2 OBJECTIVES After studying Chapter 5, the reader should be able to: 1 OBJECTIVES After studying Chapter 5, the reader should be able to: 1. Describe the role of the high-voltage (HV) battery in a hybrid electric vehicle. 2. Describe the role of the auxiliary battery in a hybrid electric vehicle. 3. Explain safety procedures that must be used while working on and around the high- voltage system in a hybrid electric vehicle. 4. Discuss the operation, testing, and service of a flooded cell lead-acid battery. 5. Discuss the operation, testing, and service of an absorbed glass mat (AGM) battery.
3 The high-voltage (HV) battery in an HEV must be able to provide large amounts of electrical current for acceleration, then recharge quickly as the vehicle is cruising and braking.
4 Battery technology evolved very slowly until later in the century when electric vehicles again were considered as an alternative for personal transportation.
5 Lead-Acid The lead-acid battery was invented almost 150 years ago, and is considered to be the original secondary (rechargeable) battery. Some experts estimate that up to 97% of all lead-acid batteries sold in the United States are recycled.
6 It performs reasonably well for starting-lighting-ignition (SLI) systems in conventional ICE-driven automobiles. The nominal (approximate) voltage for a lead-acid cell is 2.1 volts. The most glaring shortcoming is a lack of specific energy. Specific energy is measured in watt-hours per kilogram (Wh/kg), and this describes how much electrical energy the battery can store relative to its mass.
7 What Is the Difference Between Specific Energy and Energy Density What Is the Difference Between Specific Energy and Energy Density? Specific energy is measured in watt-hours per kilogram (Wh/kg), energy density, how much electrical energy can be stored relative to a battery’s volume and is measured in watt-hours per liter (Wh/l).
8 Nickel-Cadmium The nickel-cadmium (NiCD) battery was invented by Waldmer Junger in The nickel-cadmium design is known as an alkaline battery, because of the alkaline nature of its electrolyte. Alkaline batteries generate electrical energy through the chemical reaction of a metal with oxygen in an alkaline electrolyte. The nominal voltage of a NiCD battery cell is 1.2 volts.
9 How Is an Alkaline Battery Different from a Lead-Acid Battery How Is an Alkaline Battery Different from a Lead-Acid Battery? Lead-acid batteries use sulfuric acid as the electrolyte that acts as the medium between the battery’s positive and negative electrodes. Acids have a pH that is below 7, where pure water has a pH of exactly 7. If electrolyte from a lead-acid battery is spilled, it can be neutralized using a solution of baking soda and water (an alkaline solution). Alkaline batteries use an electrolyte such as potassium hydroxide, which has a pH of greater than 7. This means that the electrolyte solution is basic, which is the opposite of acidic. If an alkaline battery’s electrolyte is spilled, it can be neutralized using a solution of vinegar and water (vinegar is acidic).
10 Nickel-Metal Hydride The NiMH is very similar in construction to a NiCd, in that it uses a positive electrode made of nickel hydroxide and potassium hydroxide electrolyte. Like the NiCd, the nominal voltage of a NiMH battery cell is 1.2 volts.
11 The materials used in NiMH batteries are environmentally friendly and can be recycled. Specific energy of the NiMH is high, it exhibits excellent cycle life, and it is considered to be a very safe and durable battery design.
12 Disadvantages of the NiMH battery include a high rate of self-discharge, especially at elevated temperatures. NiMH technology is currently being used for the HV battery packs in all production hybrid electric vehicles (HEVs).
13 Lithium-Ion The positive electrode in a lithium-ion battery has lithium cobalt oxide as its main ingredient, with the negative electrode being made from a specialty carbon. A lithium-ion battery is so named because during battery cycling, lithium ions move back and forth between the positive and negative electrodes. To prevent battery rupture and ensure safety, a pressure release valve is built into the housing that will release gas if the internal pressure rises above a preset point.
15 Lithium-ion batteries show good promise for use in EV and HEV applications. They have a high specific energy, good high-temperature performance, and low self-discharge. The nominal voltage of a lithium-ion cell is 3.6 volts, which is three times that of nickel-based alkaline batteries.
16 One disadvantage of the lithium-ion battery is safety, due to the reactive nature of lithium. Also, early lithium-ion battery designs have experienced problems with thermal runaway, which has led to overheating and even explosion.
17 Lithium-Polymer Since lithium-polymer batteries use solid electrolytes, they are known as solid state batteries. Solid polymer is much less flammable than liquid electrolytes and is able to conduct ions at temperatures above 140°F (60°C). The major disadvantage with the Li-poly battery is that it is a high-temperature design and must be operated between 176° and 248°F (80° and 120°C).
18 Zinc-Air The zinc-air design is a mechanically-rechargeable battery Zinc-Air The zinc-air design is a mechanically-rechargeable battery. The negative electrode is spent during the discharge cycle, and the battery is recharged by replacing the zinc electrodes. Zinc-air is one of several metal-air battery designs (others include aluminum-air and iron-air) that must be recharged by replacement of the negative electrode (anode).
19 Zinc-air batteries can be recharged very quickly, since a full recharge is achieved through replacement of the zinc electrodes. The primary disadvantage with this design is the level of infrastructure required to make recharging practical.
20 Sodium-Sulfur Like lithium, sodium has a low atomic mass and can produce a relatively high voltage when used in a battery. Sulfur can work well as a positive electrode, and sodium and sulfur are both plentiful and cheap. However, sodium (like lithium) is highly reactive and must be used with a special electrolyte called beta alumina. The sodium-sulfur battery is a high-temperature unit that operates around 572°F (300°C).
21 Sodium-Metal-Chloride The sodium-metal-chloride battery is quite similar in construction to the sodium-sulfur battery. The major difference is that the positive (sulfur) electrode is replaced with one made from either nickel-chloride or a combination of nickel-chloride and ferrous-chloride. Sodium-metal-chloride batteries are also known as ZEBRA batteries.
22 Specific Energy (Wh/kg*) A disadvantage of the sodium-metal-chloride design is high operating temperatures. The table shows a comparison of specific energy and nominal voltage for the various battery technologies. Secondary Batteries ComparisonBattery TypeNominalVoltage (V) perCellTheoretical SpecificEnergy (Wh/kg*)PracticalSpecific Energy (Wh/kg*)Major IssuesLead-Acid2.125235Heavy, low cycle life, toxic materialsNickel-Cadmium1.224450Toxic materials, costNickel-Metal Hydride80Cost, high self-discharge rate, memory effectLithium-Ion3.6766120Safety issues, calendar life, costZinc-Air1.11320110Low power, limited cycle life, bulkySodium-Sulfur2.0792100High-temperature battery, safety, low- power electrolyteSodium-Nickel- Chloride2.578790High-temperature operation, low power*Specific energy is measured in watt-hours/kilogram
23 ROLE OF THE HV BATTERY IN THE HYBRID SYSTEM Most hybrid-electric vehicles use dual-voltage electrical systems. HEVs use high-output electric motors to drive and assist vehicle movement. If a conventional 12-volt electrical system was used to power these motors, the amount of current flow required would be extremely large and the cables used to transmit this energy would also be so large as to be impractical.
24 Automotive engineers overcome this problem by increasing the voltage provided to the motors, thus decreasing the amount of current that must flow to meet the motor’s wattage requirements.
25 Why Do Higher-Voltage Motors Draw Less Current Why Do Higher-Voltage Motors Draw Less Current? Electric motor is powered by wattage . P = I × E or Power (in watts) = Current (in amperes) × Voltage (in volts) An electric motor rated at 144 watts will consume 12 amperes at 12 volts of applied voltage (12 volts × 12 amperes = 144 watts). Some hybrid systems have motors that operate at up to 650 volts in an effort to increase system efficiency.
26 High-Voltage (HV) Battery Construction Each cell of a NiMH battery produces only 1.2 volts. In order to create a battery pack that is capable of producing high voltage (i.e.; 144 volts for Honda IMA and 330 volts for the Ford Escape Hybrid), many individual NiMH cells must be connected in series.
27 A 9-volt alkaline battery commonly used in toys and handheld games A 9-volt alkaline battery commonly used in toys and handheld games. In reality, the rectangular case of the 9-volt battery houses six 1.5-volt cells connected in series.
28 In order to build a 144-volt battery, 120 (120 × 1 In order to build a 144-volt battery, 120 (120 × 1.2 = 144 V) individual NiMH cells must be connected together in series.
29 NICKEL-METAL HYDRIDE TECHNOLOGY All current-production HEVs use nickel-metal hydride (NiMH) battery technology for the vehicle’s high-voltage battery. NiMH batteries are being used for these applications because of their performance characteristics such as specific energy, cycle life, and safety.
30 Description and Operation Nickel-metal hydride (NiMH) batteries use a positive electrode made of nickel hydroxide. The negative electrode is unique, however, in that it is a hydrogen-absorbing alloy, also known as a metal hydride.
31 NiMH batteries are known as alkaline batteries due to the alkaline (pH greater than 7) nature of their electrolyte. The electrolyte is aqueous potassium hydroxide. Potassium hydroxide does not take part in the chemical reaction of the battery, so the electrolyte concentration stays constant at any given state of charge (SOC). These factors help the NiMH battery achieve high power performance and excellent cycle life. During battery charging, hydrogen ions (protons) travel from the positive electrode to the negative electrode, where they are absorbed into the metal-hydride material. The electrolyte does not participate in the reaction and acts only as a medium for the hydrogen ions to travel through. When the battery is discharged, this process reverses, with the hydrogen ions (protons) traveling from the negative electrode back to the positive electrode.
33 What Is the Difference Between a Cell and a Battery What Is the Difference Between a Cell and a Battery? A cell is one pair of electrodes (one negative and one positive) arranged in an electrolyte solution to produce direct-current electricity. A battery, on the other hand, is a collection of individual cells, usually connected in series to achieve a higher voltage.
34 Construction There are two primary designs for a NiMH battery cell: the cylindrical type and the prismatic type. The cylindrical type has the cell’s active materials made in long ribbons and arranged in a spiral fashion inside a steel cylinder (case). Cylindrical cells are often constructed very similarly to a conventional “D” cell.
35 Cylindrical cells are most often incorporated into modules with a group of 6 cells connected in series. This creates a single battery module with a 7.2-volt output.
36 The prismatic type is a rectangular or box-like design with the active materials formed into flat plates, much like a conventional lead-acid battery.
37 A high-voltage battery pack is made using many individual NiMH modules connected in series. These modules can be constructed using cells of either the cylindrical design or the prismatic design.
39 Cooling High operating temperatures can lower performance and cause damage to a NiMH battery pack. All current-production HEVs use air cooling to control HV battery pack temperature. Cabin air or fresh air is circulated over the battery cells using an electric fan and ducting inside the vehicle.
40 Temperature sensors (thermistors) are mounted in various locations in the battery pack housing to send data to the module responsible for controlling battery temperature. These inputs are used to help determine battery charge rate and cooling fan operation.
41 In the case of the Ford Escape Hybrid, the air-conditioning system has an extra zone that cools the air being circulated over the HV battery pack.
42 State-of-Charge (SOC) Management The HV battery in a hybrid electric vehicle is subjected to constant charging and discharging during normal operation. The battery can overheat under the following conditions: 1. The battery state of charge (SOC) rises above 80%. 2. The battery is placed under a load when its SOC is below 20%.
43 In order to prevent overheating and maximize service life, the battery’s SOC must be carefully managed. In the case of Toyota HEVs, a target SOC of 60% is used, and the battery is then cycled so its SOC varies no more than 20% higher or lower than the target.
44 Safety Precautions Always keep in mind that the high-voltage batteries for an HEV can produce sufficient voltage and current to severely injure or kill. Always wear appropriate personal protective equipment (PPE) and use approved safety procedures when working around these batteries.
45 The battery case contains liquid potassium hydroxide, a strong alkali solution. Any liquid around the battery should be checked with litmus paper to determine if it is an electrolyte spill. If an electrolyte spill has occurred, be sure to disable the HV system, and then use a mixture of vinegar and water to neutralize the solution before cleaning up with soap and water. Remove any clothing that has come into contact with electrolyte and flush any exposed skin with large amounts of water. If electrolyte comes in contact with the eyes, flush with large amounts of water, but do not use a neutralizing solution. Be sure to seek medical advice to prevent further injury from electrolyte contact.
46 Service During normal vehicle operation, the charge and discharge cycles of the high-voltage battery in an HEV are monitored and controlled by a separate battery module. This module monitors battery temperature, current, and voltage to calculate SOC and determine at what rate the battery should be charged.
47 Storing a Hybrid Could Be Harmful One disadvantage of nickel-metal hydride batteries is their tendency to self-discharge. A hybrid-electric vehicle’s HV battery pack can be permanently damaged if left for long periods of time with a low state of charge. Some manufacturers recommend that their hybrid-electric vehicles be run for at least 30 minutes per month to prevent such damage from taking place.
48 ROLE OF THE AUXILIARY BATTERY IN THE HYBRID SYSTEM Most hybrid-electric vehicles (HEVs) use a dual-voltage electrical system. The vast majority of automotive electrical components are currently being built to operate on 12 volts, so the cost of building an HEV can be reduced if these same accessories can be used in its design. These vehicles will have a 12-volt auxiliary battery located either in the trunk or in the ICE compartment.
49 The 12-volt system must have an energy storage device of its own, so an auxiliary battery is installed in addition to the high-voltage battery pack. A DC-to-DC converter is often used to reduce the output of the HV battery pack to 12 volts for powering this section of the electrical system. The auxiliary batteries currently being used in production HEV models are based on lead-acid technology.
50 LEAD-ACID TECHNOLOGY Conventional lead-acid batteries are based on the flooded design, where the electrolyte is liquid and the battery plates are immersed in the electrolyte. These batteries are vented and can only be used in the upright position. An alternative to the flooded design is the valve regulated lead acid (VRLA) battery, which includes absorbed glass mat (AGM) and gelled electrolyte technology. These newer designs have become more popular in recent years due to their higher performance, lower maintenance, and increased safety.
51 Flooded Cell A flooded lead-acid cell uses a positive electrode made of lead dioxide (PbO2) and a negative electrode made of sponge lead (Pb). When the battery is discharging, sulfate ions (SO42-) move from the electrolyte solution toward the negative and positive electrodes. At the same time, oxygen is released from the positive electrode into the electrolyte and these ions combine with hydrogen (H+) to produce water (H2O).
53 When a lead-acid battery is being charged, a reversal of the cell discharge process occurs. The sulfate ion is released from both the negative and positive electrodes into the electrolyte solution. At the same time, oxygen from the electrolyte leaves the solution to form lead dioxide (PbO2) on the positive electrode.
54 Construction. A conventional (flooded) lead-acid battery’s electrodes are constructed in the form of plates. These electrode plates are made by building a grid from a lead alloy, and then pasting the active material onto the grid.
55 Conventional batteries with lead-antimony grids suffer from battery gassing, which leads to a loss of water from the battery cell over time. Low-maintenance batteries control gassing by using calcium in the plate grid alloy and decreasing the amount of antimony.
56 Each group of plates is connected together using a plate strap Each group of plates is connected together using a plate strap. Separators are placed between each of the plates, and these prevent the plates from touching but allow electrolyte to circulate freely between them. The numbers or sizes of the plates in the cells can be increased, but the voltage output will stay the same at 2.1 volts.
57 The battery case is constructed to allow the installation of six cells with a partition between each of them. The cells are then joined electrically using connectors that pass through the partition wall. The case cover is installed over this assembly, and battery posts are installed either on the top or side of the case to allow the installation of battery cable terminals. A new battery with no electrolyte in it is known as a dry-charged battery. Battery ratings.
58 Cold Cranking Amperes Automotive batteries are known as SLI batteries because they are required to provide electrical current for the starting, lighting, and ignition circuits of the vehicle. The cold cranking amperes (CCA rating) of a battery is the number of amperes that can be supplied by the battery at 0°F (-18°C) for 30 seconds while the battery maintains a voltage of at least 1.2 volts per cell.
60 Cranking Amperes The cranking ampere (CA) rating is often stamped on a battery label alongside the CCA rating. Cranking amperes are similar to CCA, except that the battery is tested at 32°F instead of 0°F.
61 Reserve Capacity The reserve capacity (RC) rating of the battery is determined by the number of minutes that a battery is able to provide 25 amperes of current to a vehicle’s electrical system and still maintain a minimum battery voltage of 1.75 volts per cell. In the case of a 12-volt (six-cell) battery, this means that the battery voltage would drop no lower than 10.5 volts during the test. The reserve capacity rating measures how long a battery could keep a vehicle running if the charging system were to fail.
62 Safety precautions. It is extremely important that no sparks be created around a lead-acid battery. Hydrogen and oxygen gases are generated while the battery is charging, and it is possible for this gas to be ignited if special precautions are not taken.
63 Connect a battery charger to the battery first before plugging the charger into a wall outlet. The idea is to do everything possible to prevent sparks around the battery. These precautions should also be observed when jump starting a vehicle. When connecting the jumper cables, be sure to make the last connection at the engine block of the disabled vehicle so that any sparks take place away from the battery itself.
64 In the case of hybrid vehicles, sometimes special terminal posts are located under the hood for attaching jumper cables, making it easy to keep sparks away from the battery itself.
65 Sulfuric acid is extremely corrosive, so spilled electrolyte should be neutralized using a solution of baking soda and water. Lead-acid batteries can be very heavy, so always lift a battery using an approved lifting device. Some lifting devices are made to grip on the battery posts, while others are made to lift on the battery case.
66 Service. A battery that is exposed to vibration or rough handling will shed the active material from its grids, which will lead to premature failure.
67 Another major cause of premature failure in lead-acid batteries is consistent undercharging or overcharging. Undercharging can lead to plate sulfation, where the lead sulfate on a discharged battery’s plates becomes crystallized over time.
68 Conventional lead-acid batteries will require the electrolyte level to be checked. When the electrolyte level is low, add ONLY distilled water to the battery cell.
69 During normal service, a moist film of acid and dirt can become deposited on the top of the battery case. Clean the top of the battery using a brush and a solution of baking soda and water to neutralize the acid.
70 In extreme situations, a battery post reamer is required to service a post that has seen severe service. Note that this tool has three parts, a section for cleaning a positive (+) post, a section for cleaning a negative (-) battery post, and a tapered reamer for cleaning the inside of a battery cable terminal.
71 Under most circumstances, a battery brush will clean the post adequately.
72 Testing. An effective method for determining the state of charge (charge level) of a lead-acid battery is to measure the specific gravity of the electrolyte in each cell. The electrolyte in a fully charged battery is made up of approximately 64% water and 36% sulfuric acid. This same electrolyte will have a specific gravity between and at 80°F (27°C).
73 The actual measurement can be done using either a hydrometer or a refractometer.
74 Many lead-acid batteries have a charge indicator built into the battery case. This device indicates battery charge through a plastic ball that floats when the specific gravity of the electrolyte rises above a certain level. When the ball floats, the color of the window will change to the color of the ball.
75 A more reliable method of determining battery SOC is to measure the battery’s open circuit voltage (OCV) using a digital multimeter (DMM) connected across the battery posts. If the vehicle was running recently, start this test by turning on the headlights for one minute to remove the battery’s surface charge. After turning the headlights off, allow the battery voltage to recover until it stops rising and then record the voltmeter reading. Battery State of ChargeSpecific GravityState-of-ChargeBattery Voltage (V)1.265Fully charged12.6 or higher1.22575% charged12.41.19050% charged12.21.15525% charged12.0Lower than 1.120Discharged11.9 or lower
76 Once it has been determined that a battery is at least 75% charged, it can then be tested for capacity using a load tester.
77 The actual load to be placed on the battery is determined by taking the battery’s cold cranking ampere (CCA) rating and dividing it by 2. Another method of assessing battery condition is to measure the battery’s internal resistance using specialized test equipment. Some types of battery testing equipment will measure a battery’s internal resistance and use this information in conjunction with voltage and current-producing ability to diagnose a battery.
78 It is also possible to diagnose a battery’s condition through measurement of its conductance. The unit of measurement for conductance is also different, as it is measured in Siemens (abbreviated with a “G”). Battery conductance testing is effective for diagnosing flooded lead-acid batteries, as well as valve-regulated lead-acid (VRLA) batteries.
79 During a conductance test, the diagnostic equipment will pass an AC current of known frequency and amplitude through the battery and then measure a portion of the AC current response signal. This data is then compared to established reference values to determine the battery’s state of health.
80 Most vehicle manufacturers require that battery conductance testers be used to determine battery warranty claims at the dealership level.
81 Valve-Regulated Lead-Acid Flooded lead-acid batteries are vented so that any gases generated above the liquid electrolyte in the battery cell are released to the atmosphere. This process has two major drawbacks. First, these batteries require periodic maintenance to replace water, second, a flooded cell battery can only be operated in an upright position. A battery design that overcomes these problems is the valve-regulated lead-acid (VRLA) battery. These are also known as sealed-valve regulated (SVR) or sealed lead-acid (SLA) batteries. The two major types of VRLA batteries are the absorbed glass mat (AGM) and the gelled electrolyte battery.
82 Absorbed Glass Mat (AGM) An AGM battery is a sealed-valve regulated lead-acid battery design with an absorbent glass microfiber separator between the battery plates. The sulfuric acid and water electrolyte solution is absorbed into this material. The separator is only 90% to 95% saturated with electrolyte, so there are pockets where gas can reside in the separator material. AGM batteries are known as recombinant batteries. The AGM battery is also known as a starved electrolyte system.
83 Since an AGM (or any other VRLA battery) does not lose electrolyte due to gassing, it does not require periodic addition of distilled water to the cells. The absorbent material used between the plates retains the electrolyte and makes the battery leak resistant and spillproof. AGM batteries are most commonly used in applications where the auxiliary battery is located in the passenger compartment.
84 Construction Many AGM batteries are constructed using flat plates, as in a conventional flooded lead-acid battery. The major difference is in the separator material, as it uses a glass microfiber material that absorbs and retains the diluted sulfuric acid electrolyte solution.
85 Other AGM batteries are built using cylindrical cells Other AGM batteries are built using cylindrical cells. The plates are supported in the compressed absorbent material, these batteries are better able to withstand vibration.
86 With the AGM and any other VRLA battery, there is a low-pressure venting system built into the battery case that will release excess gas pressure and prevent case bursting if the battery is overcharged. The valve allows internal gases to escape, but will not allow outside air to enter the battery. This limits internal corrosion and extends the battery’s service life. This one-way valve is instantly resealable and is able to perform this function as many times as required during the battery’s service life.
87 Safety Precautions An AGM battery does not work well under high heat conditions. These two factors combine to make the AGM battery very sensitive to overcharge and other conditions that would cause it to operate at a higher-than-normal temperature.
88 Service AGM batteries are sealed and do not require periodic filling of the cells with distilled water. Newer battery charging and testing equipment incorporates special settings for AGM batteries and this equipment should be utilized to maximize battery performance and service life.
90 Gelled Electrolyte Batteries Gelled electrolyte batteries are also known as gel batteries. Gelled electrolyte batteries are a type of VRLA battery that has silica added to the electrolyte. The silica causes the electrolyte to become a gel, which makes the battery leakproof and spillproof when housed in a sealed case. The gelled electrolyte battery uses a recombinant process and is built similar to an AGM battery.
91 SUMMARY 1. Battery technology is critical to the proper operation of an ICE-powered vehicle, as well as the hybrid electric vehicle (HEV). 2. Rechargeable batteries are also known as secondary cells, and nonrechargeable batteries are known as primary cells. 3. Lead-acid batteries have been in existence for 150 years and are the most common battery technology in use today for SLI (starting, lighting, and ignition) applications. 4. Lead-acid technology is not used for current HEV high-voltage battery packs due to poor cycle life and low specific energy. 5. Current HEVs use nickel-metal hydride battery technology for their high-voltage battery packs. NiMH is an alkaline battery design that operates through the movement of hydrogen ions between the battery electrodes.
92 SUMMARY 6. Lithium-ion technology is a possible replacement for NiMH in HEV applications due to its higher nominal voltage and specific energy. 7. Most HEVs use dual-voltage electrical systems, with a conventional 12-volt system operating in tandem with the high-voltage system for the traction motor. 8. Lead-acid batteries can be diagnosed using load testing, internal resistance testing, and conductance testing. 9. Valve-regulated lead-acid batteries are sealed and can be operated in any position Absorbed glass mat (AGM) batteries are a recombinant battery design that holds the electrolyte in an absorptive microfiber material.
93 REVIEW QUESTIONS 1. How does a lead-acid battery operate during the discharging and charging cycles? 2. Why are NiCD and NiMH batteries known as alkaline batteries? 3. What are the materials used in a nickel-metal hydride (NiMH) battery? 4. How is an absorbed glass mat (AGM) battery constructed differently from a flooded lead- acid battery? 5. What is a recombinant battery, and how does it operate?