Download Multi-Cell Lithium Ion Battery Management System

2010
Multi-Cell Lithium Ion Battery Management System
-For Electric Vehicles
Team: Sdmay11-04
Pramit Tamrakar
Jimmy Skadal
Matthew Schulte
Hao Wang
10/12/2010
Table of Contents
Definitions ..................................................................................................................................................... 4
Executive Summary ...................................................................................................................................... 5
Acknowledgement ........................................................................................................................................ 6
Statement and Approach ............................................................................................................................... 6
Problem Statement .................................................................................................................................... 6
Solution Approach .................................................................................................................................... 6
Operating Environment ................................................................................................................................. 6
Intended Users and Intended Uses............................................................................................................... 7
Intended Users .......................................................................................................................................... 7
Intended Uses ............................................................................................................................................ 7
Assumptions and Limitations ....................................................................................................................... 7
Assumptions.............................................................................................................................................. 7
Limitations ................................................................................................................................................ 7
Expected End Product ................................................................................................................................... 8
System Design Approach .............................................................................................................................. 8
Functional Requirements .......................................................................................................................... 8
Non Functional Requirements .................................................................................................................. 8
Market Alternatives .................................................................................................................................. 9
Proposed Approach and statement of work............................................................................................... 10
Proposed Approach ................................................................................................................................ 10
Constraints considerations ................................................................................................................. 10
Technology Consideration .................................................................................................................. 10
Technical Approach Consideration ..................................................................................................... 13
Testing Requirements Considerations ................................................................................................ 13
Security Consideration ........................................................................................................................ 14
Safety Consideration ........................................................................................................................... 14
Intellectual Property Consideration .................................................................................................... 14
Commercialization considerations...................................................................................................... 14
Possible risks and risk management ................................................................................................... 15
1
Project proposed milestones and evaluation criteria ......................................................................... 16
Project tracking procedures ................................................................................................................ 16
Hardware/Software Statement of work ................................................................................................. 17
Resources and Schedules ............................................................................................................................ 18
Resources ................................................................................................................................................ 18
Schedules ................................................................................................................................................ 19
Closure Material.......................................................................................................................................... 19
Project Team Information ........................................................................................................................... 20
Client’s Information ................................................................................................................................ 20
Faculty advisor Information .................................................................................................................... 20
Student Team Information...................................................................................................................... 20
Closing Summery ........................................................................................................................................ 21
References .................................................................................................................................................. 22
Appendix A .................................................................................................................................................. 25
2
Table of Figures
Figure 1: The NLG503-light battery charger. 1.6 kW 200-540V, $2,145 ...................................................... 9
Figure 2: Hardware Functional Block Diagram ........................................................................................... 11
Figure 3: Software Functional Block Diagram ............................................................................................. 11
Figure 4: The series scalability of the bq76pl536........................................................................................ 13
3
Definitions
Terms
EV
DC
AC
Fuel gauge
Power Converter
Feedback
Overcharging
BMS
PWM
PICCOLO
Constant Current
Constant Voltage
Thermal Runaway
Fast Charge
SPI
SMBus
Level 1 Charging
Definition
Electric Vehicle
Direct current
Alternating current
Device to measure overall status of the battery
A Device for stepping up or down the voltage
Used stabilize the signal of the output of the
device
Continuing to charge a fully charged battery
Warning! Overcharging could lead to
overheating which could cause explosion,
damage, or shortened life of the cell.
Battery Management System
Pulse Width Modulation
Digital Processing Chip
The current is held a specific value(C = 3A)
until a the adequate voltage is achieved
The voltage is held at specific value (3.6V)
until the cell is fully charged (current is 0.1C)
Overheating of cell in relation to overcharging
A condition where 10A is used to charge the
cell in 15 minutes
Serial Peripheral Interface - bus for device
communication
System Management Bus This is an industry standard Bus protocol
Ac energy to the vehicle’s onboard charger
from a typical 120V outlet. 120Vac; 16A
(1.8kW)
4
Executive Summary
Electric vehicles are one of the cleanest, most efficient, and most cost effective form of
transportation. The market demand for electric vehicles has increased ever since the
Toyota Prius was introduced as an alternative to traditional oil vehicles. (Wired) Other
vehicle manufacturers are now introducing full electric vehicles, including Tesla Motors,
Nissan, and Ford. A major complication in developing an electric vehicle is the battery
management system. This project’s goal is to find an efficient and safe way to charge
and monitor multi-cell series lithium ion batteries for an electric vehicle using AC to DC
converters and monitoring microcontrollers
The approach to accomplish this task is twofold: develop a charging system and develop
a battery management system. In order to focus more on the battery management system,
and to prevent from electric shock due to high voltage, we are scaling down the power
supply from 120V AC to about 12-16 VAC. This is an agreement with our faculty
advisor and the client. Therefore, when we have to bring the system to a large scale it will
be just the matter of building up from smaller abstract systems.
After intensive research and valuable advice from our advisor, the most efficient way to
transform the 12-16V AC to 35V DC to completely charge the multi-cell series batteries
will be by using a line filter, full-wave rectifier, AC-DC power Correction boost preregulator and a DC-DC buck converter.(UCC28019) An evaluation module from Texas
Instruments using the UCC28019 PFC boost converter controller will be used to decrease
development time for our initial project specifications. Focus will thus be directed at
developing the battery management system. This system will monitor the charge of each
individual cell in order to prevent the batteries from overheating, overcharging, or over
discharging. The result of overheating, overcharging, or over discharging can include loss
of cell functionality, fire, or even explosion. The approach used to handle this major
problem is to use a fuel gauge to monitor the charge of the batteries while monitoring the
temperature of the cells. The fuel gauge will keep track of how much power is remaining
in the batteries, sending the information to the control unit for processing.
The expected functionality of the design is to efficiently and safely charge multi-cell
lithium ion batteries. It will take about 12V AC from the source and charge the series
multi-cell lithium ion batteries while checking for overcharging and overheating. Once
the fuel gauge senses the lithium ion batteries are full, it will shut off the source of the
12Vac and monitor the charge levels of the batteries and output the signals for processing
in the central control unit.
5
Acknowledgement
We would like to acknowledge the client for this project, Adan Cervantes, from Element
1 Systems. He will be giving us direction, feedback, and guidance throughout the process
of this project. He will also be contributing the vehicle to be tested, the bank of lithium
ion batteries, the engine controller, and the engine. Adan will also provide financial aid
for this project.
Secondly, we would like to acknowledge our faculty advisor, Professor Ayman Fayed for
giving valuable guidance and advice in order to achieve the goals of our project.
Statement and Approach
Problem Statement
To develop an efficient, safe and scalable system for charging and monitoring multi-cell
series batteries in electric vehicles using a switching mode power supply and a battery
management system. The initial goal of this project was to charge the bank of lithium-ion
batteries to 324 VDC supplied from a 120 VAC wall outlet. In order to focus more on
the battery management system of this project, we are scaling down the power supply to
12-16 VAC and an output of 35 VDC, and developing a battery management system for
6-12 series batteries that can later be scaled to 90 series batteries.
Solution Approach
The first task to accomplishing this project is to find the most efficient way to bring 12V
AC to 35V DC. This is accomplished by using an AC-DC power factor correction boost
pre-regulator and a buck converter. (UCC28019) Tests will be run on the circuit design
through computer simulations and also through carefully run physical tests via the
department’s power lab. We will be using evaluation modules from Texas Instruments
and microcontrollers to monitor the batteries.
Operating Environment
This product must operate in various conditions. It needs to handle dusty conditions, due
to sitting in garages or driving down a gravel road. It will also need to be able to
withstand typical summer and winter temperatures. The batteries and controllers will be
shielded from the rain and wet conditions to prevent short circuits. The circuits and
batteries will never be thrown or dropped since they will be semi-permanently fixed in
the vehicle, but they will have to withstand shocks from the road.
6
Intended Users and Intended Uses
Intended Users
The users of the final design will be the vehicle owners, family members, friends, etc.
They could range anywhere between 14 to 100 years old and could be of any sex. As long
as they can pick up an extension cord and plug it into the outlet and to the vehicle,
anyone could use this project. However, since this project is only a scaled down version
of the prototype the intended user is the client, Element 1 Systems.
Intended Uses
The intended use of the project is to charge a bank of lithium ion batteries and manage
their charge and discharge cycles. The high voltage supply is designed to operate as a
constant-current constant-voltage lithium ion battery supply, and only for the specified
A123 battery chemistry.
Assumptions and Limitations
Assumptions




The user has a 120V outlet available to plug in the electric vehicle.
The batteries are provided by the client and capable of operating and running the engine.
The components in the design are expandable to handle more than rated power.
This is only a prototype. The client is responsible for integration into the final system.
Limitations




High voltage control.
Wall outlet power availability.
The batteries and the circuits must fit into the vehicle.
Safety: In order to develop a scalable abstract system, only 12 cells in series will be used
in our design, rather than all 90 in series and 16 in parallel. The developed solution will
be scalable in order to handle more series cells in the future.
 Parts availability.
7
Expected End Product
The expected finished product will consist of the scaled down version of the original
project, with the associated documentation, design, construction plans, and software
necessary to complete the finalized product. The completed product will be delivered to
Adan Cervantes from Element 1 Systems in May 2011.
System Design Approach
Functional Requirements
The project goal is for the charger module to charge a battery bank with sixteen parallel
branches, with each branch having 90 series cells. Our initial design will be limited to
12 series cells to ensure system functionality and scalability. A switching mode power
supply will convert the input wall outlet power from 120Vac to 324dc (or 12Vac to
32Vdc for the scaled down version). The charging and discharging of these cells will
be monitored with a battery management system to prevent overcharge, over discharge,
and over temperature situations.
Non Functional Requirements
 The scaled prototype should be usable by our client during the development of the scaled
up version.
 The system should be reliable, even in the condition of a fault.
 System maintenance should be straightforward.
 Price should be as low as possible to ensure the product is a competitive market
alternative.
 The system should be robust and long-lasting.
 Total weight should be kept to a minimum.
 The system should be using the most efficient methods available.
 The end product should be designed to ensure safety and prevent the user from coming
into contact with high voltages and currents.
8
Market Alternatives
Since only a handful of electric vehicles have been successfully implemented on a large
scale, there are few existing commercial solutions. As for the switching mode power
supply the only commercially available solution designed specifically for electric
vehicles is offered by Brusa. The NLG5 is a battery charger that provides a high voltage
power source from a 120V or 240V wall outlet. The only problem with using a Brusa
battery charger is cost; their simplest charger costs over $2,000. (Brusa)
Figure 1: The NLG503-light battery charger. 1.6 kW 200-540V, $2,145 (Brusa)
As for battery management systems, even Brusa is still developing a commercial
solution. Thus, there are no market alternatives to building a custom BMS. (Brusa)
9
Proposed Approach and statement of work
Proposed Approach
Constraints considerations
Lithium ion batteries require a protection system to maintain safe voltage and current
levels. They may suffer thermal runaway and cell rupture if overheated or
overcharged. Furthermore, over-discharge can irreversibly damage a battery. To
reduce these risks, we have to design a circuit that shuts down when the lithium ion
cells vary outside the safe range of 1.6–3.6 V.
The amount of power a typical 120V wall outlet in the United States can provide is
limited to about 1.8 kW, which is defined to be level 1 charging. (Electric) This will
constrain our charging time, as the battery will typically only be able to draw 1.8 kW.
Since our battery pack holds about 11 kW-hr, the minimum charging time will be
about 6 hours.
Technology Consideration
An MSP430 will be used to control the charging process in three stages;
As demonstrated in Appendix A:
 Slow charge: Pre- charging stage using current of 0.1C (where C is 3A).
 Constant-current charging stage: Using current of 1C.
 Constant voltage charging stage: Maintaining the nominal max battery voltage
until the minimum current is supplied, which is 0.1C
As shown in Figure 2 the microcontroller is used to control the buck converter by
increasing or decreasing the voltage to maintain the current during the slow charge and
constant current charging stages, and it will maintain a constant voltage during the
constant voltage charging stage.
10
Figure 2: Hardware Functional Block Diagram
The microcontroller software will be implemented according to the software functional diagram
in Figure 3 using either C or Assembly language.
Figure 3: Software Functional Block Diagram
11
A comparison of two similar integrated circuits from Texas Instruments:
 bq78PL114
According to the TI data sheet slua495 for the bq78PL114 “The minimum number of
parallel cells is 1. The maximum number of parallel cells is limited by the system
capacity which is a16-bit unsigned integer.” This data sheet only refers to parallel
configurations of the bq78PL114. Nowhere does it talk about an implementation of a
series configuration. The bq78PL114 with the bq76PL102 allow for charging 12 series
cells alone. The ability to charge more than 12 cells with this design may be possible, but
very difficult to achieve. With no SMBus built into this IC, a series configuration of more
than 12 cells may be very difficult to implement. Also TI would have to write a custom
TMAP file for each bq78PL114 to assign an address for SMBus compatibility. The
bq78PL114 requires communication with a computer using the .Net communication
protocol, which we would we have to use to simulate the bq78PL114 API on a
microcontroller. With these many requirement and setbacks, we looked to find a more
suitable IC that would supply features more suitable for the design. (BQ78PL114)
 bq76PL536
After running into many issues with the bq78PL114 design we decided on the
bq76PL536 to give us the needed features that the previous configuration did not.
According to the TI data sheet, “The bq76PL536 can be stacked vertically to monitor up
to 192 cells without additional isolation components between ICs. A high-speed serial
peripheral interface (SPI) bus operates between each bq76PL536 to provide reliable
communications through a high-voltage battery cell stack.”
