Lead Acid Batteries

1. Lead Acid Batteries “The motor that could, the battery that couldn’t”

2. Review of Previous Concepts • A battery is a box that segregates charges • Electrons are stripped from the anode • This leaves holes in the valence orbits of the atoms of the anode • Electrons are forced on to the cathode • This places extra electrons in the valence orbits of the atoms of the cathode

3. Review Continued • When an anode and cathode are coupled, you have a capacitor • Constructed as anode-insulator-cathode • If current is allowed to flow from the cathode to the anode, the capacitor will discharge • This tends to happen very quickly, so capacitors are not practical for the storage of large amounts of energy

4. Review Continued • For a battery to be functional, we must have a way to keep stripping electrons from the anode and adding them to the cathode • This is possible through a chemical reaction • A battery is nothing but a mechanical assembly designed to extract energy from a chemical reaction • What is this similar to? • What can we say about the efficiency of lead acid batteries?

5. Potential Difference Video • Watch the video on potential difference • What is potential difference?

6. Mechanism of Chemical Reaction • Anatomy • Anode • Made of lead dioxide PbO2 • Cathode • Made of pure lead Pb • Electrolyte • Water H2O • Sulfuric Acid H2SO4 • “Poor, poor Willy, he is no more. What he thought was H2O was really H2SO4”

7. Mechanism of Chemical Reaction Continued • Physiology • H2SO4 breaks apart into two ions when dissolved in H2O • H2 forms a positive ion • Shortage of electrons in orbits • SO4 forms a negative ion • Excess of electrons in orbits • Ions are not happy • Their valence shells aren’t full, or are over full, so they’re unstable • Ions will seek out atoms to bond with to fill their shells

8. Mechanism of Chemical Reaction Continued • Physiology of Cathode • SO4 negative ions bond with the lead of the cathode • This bond forms lead sulfate PbSO4 • This lead sulfate molecule has an extra electron • Because of the extra electrons, the cathode is negative

9. Mechanism of Chemical Reaction Continued • Physiology of Anode • SO4 negative ions wants to bond with PbO2 • The O2 must be dumped for the Pb and the SO4 to bond • O2 dissolves in the electrolyte • When the O2 leaves, it takes several electrons with it • SO4 bonds with Pb creating PbSO4 • Because the O2 took several electrons, the net result is a shortage of electrons in the anode • A shortage of electrons makes the anode positive

10. Mechanism of Chemical Reaction Continued • Physiology of Electrolyte • The electrolyte started as H2SO4 • It gave up the SO4 molecules to the anode and cathode • This leaves H2 atoms in the electrolyte • The anode started out a PbO2 • It gave up O2 to the electrolyte • The electrolyte now contains three things: • H2O • H2 • O2

11. Further Reactions in the Electrolyte • It is our hope that the dissolved H2 and O2 combine into H2O • As the battery discharges, the electrolyte goes from mostly H2SO4 to H2O

12. Discharged Battery • Discharged batteries freeze more easily • The density of the electrolyte changes • We can use a hydrometer to measure the amount of H2SO4 in the electrolyte • This is a predictor of state of charge

13. Problems in the Electrolyte • Sometimes, not all of the H2 and O2 combine into water • This results in H2 off gassing • H2 is a highly flammable gas, that will ignite with a small spark • For this reasons, sparks should be avoided around batteries • Question: How do we avoid sparks when jump starting vehicles?

14. Chemical Reactions • We know that batteries work by chemical reactions • Electrons are stripped from the anode • Leaves holes in valence shells • This lack of electrons makes the anode positive • Electrons are placed on the cathode • Places extra electrons in the valence orbits of the cathode • These extra electrons make the cathode negative • How can we predict if two chemicals will react? • If we know the pattern, we can figure out other systems

15. Driving Force Video • Watch the video on the driving force behind reactions. • When will two chemicals react? • What are some examples? • When will they not?

16. Mechanism of Chemical Reaction-Recharging • Charging a battery simply reverses the discharge process • This is possible because energy is added by the charger • The current sent through the plates break chemical bonds • PbSO4 breaks down into Pb and SO4 • SO4 is released into the electrolyte • Plates return to Pb

17. Mechanism of Chemical Reaction-Recharging • At the same time, H2O is split into H2 and O2 • H2 combines with SO4 to form sulfuric acid • O2 combines with the Pb in the anode to form lead oxide PbO2 • The reaction has now been fully reversed, and the battery is recharged

18. Testing State of Charge • Because some of the H2O in the electrolyte has been converted to H2SO4, the density of the electrolyte increases. • This will show up on a hydrometer test

19. The Entire Cycle Discharging Recharging

20. Characteristics of the Reaction • There are three basic ways to measure the characteristics of the reaction taking place • Potential difference developed • The rate at which the reaction happens • The overall energy released by the reaction

21. Measuring Potential Difference • Potential Difference is the amount of “push” that makes the electrons want to flow through the wire • “Push” as you will recall is voltage • Voltage can be measured with a voltmeter

22. Measuring Potential Difference-Continued • The potential difference depends on the chemicals reacting • The best possible combination is Lithium and Flourine • This would produce a potential difference of 5.9 volts • These material are unrealistic • Common batteries use lead and sulfuric acid • These reactants produce 2.1 volts potential difference

23. Getting 12 volts • Each cell of a battery produces 2.1 volts • Cells are stacked up in series to add voltages • 6 cells are stacked to make 12.6 volts in a healthy battery • Batteries showing less than 12.6 volts should be charged before further testing

24. Critical Question • If the chemical reaction produces 12.6 volts, how is it possible to get 9.6 volts under load?

25. Characteristics of the Reaction - Rate • Voltage is not a good predictor of the health of a battery • Would a 9 volt battery crank your engine? • Why not? • What is really important is the power available • One definition of power is the rate at which energy is being converted.

26. Characteristics of the Reaction - Rate • When we measure the power output of the battery, we are measuring the rate at which the chemical reaction is happening. • The rate of the chemical reaction determines the electrical power available from the battery. • There are a number of factors that affect the rate of a chemical reaction.

27. Chemical Kinetics Video • Watch the video on chemical kinetics. • What factors affect the rate of the chemical reaction? • How are these factors optimized in lead-acid batteries?

28. Energy Content Testing • Batteries are rated for energy content in Amp-Hours • Testing energy content involves measuring current vs. time and voltage vs. time. • Current and voltage are then multiplied to find power. • Most tests are conducted at a constant current.

29. Energy Content Testing- Current • Amperage is held constant

30. Energy Content Testing- Voltage • Voltage is allowed to vary throughout the test • Test stops when voltage drops to 10.5 volts

31. Energy Content Testing- Power • Power = Volts X Amps

32. Energy Content Testing- Energy • Energy is defined as the potential to do work • Energy = Power X Time • How is this reflected in the graph?

33. Finding Area Under the Curve • How might you find the area under the curve? • See if you can figure it out on the worksheet!

34. Energy Density • Now that you know the energy in the battery, we can calculate energy density. • We know that the battery contains about 1.2 kilowatt-hours of energy • To make our comparison, it is necessary to convert that number to kilojoules • To get kilojoules from kilowatts, multiply by 3600 • 1.2 kilowatt-hours X 3600 kilowatt-hours per kilojoule = 4,320 joules

35. Energy Density-Continued • To find the energy density, divide the energy content by the mass • Our battery has a mass of about 15 Kg • 4,320 kilojoules / 15 kilograms = 288 Kilojoules per kilogram • Gasoline has an energy density of 46 megajoules per kilogram • What can we say about the viability of the lead acid battery in weight sensitive applications?