DOT-FTA-MA-26-7071-03-1
DOT-VNTSC-FTA-03-05
DOT Insignia
U.S. Department of Transportation
Federal Transit Administration 		Clean Air Program

Design Guidelines for Bus Transit
Systems Using Electric and Hybrid Electric
Propulsion as an Alternative Fuel

March 2003
Final Report

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NOTICE

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof.

 

NOTICE

The United States Government does not endorse products or manufacturers. Trade or manufacturers' names appear herein solely because they are considered essential to the objective of this report.

 

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE
March 2003 		3. REPORT TYPE AND DATES COVERED
Final Report - March 2003
4. TITLE AND SUBTITLE
Design Guidelines for Bus Transit Systems Using Electric and Hybrid-Electric Propulsion as an Alternative Fuel 		5. FUNDING NUMBERS
U3077/TT39
6. AUTHOR(S)
William P. Chernicoff,* Thomas Balon**, and Phani Raj***.
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Volpe Center                        * MJ Bradley and Associates           ** TMS Inc***
Cambridge, MA 02142           Manchester, NH                            Burlington, MA 		8. PERFORMING ORGANIZATION
REPORT NUMBER
DOT-VNTSC-FTA-03-05
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
U.S. Department of Transportation Federal Transit Administration Office of Research Demonstration and Innovation 		10. SPONSORING/MONITORING AGENCY REPORT NUMBER
FTA-MA-26-7071-03-1
11. SUPPLEMENTARY NOTES
This work performed under contract to:
     U.S. Department of Transportation
     Volpe National Transportation Systems Center
     Cambridge, MA 02142
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This document is available to the public through the National Technical Information Service, Springfield, VA 22161 		12b. DISTRIBUTION CODE
13. ABSTRACT (Maximum 200 words)
The use of alternative fuels to power transit buses is steadily increasing. Several fuels, including Compressed Natural Gas (CNG), Liquefied Natural Gas (LNG), Liquefied Petroleum Gas (LPG), and Methanol/Ethanol, are already being used. At present, there are no available comprehensive facility guidelines to assist transit agencies contemplating converting from diesel to electric of hybrid electric propulsion. This document addresses that need. This guidelines document presents various facility and bus design issues that need to be considered to ensure safe operations when using electric or hybrid electric propulsion. Fueling facility, garaging facility, maintenance facility requirements and safety practices are indicated. Among the issues discussed are electric storage device properties, potential hazards, requirements for specified level of service, and applicable codes and standards. Critical fuel related safety issues in the design of the related systems on the bus are also discussed. A system safety assessment and hazard resolution process is also presented. This approach may be used to select design strategies which are economical, yet ensure a specified level of safety. This report forms part of a series of published by the U.S. DOT/FTA on the safe use of alternative fuels. Documents similar to this one in content have been published for CNG, Hydrogen, LPG, LNG, and Methanol/Ethanol.
14. SUBJECT TERMS
Electric propulsion, Hybrid-electric propulsion, electric drive, transit bus, transit facility design, system safety, alternative fuel 		15. NUMBER OF PAGES
160
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Acknowledgements

The work reported in this document was performed by Technology & Management Systems, Inc. and M.J. Bradley & Associates, Inc., under contract DTRS57-02-P-80173 from the U.S. Department of Transportation, John A. Volpe National Transportation Center ("Volpe Center") in Cambridge, Massachusetts. William Chernicoff was the Project Manager at the Volpe Center.

The Volpe Center wishes to thank the subcontractors, companies, and individuals that provided assistance to this project.

Companies and organizations involved in the review process included:



Table of Contents



Chapter 1: Introduction

1.1 Background

1.2 Disclaimer

1.3 Document Objective

1.3.1 Description of the Contents

Chapter 2: Overview of Battery Electric and Hybrid-Electric Bus Technologies

2.1 What is an Electric Bus?

2.2 What is a Hybrid-Electric Bus?

2.3 Hybrid-Electric Configurations

2.3.1 Series Hybrid

2.3.2 Parallel Hybrid

2.4 Electric and Hybrid-Electric Bus Components

2.4.3 Battery Thermal Management System

2.4.4 Replacement of the Battery Pack

Chapter 3: Electric and Hybrid-Electric Bus Safety Issues

3.1 Electric Shock

3.1.1 Enclose and Label High Voltage Parts

3.1.2 Electrical Isolation

3.1.3 Additional Recommendations

3.1.4 Battery Box

3.1.5 Automatic Disconnect Devices for Energy Storage Systems

3.1.6 On-Board Charging

3.1.7 Power Train and Control Systems

3.2 Hydrogen Gassing

3.3 Fire/Toxic Fumes

3.4 Electrolyte (Acid) Spills

3.5 Vehicle (Electrical System) Maintenance Issues Related to Safety

Chapter 4: Safety Issues in the Maintenance/Storage Facility

4.1 Battery Off-Loading & Handling

4.2 Battery Storage

4.3 Battery Charging

4.3.1 Battery Charger Safety and Location

4.4 Facility Fire Detection and Protection Systems

Chapter 5: Personnel Training

5.1 Training of Transit Vehicle Operators

5.2 Training of Maintenance Personnel

5.3 Emergency Response Personnel Training

Chapter 6: REFERENCES

Chapter 7: APPENDIX A: APPLICABLE REGULATIONS, CODES, STANDARDS & RESOURCES

Regulations

Codes & Standards

Resources

 

List of Tables



2.1 COMPARISON OF HYBRID CONFIGURATION

2.2 ELECTRIC MOTOR COMPARISON

2.3 PERFORMANCE OF SOME ADVANCED ELECTRIC BUS BATTERY SYSTEMS

4.1 BATTERY CHARGER REQUIREMENTS

 