This device can run at a Continuous 36 V Peak with respect to the voltage of the bottom
most cell in the series. Charging 6 cells at 3.6 V the voltage required is 21.6 V, and so the
bq76PL536 can handle the capacity of our supplied cells. With the ability to connect up
to 192 cells in series, the bq76PL536 is the perfect choice for this project. Also another
advantage to using the bq76PL536 is that there is already a SMBus built in to this IC
which will allow easy communication of series configurations. With the bq76PL536
hooked up in series one Host interface is required to communicate with the system which
is a lot easier than the previous design. (Battery)
12
Figure 4: The series scalability of the bq76pl536
Technical Approach Consideration
Since the bq76PL536 is designed to balance 6 series battery cells we have to combine
15 of them to monitor and manage 90 series cells.
The battery management system relies on the MSP430 and the Texas Instruments
bq76PL536 microcontroller to implement the design requirements. These
microcontrollers and integrated circuits exceed all of our requirements for undervoltage, over-voltage, and over-temperature protection. Programming of these
integrated circuits includes learning the SMBus protocol which is a set of industry
defined standards allows a host controller to easily obtain large amounts of data from
the battery over a single communication interface. In our case, we will be using SPI
with an MSP430 to gather the data and make decisions based on battery conditions.
Testing Requirements Considerations
The input voltage is 120 Vac. Since we are scaling the conditions down by a factor of
10, the output for our testing simulation with 6 cells will be between 9.6V and 22.8V.
The goal of scaling this system means we will also be testing with an additional 6 cells
managed by a separate bq76PL536 circuits. The battery voltage range for this
configuration will be between 19.2V and 45.6V.
13
The charging current C is defined to be 3A. This is a high current, and our initial
design will be limited to 1A to ensure components do not unnoticeably overheat or
explode in the case of an accident.
Security Consideration
The bq76PL536 integrates dedicated overcharge and under-voltage fault detection for
each cell and two over-temperature fault detection inputs for our device. The protection
circuits use a separate band-gap reference from the ADC system and operate
independently. The protector also uses separate I/O pins from the main
communications bus, and therefore is capable of signaling faults in hardware without
intervention from the host MSP430.
Safety Consideration
When dealing with high voltages:
1. Keep one hand in a pocket to prevent conduction channel through the heart.
2. Set up a work area away from possible grounds.
3. If circuit boards need to be removed from its mountings use insulating material.
4. Discharge high voltage capacitors appropriately.
5. Remove metal objects such as jewelry.
6. Prove that exposed metal surfaces are grounded, as are outlet grounds.
7. Do not assume insulation integrity.
8. Do not leave an experiment unattended.
9. Do not work on an experiment while tired or not alert.
10. Ensure that someone is trained in CPR.
(Notes)
Intellectual Property Consideration
The design for this project could potentially be patented, trademarked, and copyrighted.
Patent protection can be applied for with the U.S. Copyright Office, which handles
copyright registration in order to ensure a market claim to the product. However, we
need to check to see if a patent is already registered.
Commercialization considerations
Our design is a prototype for our client; it could potentially become a commercial
venture.
14
Possible risks and risk management
Risks have been identified throughout the project and tracked for resolution and
mitigation. A risk register is used to identify risk to the project:
Risk Register:
No
Risk
Risk Description
1
High power systems
Generate 324
VDC for
electrical car
motor
2
Electric shock
3
IC replacement
The risk of
electric shock is
possible when
working with a
charging system
In case any small
part of the system
malfunctions.
4
Over-temperature
Over-voltage
Under-voltage
5
Weight
6
Cost
Cells may suffer
thermal runaway
and cell rupture if
overheated or
overcharged
The mass of the
system.
Cost should be as
low as possible
Mitigation
Run simulations before physically
testing.
Start at a lower voltage level to test the
components before using it at the higher
voltage level.
Be careful when doing the circuit
testing, follow the rules listed in the
safety consideration.
Disconnect the problem section and
determine if further shut down is
necessary. Replace the faulty
component.
The bq76PL536 provided by TI is
available for protecting our system.
Ensure lightweight components are
utilized when possible
Only purchase necessary components.
Introduces risk with schedule if
replacement contingency parts are not
available.
15
Project proposed milestones and evaluation criteria
We have lists of weekly tasks that need to be accomplished which are then checked off
by the group members. The tasks accomplished by the group are evaluated by the team
and our advisor.
Project tracking procedures
Project planning will be performed using the Gantt chart schedule and our progress will be
tracked using the following schedule:
16
Hardware/Software Statement of work
Task 1 - Problem Definition:
Design a charger and battery management system operating from a 12 Vac source.
Subtask 1a – Define the problem
Subtask 1b – Identify the intended audience and End-Use
Subtask 1c – Define the constraints
Task 2 – PFC Boost Converter design:
Boost the input 12Vac to 35Vdc.
Subtask 2a – Compare similar topologies.
Subtask 2b – Design or purchase the PCB and schematic
Task 3 – Buck Converter design
Buck the output of the PFC boost from 35V to between 14.4V and 32.4V.
Subtask 3a – