its open-circuit voltage is less than the nominal value. As Figure 7 shows, the value of the open-circuit voltage varies linearly with the state-of-charge (residual capacity) of the battery. It can therefore be used to approximate the state-of-charge of a battery. 6 Festo Didactic

5 Exercise 1 Battery Fundamentals Discussion Open-circuit voltage (V) State-of-charge (%) Figure 7. Typical open-circuit voltage versus state-of-charge of a 12-V lead-acid battery. A more precise method of measuring the state-of-charge of a battery is to measure the specific gravity () of the electrolyte using a hydrometer. The specific gravity refers to the weight of a solution (electrolyte), with water having a reference rating of g/cm 3. Table 2 shows the specific gravity of an SLI leadacid battery electrolyte at different states-of-charge. Table 2. State-of-charge versus specific gravity of the electrolyte (SLI battery). State-of-charge (%) Specific gravity 100 (fully charged) (discharged) Voltage regulation and internal resistance The voltage measured at the terminals of a battery to which a load is connected (working voltage), is always lower than the open-circuit voltage. The difference is due to the internal resistance of the battery. An ideal battery has negligible internal resistance, so it can maintain a constant voltage no matter what the load current value is. See Figure 8. Festo Didactic

6 Exercise 1 Battery Fundamentals Discussion Voltage (V) Ideal battery Actual battery Current (A) Figure 8. Voltage regulation curve. In Figure 9a, the battery is represented by an ideal voltage source (12 V) to which a load is connected. Now suppose a real battery has an internal resistance of 0.2 as shown in Figure 9b. The internal resistance adds to the load resistance. For instance, when a load having a 1 resistance is connected to the battery, the result is a circuit current of 10 A (12 V 1.2 ) and a load voltage of 10 V (10 A 1 ) instead of 12 V. This is illustrated in Figure 8. = 12 A = 10 A = 0.2 = 12 V = 1 = 12 V = 1 = 10 V = 12 V (a) (b) Figure 9. Equivalent circuit of a battery. 8 Festo Didactic

7 Exercise 1 Battery Fundamentals Discussion Notice that the voltage drop caused by the internal resistance increases with the load current, as is shown in Figure 8. Because the internal resistance of leadacid-batteries is rather low, the voltage variation that occurs when the load current varies is also low. For this reason they are said to have a fairly good voltage regulation. Battery capacity The capacity () of a battery is a measure of the amount of energy that can be delivered by a battery when it is fully charged. The capacity of a battery is determined by the type of active material used (which determines the voltage), the amount of active material (the amount of electrolyte), and the surface area of the plates that constitute the electrodes. So the larger the battery dimensions and the heavier the battery weight, the higher the capacity. One ampere-hour (Ah) corresponds to the electric charge transferred by a steady current of one ampere for one hour. Note that the symbol A h is also widely used to represent the ampere-hour. The capacity of a lead-acid battery is usually determined by measuring the average load current during a discharge test that lasts a specific time and brings the battery voltage down to a preset end value at which the battery is considered to be completely discharged. This final voltage is commonly referred to as the cutoff voltage because it is the voltage at which battery discharge must be stopped to avoid damage to the battery (e.g., subsequent plate sulfation when battery is left discharged). The capacity rating is expressed in amperehours (Ah). For instance, a lead-acid battery rated for 200 Ah will deliver 10 A of current for 20 hours under standard temperature conditions or 50 A for 4 hours, and so forth. Figure 10 shows the discharge curves of a 200 Ah lead-acid battery at the above currents. Notice that an ideal battery is assumed here. Terminal voltage (V) 50 A 10 A Minutes Hours Duration of discharge Figure 10. Discharge curves of an ideal 200 Ah lead-acid battery. When the capacity is determined using a test that lasts 20 hours, it is referred to as the capacity. Similarly, when the capacity is determined using a 10 h test, it is referred to as the capacity. For example, if the average current value during a 20 h discharge test is 2 A, then the battery capacity is 40 Ah. The capacity of lead-acid batteries is commonly defined using a 20 h discharge test (capacity ), and sometimes using a 10 h discharge test (capacity ). Festo Didactic

8 Exercise 1 Battery Fundamentals Discussion It is common practice to express the load current as a discharge rate related to the battery capacity (usually the capacity in the case of lead-acid batteries). For instance, if the battery capacity is 20 Ah, a discharge rate of will correspond to a 4.0 A load current. Depth of discharge As its name indicates, the depth of discharge (D.O.D.) is a measure of how deeply a battery is discharged. It corresponds to the amount of energy that has been removed from the battery, and is usually expressed as a percentage of the total capacity of the battery. For example, an 80% depth of discharge means that 80% of the energy has been removed, so the battery now holds only 20% of its full charge. The depth of discharge affects the number of charge-discharge cycles in a battery life. The deeper the batteries are discharged on average, the shorter the battery life. Cycle life The number of charge-discharge cycles a lead-acid battery can perform before its capacity drops to a given percentage (typically 80%) of its initial specified capacity is the cycle life. Many factors affect the cycle life of a battery: ambient operating temperature, type of storage, and discharge rate, but generally speaking, the two most important factors are the depth of discharge and the charging conditions. Figure 11 shows a typical cycle life versus depth of discharge characteristic of a lead-acid battery. This figure clearly shows that the cycle life decreases when the depth of discharge is increased. Percentage of capacity available 100% D.O.D. 50% D.O.D. 30% D.O.D. Number of charge-discharge cycles Figure 11. Cycle life versus depth of discharge characteristic. For cyclic applications where the batteries are regularly discharged and recharged, such as forklifts, golf carts, electric bikes, and electric cars, the batteries are specially designed to withstand frequent deep discharging. On the other hand, SLI batteries designed for starting automotive engines are not designed for deep discharges, and consequently, are quickly ruined by repeated discharges. 10 Festo Didactic

9 Exercise 1 Battery Fundamentals Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Battery state-of-charge (residual capacity) evaluation Battery voltage regulation curve Battery internal resistance evaluation Battery voltage and energy supplied during a discharge at 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 the O (off) position, then connect its 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 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. Battery state-of-charge (residual capacity) evaluation In this part of the exercise, you will evaluate the state-of-charge of each battery in the Lead-Acid Batteries module by measuring the open-circuit voltage. This evaluation is necessary to ensure that the batteries are fully charged at the moment when you begin to perform the next steps. Festo Didactic

10 Exercise 1 Battery Fundamentals Procedure 4. Using a multimeter, measure the open-circuit voltage across each battery of the Lead-Acid Batteries module. Left battery voltage: V Right battery voltage: V 5. Determine the state-of-charge of each battery (expressed in percentage) using the open-circuit voltages measured in the previous step and the stateof-charge versus open-circuit voltage curve shown in Figure Open-circuit voltage (V) State-of-charge (%) Figure 12. State-of-charge versus open-circuit voltage of the batteries in the Lead-Acid Batteries module. State-of-charge of the left battery: % State-of-charge of the right battery: % a The open-circuit voltage can sometimes slightly exceed 13.0 V at 100% stateof-charge. If the open-circuit voltage of either one of the batteries is lower than 12.9 V, ask your instructor for assistance as the corresponding battery is probably not fully-charged. 12 Festo Didactic

11 Exercise 1 Battery Fundamentals Procedure Battery voltage regulation curve In this part of the exercise, you will measure the battery voltage at various load currents to plot the voltage regulation curve of the battery. 6. Record the nominal voltage and the rated capacity of the batteries indicated on the front panel of the Lead-Acid Batteries module. a The capacity indicated on the front panel of the Lead-Acid Batteries module has been rated using a 20-hour test (capacity ). Nominal voltage: Capacity ( ): Ah V 7. Using the battery capacity 20 that you have recorded in the previous step, calculate the load current corresponding to the following discharge rates:,,,,,, and. Record your results in the Load current row of Table 3. Table 3. Battery voltage versus load current. Discharge rate Load current (A) Battery voltage (V) 8. In LVDAC-EMS, open the Four-Quadrant Dynamometer/Power Supply window and make the following settings: Set the Function parameter to Current Source (-).This setting makes the internal power source operate as a negative current source. When the Four-Quadrant Dynamometer/Power Supply operates as a negative current source, the current enters (sinking) through the yellow terminal and exits through the white terminal (neutral terminal N). Set the Current parameter to 0 A. This sets the current of the negative current source to 0 A. 9. Connect the left battery in the Lead-Acid Batteries module to the Four- Quadrant Dynamometer/Power Supply as shown in Figure 13. Festo Didactic

12 Exercise 1 Battery Fundamentals Procedure Four-Quadrant Dynamometer/Power Supply * * 12 V Lead-acid battery N (*) Meter in the Negative Current Source window of LVDAC-EMS Figure 13. Battery connected to the Four-Quadrant Dynamometer/Power Supply operating as a negative current source. 10. For each load current indicated in Table 3 perform the following steps: In the Four-Quadrant Dynamometer/Power Supply window, set the current to the required load current value then start the negative current source. Wait 30 s for the battery voltage to stabilize. Record the battery voltage (indicated on the display of the Four- Quadrant Dynamometer/Power Supply) in the Battery voltage row of Table 3. In the Four-Quadrant Dynamometer/Power Supply window, stop the negative current source. 14 Festo Didactic

13 Exercise 1 Battery Fundamentals Procedure 11. Using the values in Table 3, plot the battery voltage versus load current curve (voltage regulation curve) in Figure Battery voltage (V) Load current (A) Figure 14. Battery voltage versus load current (voltage regulation curve). 12. Does the battery voltage versus load current curve in Figure 14 confirm that the voltage variation that occurs when the load current varies is rather low? Yes No 13. Does this confirm that lead-acid batteries have good voltage regulation? Yes No Battery internal resistance evaluation In this part of the exercise, you will calculate the internal resistance of the battery by using the battery voltage and load current measured during discharge at rates of 20 and. 14. Calculate the internal resistance of the battery using the voltage and load current measured at and indicated in Table 3, and the following suggested equation: (voltage at voltage at ) / (load current at load current at. Battery internal resistance: Festo Didactic

14 Exercise 1 Battery Fundamentals Procedure 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 battery during a discharge cycle at. 15. In the Four-Quadrant Dynamometer/Power Supply window, make the following settings: Set the Function parameter to Battery Discharger (Constant-Current Timed Discharge with Voltage Cutoff). When tthis function is selected, the Four-Quadrant Dynamometer/Power Supply operates as a negative current source whose operation is controlled by parameters associated with battery discharge: discharge current, discharge duration, and cutoff voltage. The positive terminal of the battery to be discharged connects to the yellow terminal and the negative terminal of the battery connects to the white terminal (neutral terminal N). Set the Discharge Current parameter to 1.15 A ( 20 ). This sets the discharge current of the Battery Discharger to 1.15 A. Set the Discharge Duration parameter to 90 min. This sets the discharge duration to 90 min. a Set the Cutoff Voltage parameter to 9.8 V. This sets the cutoff voltage to 9.8 V. The setting of the cutoff voltage corresponds to the nominal cutoff voltage of the batteries in the Lead-Acid Batteries module for a discharge rate at (battery cutoff voltage is explained in more detail in the next exercise). 16. Replace the battery connected to the Four-Quadrant Dynamometer/Power Supply with the right battery (fully charged) of the Lead-Acid Batteries module. 17. In LVDAC-EMS, open the Data Table window. In the Timer Settings window of the Options menu, set the timer to make 180 records with an interval of 30 seconds between each record. This setting corresponds to a 90-minute period of observation. In the Record Settings window of the Options menu, select Voltage, Energy, Current, and Time Data as parameters to record. 18. 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. 16 Festo Didactic

15 Exercise 1 Battery Fundamentals Procedure 19. Export the recorded data to a spreadsheet application, and plot the battery voltage versus time curve at a discharge rate of. Note that the recorded data will be used in the next exercise. 20. From the battery voltage versus time curve, determine the duration of the discharge cycle at a rate of. Duration of a discharge cycle at a rate of : minutes 21. What caused the discharge cycle to end? 22. Describe how the voltage varies during the discharge cycle. 23. Describe how the battery voltage varies during the first fifteen minutes after the end of the discharge cycle. 24. Record the open-circuit voltage at the end of the period of observation. Open-circuit voltage at the end of the period of observation: V 25. Determine the state-of-charge of the battery (expressed in percentage) corresponding to the open-circuit voltage at the end of the period of observation. State-of-charge of the battery at the end of the period of observation: % 26. Does this correspond to a fully-discharged battery? Yes No 27. Close LVDAC-EMS, then turn off all equipment. Remove all leads and cables. Festo Didactic

16 Exercise 1 Battery Fundamentals Conclusion CONCLUSION In this exercise, you were introduced to lead-acid batteries. You saw that there are three major types of batteries: primary, secondary, and reserve. You learned that batteries consist of a series/parallel arrangement of several cells. You learned that electricity is produced by a chemical reaction between a positive electrode (cathode) and a negative electrode (anode) submerged in an electrolyte which is sulfuric acid. You saw that the battery open-circuit voltage can be used to approximate the state-of-charge of a lead-acid battery. You learned that lead-acid batteries have a good voltage regulation because they have a rather low internal resistance. You also learned that the battery capacity is a measure of the amount of energy that can be delivered by a battery when it is fully charged. REVIEW QUESTIONS 1. What happens when a load is connected to a lead-acid battery? Briefly explain why. 2. Give a brief description of the three major types of battery. 3. What is the effect of reducing the depth of discharge on the cycle life of a lead-acid battery? 4. Explain why the voltage measured at the terminals of a battery to which a load is connected is always lower than the open-circuit voltage. 5. Explain how the capacity of a lead-acid battery is usually determined. 18 Festo Didactic