List of Figures



2.1 Schematic of an Electric Vehicle

2.2 Series Configuration

2.3 Parallel Configuration

2.4 Effects of Temperature on Batteries

 

Acronyms



AC Alternate Current

APU Auxiliary Power Unit

BMS Battery Management System

CNG Compressed Natural Gas

DC Direct Current

EPRI Electric Power Research Institute

FTA Federal Transit Administration

FTP Federal Transient Procedure

Li Lithium

LNG Liquid Natural Gas

NEC National Electrical Code

NFPA National Fire Protection Association

NiCd Nickel Cadmium

NiMH Nickel Metal Hydride

NOx Nitrogen Oxides

NFPA National Fire Protection Association

NYC New York City

PM Particulate Matter

SAE Society of Automotive Engineers

UL Underwriters Laboratories

US United States

V Volt

Volpe Center US Department of Transportation, John A. Volpe National Transportation Center

Wh/kg Watt-hours per kilogram

 

Chapter 1: Introduction

1.1 Background

The Office of Research, Demonstration, and Innovation of the Federal Transit Administration (FTA) funded a number of research and demonstration projects involving the application of alternative fuel technologies to transit buses. FTA has also funded a number of electric and hybrid-electric bus demonstration projects. At present, there are approximately 220 electric buses, 90 hybrid-electric buses and trolleys, and 6 fuel cell buses operating in the US.1 Electric and hybrid-electric buses offer transit agencies a way to reduce local emissions without potentially costly alternative fuel infrastructure costs.

FTA has published a set of five guideline reports for transit agencies planning to incorporate alternative fuel vehicles in their fleets. These documents are listed in Appendix A and are available for free to the public. FTA received positive feedback from the transit industry on the usefulness of these guidelines for operations, training, and bus procurement activities. Therefore the FTA initiated the development of this document for electric and hybrid-electric buses, similar in format and scope to the previous publications.

1.2 Disclaimer

While this guidance document was reviewed by a broad-based and representative group of individuals, none of the participating organizations were asked to, nor have they necessarily, endorsed or adopted the recommendations included in this guidance document. Nor does FTA endorse any company or individual who supported this effort or products that may be mentioned in the document.

This document is intended to be a guidebook on bus and facility design issues and SHOULD NOT be considered a specification manual or a substitute for existing local, state, or national codes and regulations. In addition, the reader should consider the following issues when reading this document.

1.3 Document Objective

The principal objective of this document is to provide transit agencies with an overview of the technology, recommended safety specifications in bus design, and training for personnel that will enable them to understand the implications of purchasing, operating, and maintaining electric and hybrid-electric buses. In addition, the document is intended to provide basic information on electrical and operational safety for transit and non-transit personnel, such as emergency responders to an accident.

This document was not developed under a consensus format. Rather, the document highlights issues that transit agencies should be aware of and directs readers to review established industry standards, where available, for minimum specification requirements. Since many technology alternatives are being explored, it is difficult at this time to determine uniform standards that overlap the various technologies and configurations available in the electric and hybrid-electric bus market. This report should be used by transit systems to survey the various levels of safety and training options available and initiate discussions with manufacturers regarding these areas.

1.3.1 Description of the Contents

Chapter 2 outlines the principles of electric and hybrid-electric technologies and the major components as they apply to transit buses. This report does not include information on electric buses whose power source is external, such as an electric trolley bus. Chapter 3 examines safety issues that high voltage electric buses pose. Safety in garages and maintenance facilities is covered in Chapter 4. The types of training for personnel and emergency responders for transit agencies with electric and hybrid-electric buses are discussed in Chapter 5. Appendix A provides a list of rules, regulations, and standards that should be consulted to understand the requirements for electric and hybrid-electric buses and infrastructure. Additionally, this appendix includes recommended resources for more detailed information.

Chapter 2: Overview of Battery Electric and Hybrid-Electric Bus Technologies

To understand what makes the electric bus and hybrid-electric bus designs unique, it is helpful to compare them to a conven­tional bus. In a conventional bus design an internal combus­tion engine provides power. The engine is tied mechanically to the wheels via a transmis­sion, allowing the vehicle to utilize the engine's energy directly. Fossil-fueled en­gines emit air pollution, such as nitrogen oxides (NOx) and particulate matter (PM). One of the goals of electric and hybrid-electric buses are to reduce or eliminate these exhaust pollutants.

Figure 2.1: Schematic of an Electric Vehicle


2.1 What is an Electric Bus?

he operating principle of a battery-powered electric vehicle is simple. An energy storage device located on the vehicle supplies all of the motive energy in the form of electricity to a traction motor or motors. The rotary motion of the electric motor translates rotary motion to the vehicle wheels either by direct drive or through a mechanical transmission. The speed of the motor is controlled by an on-board electronic controller, which functions primarily based on the position of the accelerator pedal. The energy storage device, typically batteries, is recharged from an external electrical source when its charge is depleted. This mode of operation has an advantage over conventional and hybrid-electric buses, as it eliminates local air pollutants and engine noise. Figure 2.1 shows the important components of a battery-powered electric bus. These components are described in more detail later in the chapter.

Energy Storage Device: A component or system of components that stores energy and for which its supply of energy is rechargeable by an electric motor system, an off-vehicle electric energy source, or both.

Most electric vehicles also recharge the energy storage device during braking by recovering part of the vehicle's kinetic energy. Termed regenerative braking, the traction motor acts as a generator, and as a brake on the vehicle, with the electrical energy generated during braking fed into the energy storage device. There are a number of variations in the application of the above basic principle depending upon the vehicle size, duty cycle, and other technical and economic considerations.

Regenerative Braking: Deceleration of the vehicle caused by operating an electric motor system, thereby returning energy from the vehicle propulsion system to the energy storage device or to operate auxiliaries.

 

2.2 What is a Hybrid-Electric Bus?

A hybrid-electric bus carries at least two sources of motive energy on board the vehicle. The non-electric source is typically referred to as an auxiliary power unit (APU)2, which converts replenishable fuel into energy. Examples of APUs are internal combustion reciprocating engines, microturbines, or fuel cells. Depending upon the design of the hybrid-electric bus, an APU generates energy either continuously or intermittently. The energy is then used either to drive the wheels or stored for later use. The electrical energy produced by the generator may be used to charge the energy storage device or is directly fed to an electric motor to provide energy for motive power. The energy storage system may also be recharged by the energy recovered by regenerative braking. An electronic controller controls the "flow of current" from the batteries and/or the APU-generator set to the traction motor. The electronic controller is continually monitoring the energy storage device's state of charge to determine whether engine operation is needed to recharge the batteries independent of the driver signals. Introducing electrical energy as a means to provide a portion of the energy for vehicle motive power allows for a decrease in the total necessary energy and severity of transient operation required by the conventional reciprocating engine, therefore, reducing the amount of pollution emitted.

Auxiliary Power Unit: Converts fuel into electrical energy. May take the form of an engine/generator, fuel cell, or turbine.

The principal hybrid-electric bus components include: (a) a drive motor, (b) a controller and inverter, (c) an energy storage device, (d) an APU, and (e) other auxiliary systems, such as air conditioning and lighting. An advantage of a hybrid-electric bus over a conventional bus is theoretically better fuel economy and lower exhaust emissions. A previous analysis of in-use data indicates that engines in series hybrid-electric vehicles exhibit less aggressive transient behavior with engine operating points closer together in a much smaller range.3 This is because the energy storage system provides the extra power for acceleration and grade demands, allowing a less precise relationship between engine speed and wheel speed. Avoidance of certain engine operating regimes minimizes PM and NOx emissions and allows for the engine to be operated for optimum steady-state efficiency. Additional information on the increased efficiency and emission reductions can be found in the Northeast Advanced Vehicle Consortium's Hybrid-Electric Drive Heavy-Duty Vehicle Testing Project report.4

2.3 Hybrid-Electric Configurations

2.3.1 Series Hybrid

Figure 2.2: Series Configuration

Hybrid-Electric technology is typically divided into two general types of drive configurations-series and parallel with a number of subcategories and even combinations of the two. In the series hybrid, similar to an electric bus, either a single electric motor drives the wheels through a mechanical transmission or an independent wheel motor drives each drive wheel. The electric motor(s) may draw energy from either the energy storage device or from the APU as determined by the controller. Figure 2.2 shows the series-hybrid system in a hybrid-electric bus.

The two main variants of a series hybrid depend upon whether the APU or the battery dominates the system. In the engine-dominant hybrid, also referred to as charge sustaining, the engine provides a significant part of the drive power; that is, the energy is immediately utilized, minimizing efficiency losses that occur from energy storage. The size of the battery can therefore be small; however, the range in an all-electric mode would be relatively short. In the battery-dominant configuration, which can be charge sustaining or charge depleting, a greater distance can be traversed on the battery power alone and a larger quantity of the regenerated electrical energy can be stored. The disadvantage is that the size of the battery pack, and hence its weight may be significantly larger and there may be increased energy losses due to energy being routed through the batteries.

Figure 2.3: Parallel Configuration

 

As noted, the determination of APU or energy storage system dominance is generally linked to whether the vehicle is charge sustaining or charge depleting. A pure electric bus is both charge depleting and battery dominant as it derives all of its motive energy from batteries and needs to be recharged from an external sources, such as the electricity grid. A charge depleting hybrid-electric bus is similar in that a majority of the energy is derived from the electricity grid and only minimal energy is supplied from the APU to extend vehicle range. At the other end of the spectrum, a charge-sustaining hybrid derives a majority of its energy from the APU and connection to the electricity grid is only used for battery conditioning if necessary.

2.3.2 Parallel Hybrid

In a parallel-hybrid system the electric motor and the APU are both connected to the vehicle drive wheels. This system is shown schematically in Figure 2.3. The electric drive motor draws energy from the energy storage device, to supply additional tractive effort and also recovers energy from regenerative braking and supplies this energy back to the energy storage device. It is also possible to configure the drive system such that the APU can move the vehicle while simultaneously recharging the energy storage system. The parallel-hybrid configuration can be designed in either an engine-dominant or a battery-dominant subtype.

Each hybrid configuration has its own advantages and disadvantages, as listed in Table 2.1. The choice in hybrid-electric bus design is usually determined based on its intended duty application, such as central city urban versus long distance arterial service routes.

 

Table 2.1: Comparison of Hybrid Configurations

Type of Hybrid Configuration Advantages Disadvantages
Series ● Allows APU to operate independently from the driver's commands for power, which reduces emissions
● Fuel cell compatible
● Energy efficient system when the vehicle is operated in stop and go modes
● Transmission is eliminated
● Electric only capable 		● Mechanical energy of the engine is converted into electrical energy and then reconverted to mechanical in the drive motor
● Less suitable for high-speed highway cruising if equipped with a small APU
Parallel ● The battery provides additional power during accelerations. Hence, the engine can be sized smaller than in conventional diesel buses for comparable accelerations
● Direct mechanical drive path is more efficient in certain drive modes, particularly high-speed steady state, such as highway cruising 		● Does not typically facilitate the installation of a non-mechanical APU, such as a fuel cell
● Less capable of capturing all available regenerative braking energywhen small battery packs are used in engine dominant designs

2.4 Electric and Hybrid-Electric Bus Components

Auxiliary Power Units: APUs used in hybrid-electric buses are available in a number of configurations including reciprocating internal combustion engines, fuel cells and microturbines, and with different fuels, such as diesel, gasoline and compressed natural gas (CNG), liquid natural gas (LNG) and propane. The choice of the APU affects the performance of the bus, overall efficiency and emissions as is true with a conventional bus. The following describes four types of APUs.

Internal Combustion Engines: Engines utilized in hybrids can be the same engines used in conventional buses. However, they tend to be smaller, because the bus does not rely entirely on the engine for peak power output at the axles. Instead, the engine is typically sized for the average bus power demand, not peak power demand since the energy storage device provides supplementary power. The advantage of hybrids is that for the same output, a smaller engine operating at a higher percentage output can be more efficient than a larger engine operated at a lower percentage output due to internal engine losses. Engines in hybrid configurations also operate over a narrower range of load and speed combinations compared to engines in conventional buses. The power rating for diesel engines for 40 foot buses operating in a hybrid configuration range can be as small as 150 horsepower, compared to 250 - 275 horsepower in the same size conventional bus. In a series application the engine can be determined on its power, not its torque, which may further increase the system fuel efficiency and fuel options.

One of the obstacles that hybrid-electric bus manufacturers face is that the engines for heavy-duty transit buses must be certified for both emissions and durability using the Federal Transient Procedure (FTP). However, the smaller sized engines that are optimized for series hybrid-electric buses are not typically utilized or certified for use in conventional buses. Thus, even though they meet the emission requirements in a hybrid configuration, the engine may not by itself meet the durability requirements.

Microturbines: While a microturbine could not be utilized in a conventional bus, it is suitable for a hybrid bus service because its power output can be directly fed to an electric generator. Microturbines have the advantage of few moving parts, which reduces maintenance requirements and noise, and they tend to be lighter than diesel engines of similar power rating. Peak efficiency is maintained at near steady-state operating conditions and they can achieve more complete combustion, which means fewer emissions. However, they typically achieve lower overall fuel efficiency than other APUs due to limited overall effective combustion ratio. The high microturbine operating temperature also requires the installation of a heat recovery system. The special materials required to withstand the high temperatures and the precision in manufacturing of parts causes microturbines to be more expensive than comparable power diesel engines, which may be offset with the microturbine's lower maintenance requirements.

Fuel cells: Fuel cells generate direct current (DC) electricity from the chemical reaction between hydrogen and oxygen ions in a cell, facilitated by a catalyst. Fuel cells are attractive as APUs because there are relatively few moving parts, which reduce maintenance costs, and the only by-products are water and thermal energy when utilizing a pure hydrogen fuel. In theory, fuel cells also have a high efficiency for converting chemical energy into electricity. However, energy consumption for producing pure hydrogen fuel can reduce the overall energy efficiency when considered from a well to wheels basis.

Traction Motors: Two primary types of electric motors can be used in electric vehicles, DC motors and alternate current (AC) motors. On a power comparative basis, an AC motor generally exhibits higher efficiency, has a favorable power to size/weight ratio, is less expensive and generates regenerative braking energy more efficiently than a DC motor. However, AC motors require an inverter and more expensive controller, increasing the associated cost. An electric vehicle power train design based on a DC motor may be slightly less efficient overall and the DC motors themselves are more expensive. However, the controllers for DC motors are generally less expensive making the total cost compare between the two types of motors.

 

Table 2.2: Electric Motor Comparison

AC Motor DC Motor
Single-speed transmission Multi-speed transmission
Less expensive More expensive
95% efficiency at full load 85 - 95% efficiency at full load
Motor/controller/ inverter more expensive Motor/controller less expensive

Source: US Department of Energy, "Electric Bus Power Systems," National Alternative Fuels, 2002.

 

A motor is typically chosen based on specific considerations of the vehicle application, such as the road-load profile, maxi­mum speed of the duty cycle and the maximum grade. Where a regenerative brak­ing system is employed, the motor also doubles as a generator producing electric­ity during vehicle braking. AC motors are more com­mon in buses than DC motors. Table 2.2 shows the characteristics of the two types of motors used in electric buses and hybrid-electric buses.

Even efficient motors lose some energy input as heat. If this heat is not dissipated, the motor will overheat and fail or operate at a reduced effi­ciency. Either air-cooling or water/fluid cooling can be employed depending on the design, size and power rating and operating conditions of the motor. While water/fluid cooling is more efficient, potential disadvantages are increased system complexity and maintenance.

Electric drive motors are connected to the vehicle wheels either directly, referred to as wheel motors, or through a transmission and ring and pinion/differential assembly. Wheel motors are more efficient both in drive cycle and in the regenerative cycle by eliminating the losses in the mechanical transmission and the differential. The use of wheel motors has an added benefit of accommodating the "full" low floor bus design. However, wheel motors are expensive, have somewhat lower reliability to date, and the cooling systems tend to be more elaborate and complex in design. There is also a concern about heavy un-sprung mass that must be accounted for in suspension design. At least two wheel motors are needed to drive a vehicle, whereas drives through transmission and differential systems may be designed with a single motor. Improvements in cost, durability, and efficiency in both AC and DC motor technology are constantly occurring.

Controller and Inverter/Rectifier: The electronic controller regulates the amount of energy, (DC power in the case of batteries), that is transferred or converted to AC power by the inverter (in AC motors) for acceleration. It also ensures that voltage is maintained within the specifications required for operating the motor. An electronic controller can also recover electrical energy by switching the motor to a generator in order to capture the vehicle's kinetic energy via regenerative braking. The controller also ensures that the regenerative current does not overcharge a battery.

Inverters are also used on board pure electric vehicles when the drive motor is an AC motor; the DC voltage of the battery is converted into AC for powering the motor. In such a design, the regenerative breaking energy generated is in the form of AC current and is "rectified" or converted to DC by the "rectifier." The batteries always require DC current to recharge.

A loss of energy as heat is inevitable in both inversion, as well as rectification. Hence, for optimal performance, the inverter and rectifier must be either designed to withstand high temperatures or be provided with active cooling by a fan or liquid cooling. The conversion efficiency of these devices ranges from 87 percent to 96 percent.

Energy Storage Devices Battery: A device that stores chemical energy and releases electrical energy. Capacitor: A device that stores energy electrostati­cally and releases electrical energy. Electromechanical Flywheel: A device that stores rotational kinetic energy and can release that kinetic energy to an electric motor system, thereby produc­ing electrical energy.

Additionally, in grid-connected electric vehicles, the controller acts as an interface between the external charger and the on-board battery pack to ensure the optimal strategy for charging the batteries. It monitors the voltage and temperature of the battery pack and maintains the temperature within specified limits to ensure optimal battery performance. Monitoring, in more advanced controllers, can also include tracking of individual cell voltages in the battery to ensure a balanced battery operation.

Energy Storage Devices: Energy storage devices provide all of the energy in electric buses, and are also necessary in hybrid-electric buses to supplement the APU energy when there is a high demand (e.g., acceleration from stop, speed acceleration, climbing an up-hill gradient) and to recover and store the energy generated during deceleration (e.g., braking, down-hill coasting). The major challenge when choosing an energy storage device is to minimize size and weight, while maintaining or improving vehicle performance and efficiency. Batteries represent the single most reliable and proven technology among the energy storage devices commercially available in the transportation industry, although they are not the most efficient manner for energy storage. Considerable research is underway to utilize alternate types, including flywheels and ultracapcitors.

A battery system used in a vehicle consists of several individual batteries or modules connected in series to provide the required voltage for the vehicle. Each battery further consists of individual cells connected serially; any imbalance in the cell voltages may result in degradation in the overall battery performance. Battery technology is still evolving with a number of chemical combinations. The most common types used in transportation include lead-acid, nickel-metal hydride (NiMH), nickel cadmium (NiCd) and lithium (Li). Each one of these battery types has specific characteristics that can improve or decrease the performance of an electric vehicle. Lead-acid batteries have been utilized traditionally because of their proven reliability and relatively low cost. More recently a trend has been leaning towards lithium battery chemistries, which have been found to be more energy efficient with longer lifespans, but with a higher associated cost.In practical terms, the choice of a battery often comes down to a trade-off between performance and cost.

Below are some important issues related to the selection of batteries.

1. Energy Density: Battery weight can be a significant part of the overall weight of a bus. For example, a lead-acid battery pack with its corresponding bus packaging can weigh as much as 4,000 to 5,000 pounds. Thus a battery system with a high gravimetric energy density, rated in terms of watt-hours per kilogram (Wh/kg), will provide for better performance and range. Lead-acid batteries have a relatively low energy density (30 to 40 Wh/kg), while NiMH and NiCd have a higher range (50 to 60 Wh/kg).

2. Battery Lifecycle: Batteries are a high replacement cost item and factor greatly into the reliability and operating cost of the bus. Low cost batteries usually need to be replaced more often. It is prudent to look at the total lifecycle cost of the batteries and not simply the price of the pack or individual module.

3. Recharge Time: Once battery packs are depleted of energy, the recharge time for lead-acid batteries can be lengthy (six to eight hours), although new charging technologies may be able to decrease this time. Other types of battery chemistry, such as NiCd and NiMH can sustain higher voltage recharging methods, which can reduce the recharge time to less than an hour.

4. Power Density: The power of a cell is the ability to discharge and accept energy at a given rate. This parameter indicates how rapidly the cell can be discharged and how much power is generated and is expressed in units of watts per kilogram. This characteristic helps determine the magnitude of acceleration of a vehicle. The higher the power density of a battery is, the higher the acceleration that can be achieved. However, the higher the power density is, the quicker the battery may be discharged and, therefore, the shorter the duration over which the battery is effective. In general, batteries with higher energy densities exhibit significant voltage and capacity drops at higher discharge rates and therefore have a lower power density.

5. Charge/Discharge Efficiencies: The efficiency of a battery is defined as the ratio of the energy delivered by a battery during discharge to the total energy required to restore it to a full state-of-charge. The battery efficiency decreases with usage and is a function of the number of charge-discharge cycles.

6. Maintenance: Both flooded lead-acid and NiCd batteries need to be refilled regularly to replenish the water that is lost to electrolysis during the battery charge and discharge cycles. Flooded lead-acid batteries need distilled water replenishing about twice per week and NiCd about once every 10 days.

7. Cost: Due to the use of unique metals, production volumes, or the status of the battery technology, the price range of batteries varies widely. Lithium batteries are the most expensive, while lead-acid batteries are at the lower end of the price range.

8. Recycling: Many manufacturers provide detailed information on the disposal or recycling of batteries. These protocols should be followed to ensure that hazardous materials in batteries, such as lead, cadmium, and corrosives are not released into the environment when the useful life of the battery is completed. Fluids used for the cooling of components are generally similar to other vehicle fluids, such as water, oil, and glycol, and should be disposed of properly.

9. Interchangeability: When replacing batteries of one manufacturer or model with that of another, care should be exercised to ensure that the battery specifications are the same and that they have the same ratings in power, power density, size and voltage. Battery switching may require checking with the original equipment manufacturer to ensure compatibility with the warranty. Generally, all batteries in a pack should be of the same type, manufacturer and condition.

10. Safety: The precautions to be taken in placement of the battery pack and personnel training depend on the type of battery. For example, lead-acid and NiMH batteries can potentially release hydrogen when overheated. Hence, the storage box needs to be well ventilated and the temperature needs to be monitored. Also, there is the potential for electrolyte leaks from batteries.

11. Application Drive Cycle: The magnitude of accelerations and decelerations experienced by a bus and hence, the rates of energy drawn from and retained to the batteries greatly affects the overall range and efficiency performance of the bus. The distance to which a bus can be driven on a fully charged battery pack is dependent upon a driver's ability to operate the bus; the greater the accelerations and decelerations, the lower the range on a single charge. This is because: (1) fast accelerations draw energy from the battery more quickly, resulting in lower "effective" battery capacity, and (2) quick decelerations are not efficient for capturing regenerated energy due to limitations on acceptance by the battery pack.

While the range of a typical hybrid-electric bus may not be significantly affected by battery capacity, battery efficiency will impact fuel economy and emissions. In an electric bus with a slow operating cycle, a lead-acid battery may be a better choice, while a transit system with a fast transient cycle application might choose NiMH.

12. Warranty: A transit agency should check with the battery manufacturer that the transit system's battery charger and charging procedure is appropriate, and will not void the battery warranty. For example, some battery manufacturers are hesitant about fast charging battery packs.

Table 2.3 provides additional information on different types of batteries. A good source of information on batteries is the United States Advanced Battery Consortium, whose contact information is listed in Appendix A.

 

2.4.3 Battery Thermal Management System

Batteries also require a thermal management system because the discharge and recharge processes result in a net production of heat. If this heat is not dissipated quickly, the battery temperature can increase substantially resulting in accelerated self-discharge, degraded battery cycle life or battery failure. The efficiencies of a battery to accept or discharge current are also dependent on battery temperature. For example, lead-acid batteries are negatively affected by the cold, but other battery types may have issues with hot temperatures. This is also true of conventional buses, which see a degradation in efficiency in cold temperatures.

 

Table 2.3: Performance of Some Advanced Electric Bus Battery Systems

Battery Mfgr/Types

Energy Density (Wh/kg)

Power Density (W/kg)

Life Cycles per battery

Advanced Lead Acid

48

150

800

GM Ovonic NiMH

70

220

>600

SAFT NiMH

70

150

1500

SAFT Lithium Ion

120

230

600

Lithium Polymer

150

350

<600

Zebra Sodium-Nickel Chloride

86

150

<1000

USABC Short-Term Goals

86

150

600

USABC Long-Term Goals

200

400

1000

Source: US Department Of Energy, "Electric Bus Batteries," DOE Fields Program, 2002, available at http://ev.inel.gov/fop/general_info/battery.html

 

Figure 2.4 shows lead-acid and NiMH battery behaviors at lower than normal temperatures. It is recommended that batteries used on electric and hybrid-electric vehicles have a thermal management system, which regulates the temperature in the battery pack within tolerable limits, which is important for warranty and life expectancy.

 

 

 

Figure 2.4: Effects of Temperature on Batteries (Normalized to 21°C)Figure 2.4: Effects of Temperature on Batteries (Normalized to 21 degrees Celcius)

Source: EVermont, Advanced Battery Management and Technology Project, NAVC1096-PF009524, August 1999.

 

It is also recommended that the battery pack have a battery management system (BMS). This system consists of a microprocessor that monitors energy, as well as temperature, individual cell or module voltages, and total pack voltage. The BMS can adjust the control strategy algorithms to maintain the batteries at uniform state of charge and optimal temperatures.

2.4.4 Replacement of the Battery Pack

Eventually an electric bus or hybrid-electric bus operator will need to replace the bus battery pack or modules within the pack. If the electric bus or hybrid-electric bus experiences reduced battery range or voltage fluctuations before the predicted end of life and while operating within normal parameters, such as temperature, then individual module failures, or even poor connections, may be the cause of the malfunction. Swapping individual modules will require that the new replacement modules be at the same approximate state of charge as the rest of the pack or that the bus is equipped with a battery management system. Analyzing an entire battery pack is labor intensive, so current wisdom dictates that a malfunctioning battery pack more than halfway through its life cycle should be replaced and recycled.

Rectifier/Charger (external to vehicle): In general, 3 phase, 240/480 Volt (V) AC current is supplied to the batteries from a wall transformer/rectifier unit, which converts the AC electricity from the electric utility grid to DC. Normal electric bus battery pack voltage ranges from 240 to 360 VDC, although electric vehicles can have voltages between 100 V and 600 V (nominal). Battery charger designs come in a number of different charging strategies based on the way they control the recharge rate, such as current charge, constant voltage, or fast charge followed by cell voltage balancing.

Total battery recharge time is dependent on voltage, current, and the charging algorithm. Batteries can be damaged from the heat produced from charging at a high current, or from overcharging. Fast chargers have advanced in recent years, generating less battery heating and, therefore, lowering the risk of battery overheating. Slow chargers are less expensive and their low current rate minimizes potential damage to the battery cells. Most charging systems connect the supply and the bus using a conductive cable, although advances are also being made with inductive charging.

A manufacturer-supplied charging protocol generally involves a rapid charge period followed by a slower "cell voltage balancing" charge. In fleet operations, electric bus batteries are typically charged overnight when the electricity rates are lower. In several areas, such as California, New York, and Arizona, the demand charge for using electricity during peak daytime hours may be ten to a hundred times more expensive than during the night hours. Given high demand charges it may be advantageous to charge as slowly as allowable for a given application and to minimize opportunity charging during the day. Load leveling chargers may also be used that draw a steady current from the electricity grid, store that energy and deliver that energy to the vehicle as a fast charge.

Transit systems that do not experience daytime peak electricity charges may be interested in "opportunity charging" of batteries during service time. Other transit systems buy multiple battery packs to swap in freshly charged battery packs during a 15-minute break when battery energy is running low.

 

Chapter 3: Electric and Hybrid-Electric Bus Safety Issues

The introduction of electric drive and energy storage systems into buses adds several new safety considerations beyond the risks of conventional buses. A major difference is that both the electric bus and hybrid-electric bus have high voltage systems, whereas conventional buses typically have only a 24 V battery system. In addition to design considerations, electrical power systems have profound implications for maintenance operations. The potential additional hazards that electric bus and hybrid-electric bus generate are:

Electric and Hybrid-Electric Training Centers

Mid-Del Technology Center
Oklahoma
405.672.6665
www.evtraining.com

National Alternative Fuels Training Consortium
West Virginia/Nationwide
304.293.7882
www.naftp.nrcce.wvu.edu


In general, most transit personnel are unfamiliar with high voltage safety procedures or they have not undergone the necessary training, and may also lack experience in servicing high-voltage systems and electric drive technology. Transit agencies should include certain safety precautions in bus specifications to minimize risks to passengers, drivers and maintenance personnel. Training for emergency response personnel is also a very important step when incorporating advanced vehicles into fleets. Typically this means going beyond the basic training that manufacturers may offer. Several organizations, such as Mid-Del Technology Center and National Alternative Fuels Training Consortium offer advanced training, and other transit agencies with these types of vehicles may offer the opportunity for personnel to train with them.

The general safety requirements that transit buses, including alternative fueled ones, must meet are the Federal Motor Vehicle Safety Standards (49 CFR 571) issued by the National Highway Traffic Administration. It is also recommended that transit agencies review the American Public Transit Association's Standard Bus Procurement Guidelines, which provides crash-worthiness criteria advice when purchasing buses5. Other equipment codes and specifications are available from Underwriters Laboratory (UL) and Society of Automotive Engineers (SAE).6

In addition to federal regulations, the National Fire Protection Association establishes the National Electric Code (NEC) standards for electrical construction and operation. Articles 511 and 625 apply to the infrastructure (garages and charge equipment) necessary for electric and hybrid-electric fleets. Additionally, states and local cities/counties administer regulations related to electric vehicles and electricity infrastructure, which tend to be consistent with NEC. Transit agencies should check with the state and local building code commission, and state and local fire marshall office for other applicable regulations.

3.1 Electric Shock

The single most important difference between a conventionally fueled bus and an electric bus or a hybrid-electric bus is that, in the latter, there are high voltage circuits that may be active even when the bus is not operating. The effective operating voltage range for electric buses is about 300 V to more than 600 V, but can be as high as 800 V. Before any of the components in an electrical circuit are probed or maintained, personnel must be properly trained to avoid a high voltage shock.

Generally, the electrical systems in electric buses and hybrid-electric buses contain: (1) a low voltage (12 V - 24 V) system for such accessories as the lights, wiper, horn, sensors and instruments, and (2) a high voltage system (50 - 800 V DC and AC) including the traction motor, traction battery pack, rectifying and inverter systems and charging. The low voltage system should be independent of the high voltage system, so that in the event of a high voltage failure, emergency lighting and other accessory devices still operate.

A shock hazard can occur by:

The risk of electric shock can be mitigated through proper engineering, labeling, and safe maintenance practices. In addition to complying with federal and state regulations, the design of an electric or hybrid-electric bus should meet the following criteria:

3.1.1 Enclose and Label High Voltage Parts

Refer to SAE J1673 for specific minimum requirements. The bullets listed below highlight important areas from this recommended practice. Note that SAE J1673 references UL specifications, as well as other SAE recommended practices.

Transit agencies should also request non-proprietary manuals for parts, service, operation and maintenance, interconnection wiring diagrams and schematics. The schematic should show all major components, switches, fuses and circuit breakers.

3.1.2 Electrical Isolation

Another important safety issue of concern when using high voltage electrical systems on vehicles is the potential for occurrence of fires caused by short circuit (from cables whose insulation is damaged) or from overheating due to excessive currents through the wires or the machinery. See SAE J1766 for specific recommendations.

3.1.3 Additional Recommendations

Energy storage devices should be provided with cut out switch(es) to provide electrical isolation of the high voltage system power from all other electrical systems in the vehicle. The design of these switches should provide for their remote operation or hand operation. The cut out switch(es) should be installed at a location that is easily accessible to emergency responders. They should be labeled and easily understood by an individual unfamiliar with electric vehicles, and optionally include physical lock-out/tag-out features for maintenance. The electrical system should conform to SAE standards for wiring (J1654 and J1673) and connectors (J1742).

3.1.4 Battery Box

3.1.5 Automatic Disconnect Devices for Energy Storage Systems

Buses should have an automatic method to disconnect an energized system from the electricity source in case of an overload or short circuit. The disconnect systems include circuit breakers and battery isolation devices. The electrical system design should:

3.1.6 On-Board Charging

The potential for electric shock also exists during battery recharging. The on-board charging system design should comply with UL-2202 and take into consideration the following issues. Transit agencies should consider the following design options. The charger should:

External charging requirements are discussed in the next chapter.

3.1.7 Power Train and Control Systems

Thermal degradation of the vehicle systems may also lead to failure of the components and a resulting loss of electrical isolation.


3.2 Hydrogen Gassing

Another safety issue of great importance is related to the charging of batteries. When some batteries are charged, the evolution of hydrogen occurs due to the dissociation of water in the electrolyte (especially in flooded lead-acid and NiCd batteries). Even sealed batteries, such as NiMH can discharge hydrogen due to overcharging. Prevention of potentially explosive reaction between hydrogen and oxygen gases is of paramount importance; this is achieved by performing battery charging operations in well ventilated areas to ensure that hydrogen concentrations remain below the lower explosion limit.  Additionally, maintenance personnel should ensure that no ignition sources are near the battery charging stations. Use of listed battery charger/battery pack combinations that have been evaluated using SAE J1718 test method and identified as suitable for charging indoors is recommended.

Or

3.3 Fire/Toxic Fumes

In an electric bus or a hybrid-electric bus a significant amount of energy is stored, which creates the potential for energy to be released accidentally by a short circuit. While the short circuit carries electric risk, there also exists a potential for creating a high energy arc leading to the ignition of combustibles. Battery components, other than the plastic casing, are less susceptible to burning in a fire. However, many battery components (electrodes and electrolytes) are made of substances that when heated give off toxic fumes. In addition, several metals used in some batteries, lithium in particular, have the potential to burn at very high temperatures when ignited. Electrical isolation of the energy-storage system before any maintenance occurs on the batteries, cables and circuits is essential. When battery packs are removed or replaced significant care needs to be exercised to ensure that electrical systems are isolated and that no electrolyte leak occurs within the battery compartment. Additional inspections should make certain that no pathway for arcing is possible due to collection of dust or other contaminants.

A fire created by an electrical short circuit cannot be extinguished until the source of electrical energy is disconnected. A fire retardant barrier or coating between the batteries and battery box or the bus itself should be used to prevent, or at the very least delay, the spread of fire. The next level of safety would involve the installation of a fire suppression system to reduce the risk of the fire from spreading to other parts of the vehicle. Fires supression is not currently a requirement on diesel fueled buses, nor would it suppress an electrical short circuit arc.

It is recommended that transit agencies consider specifying that the vehicle have:

3.4 Electrolyte (Acid) Spills

SAE and existing federal regulations require that batteries be designed to minimize the amount of battery electrolyte that could be spilled during a collision. Other safety measures transit agencies should consider:

3.5 Vehicle (Electrical System) Maintenance Issues Related to Safety

Maintenance of the electrical and traction systems in an electric or hybrid-electric vehicle is an extremely important function, which ensures not only the availability of the vehicle, but also its safe operation. Different items require different intervals of maintenance schedule. The inspection and maintenance protocol for the electrical and associated systems should include:

Chapter 4: Safety Issues in the Maintenance/Storage Facility

The intent of this section is to outline potential facility safety risks, and provide guidance on managing and mitigating them. Information is derived from existing codes and standards, industry input, and operational experience. The information is presented as a recommend action.

The general construction standards applicable to the bus maintenance/storage facilities (e.g., National Fire Protection Association (NFPA) 88A for vehicle storage and NFPA 30A for repair garages)8 should be followed for the design and/or construction of buildings in which electric or hybrid-electric buses are parked and serviced. In addition, because of the prevalence of high voltage electric cables, wires, outlets and high power battery chargers in the garages servicing electric bus and hybrid-electric bus, applicable sections of the National Electric Code (NFPA 70) should be followed. Extra precaution should be taken in handling, storing and off-board charging of battery packs. Some of the related safety issues are indicated in the next sections. It is important that transit agencies adhere to the most current applicable codes and standards to ensure safety.

4.1 Battery Off-Loading & Handling

Battery packs in electric buses and hybrid-electric buses are bulky and typically weigh 600 - 2,000 pounds. Removing and reinstalling packs has to be conducted with care and precision. In performing these functions, the following safety issues need to be observed:

4.2 Battery Storage

4.3 Battery Charging

4.3.1 Battery Charger Safety and Location

Transit agencies should discuss their charger choice with their electricity provider to ensure that the type of electricity is available. Level 3 charging often requires wiring upgrades. Level 2 and Level 3 (typically fast charging) require the elements listed in Table 4.1.

 

Table 4.1: Battery Charger Requirements

  Voltage (VAC) Current (amps) Power (kVA) Frequency (Hertz) Phase
Level 2 208/240 32 6.7/7.7 60 Single
Level 3 208/240 or 480 400 192 60 Three

Source: Massachusetts Division of Energy Resources, 2000

Battery chargers should:

The location of battery charging equipment should be taken into consideration. For example, if the charger is exposed to weather, it should be UL listed for outdoor use. The charging cables, wiring, materials should be sunlight (UV) resistant.

Charging equipment can produce considerable heat during the charging process. This heat must be dissipated by water or air-cooling of the equipment. Sufficient safeguards must be engineered into the charging units that if the temperature exceeds a set safe limit the charging is automatically shut off.

4.4 Facility Fire Detection and Protection Systems

The fire protection system provided in the bus garage should be commensurate with the electrical charging activity and battery storage that occurs in the maintenance facility. NFPA 72 provides some guidance in this area. At the very minimum, the facility fire protection system should include the following:

Chapter 5: Personnel Training

Training of transit personnel involved in all levels of operation of electric or hybrid-electric buses is an important part in achieving a safe, economic and successful bus operation. Because it will require additional effort initially, transit agencies should address concerns from personnel and educate them as to why electric and hybrid-electric vehicles are an important element of the transit bus fleet. Personnel can influence future purchases if initial experiences are not positive, which can occur without proper training and support.

Additionally, transit agencies should be aware that new vehicle technology will require an investment of time, particularly in trouble shooting tasks, and in securing additional unique replacement parts. Transit agencies with electric and hybrid-electric technology, such as New York City (NYC) highlighted on the next page, reported that personnel experienced a learning curve. Hence, the more experience and training provided in the beginning, whether attending classes or shadowing personnel at a facility with the technology, the better equipped transit agency personnel will be in implementing a smooth transition.

While electric and hybrid-electric buses are driven and operated similarly to conventional buses, transit agencies are still encouraged to allow for one week minimum for personnel training upon the arrival of new electric and hybrid-electric buses. In addition to training its own personnel, a transit system should provide information and some basic training to local emergency response personnel who may be required to respond to accidents involving buses with high voltage systems.

Some of the training issues that a transit system should consider implementing are discussed below.

5.1 Training of Transit Vehicle Operators

The efficiency of an electric bus depends significantly upon the extent of training received by bus drivers on the proper operation of the electric traction system and its details, such as regenerative braking. The training of an operator should consist of at least, the following elements: