Procedures and Plans
Advanced Battery Research Semester Milestone Layout
Starting Date: September 1, 1998
Customer Requirements - (9/1/98) - 3 days
General Research - (9/4/98) - 7 days
Milestone #1 (9/10/98) Determine Battery Types for Research
chemistry (9/11/98) 10 days
chemical reaction
reaction diagram
half-cell reactions
physical characteristics (9/20/98) 6 days
weight
potential
illustrations and schematics (9/26/98) 3 days
advantages and disadvantages ( 9/11/98) 19 days
environmental effects
comparisons with other electric vehicle batteries (9/11/98) 19 days
specific power
cycle life
Milestone #2 (9/29/98) Lead-Acid Battery Presentation
compile data (9/30/98) 4 days
organize data (10/3/98) 4 days
analyze data (10/7/98) 4 days
report and presentation (10/11/98) 4 days
Milestone #3 (10/15/98) Mid-term Report and Presentation
chemistry (9/30/98) 20 days
chemical reaction
reaction diagram
half-cell reactions
physical characteristics (10/20/98) 11 days
weight
potential
illustrations and schematics (10/31/98) 11 days
advantages and disadvantages ( 9/30/98) 42 days
environmental effects
comparisons with other electric vehicle batteries (9/30/98) 42 days
specific power
cycle life
Milestone #4 (11/10/98) Nickel-Metal Hydride Battery Presentation
chemistry (10/27/98) 18 days
chemical reaction
reaction diagram
half-cell reactions
physical characteristics (10/20/98) 7 days
weight
potential
illustrations and schematics (10/31/98) 6 days
advantages and disadvantages ( 10/27/98) 31 days
environmental effects
comparisons with other electric vehicle batteries (10/27/98) 31 days
specific power
cycle life
Milestone #5 (11/24/98) Lithium Polymer/Ion Battery Presentation
compile data (9/30/98) 1 days
organize data (10/3/98) 1 days
analyze data (10/7/98) 5 days
report and presentation (10/11/98) 7 days
Milestone #6 (12/1/98) Manager's Presentation
Objective:
The objective of the Battery Research is to thoroughly research battery systems applicable to electric vehicle systems. These battery systems will include state-of-the-art lead acid batteries and any or all other battery systems that are currently being used or have the potential of being used in electric vehicles. Several types of batteries are being used by car companies in electric vehicles today, and more are under development. Advancements continue at a rapid pace in the quest for increasingly better battery solutions that provide sufficient energy for electric vehicles, at a practical size and weight, and at a reasonable price. Other alternative battery systems include nickel-metal hydride, nickel-cadmium, lithium polymer, and zinc-air.
Background:
An electrical battery produces electric current by means of a chemical reaction. The chemical reaction occurs because potentials exist between two dissimilar materials (lead plate and lead-dioxide plate) placed in an electrolyte (sulfuric acid). Figure 1 illustrates the components of a typical battery cell.
Batteries are produced in two basic forms, primary and secondary. All chemical batteries, as opposed to mechanical batteries, use chemical energy that is directly changed into electrical energy by means of chemical reactions. In primary batteries, the chemical reaction is irreversible thus, the battery is exhausted after a single use. The batteries found in a flashlight, calculator, or remote control are common primary batteries.
Secondary batteries also create electric energy by means of chemical reactions, although these reactions are reversible. Because the reactions are reversible the batteries can be charged and discharged. Electric Vehicles use secondary batteries because they posses the ability to be charged and discharged. Rechargeable batteries are also used because of their convenience and for economic purposes.
Figure 1. Typical Battery Cell
Lead-Acid Battery:
Lead-Acid technology is the oldest and most cost efficient form of battery. The majority of today's electric cars run on this type of battery since more energy efficient forms of batteries are still being researched. The type of battery used in electric vehicles is really only a larger version of the type of battery already in modern automobiles. However, the typical lead-acid battery used in an electric vehicle only has a range of 120 miles, followed by an eight-hour recharge. Another disadvantage is that the lead-acid battery only has a lifetime of 25,000-30,000 miles before needing to be replaced. The cost of replacement is about $1,500 every two to three years.
Lead-Acid Chemistry:
The lead-acid cell consists of a metallic lead anode and a lead dioxide (Pb02) cathode, which are immersed in a dilute sulfuric acid solution (H2SO4). When discharged, the lead plate and acid interact, producing lead sulfate and some free electrons. The electrons run through an external wire to the other plates, where they react with the lead oxide and sulfuric acid to create lead sulfate. The electrons running through the outside wire are what enable the car to run. After this process, all that remain are plates of lead sulfate immersed in water. The balanced chemical reaction for the discharge of a lead-acid battery is:
PbO2 + Pb + 2H2SO4 = 2PbSO4 +2H2O
239.2g PbO2 + 207.2gPb + 196gH2SO4 = 642.4gPbSO4 +36H2O
This process consumes one gram equivalent each of lead and lead dioxide, and two gram equivalents of sulfuric acid. The consumption of these produces two gram equivalents each of lead sulfate and water. As indicated by the arrows in the chemical equation the process is reversible. The reverse reaction is a recharge of the battery.
The reaction above is a reduction-oxidation reaction, which can be divided into two half-cell reactions. The reactions for the anode and cathode are as follows:
Pb + SO42- (aq)= PbSO42- (sol) + 2 e- (anode)
PbO2 + 4 H+ (aq) + 2 e- + SO42- (aq)= PbSO42- (sol) (cathode)
Note that both electrodes dissolve into the electrolyte during the discharge reaction. When charged the reverse reactions occur, although overcharge will lead to the electrolysis of water and consequent production of (hazardous) H2 (g) at the cathode. The excess electrons are captured by the H+ ions, forming hydrogen atoms. The hydrogen atoms then combine to form potentially explosive H2. This is the reason for the baffled vents on the top casing of the lead-acid battery.
Physical Characteristics:
A typical lead acid battery consists of the following components.
Plate grid - The grids are the supporting framework for the active material of the plates. They also conduct the current to and from the active plates. The grid is typically made of a lead alloy, usually one strengthened with antimony. The grids are subject to corrosion into lead oxide, thus reducing the life of the battery. Other lead alloys include calcium and strontium. Examples of corrosion growth curves are shown below for alloys. As can be seen, the growth is heavily dependent on the alloy type.
Figure 2. Corrosion Induced Growth on the Plate Grid
Positive and Negative Plates - Lead and Lead Dioxide.
Separators - These are porous insulated material that separate the plates in a cell. They prevent short circuit between positive and negative plates.
Cell Connectors - The connectors conduct the current to the battery post on the exterior
of the battery case.
Container - The container is usually polypropylene or rubber rectangular casing with the appropriate number of molded-cell locations.
Vent Plugs - The vent plugs are what allow any produced hydrogen gas to escape the battery casing.
Terminals - The terminals are protrusions on the exterior of the battery to which the electrical cables from the vehicle connect.
Figure 3. Lead-Acid Battery
Typical construction of a lead acid battery is shown on the above diagram.
Advantages and Disadvantages
The use of lead-acid batteries leaves an extreme environmental problem because this type of technology requires lead mining, processing, manufacturing, and recycling and disposal, which would cause massive air pollution. In the near future, if all motor vehicles continue to use lead-acid batteries, this will result in 60 times as much lead being emitted per mile driven than was the case with leaded gasoline.
The cells in a lead-acid battery have a nominal discharge potential of 2 V, are inexpensive, and are capable of high power densities (necessary for starting a car, these can be as high as 600 W / kg). However, the cells undergo relatively rapid self-discharge are environmentally unfriendly, have low energy densities - typically less than 100 W hr / kg due to the high density of lead. Finally, lead-acid cells can only be cycled a few hundred times, and far fewer cycles are possible if the battery is fully discharged. As many people who need to start their cars in the winter know, cell performance is markedly poorer at low temperature.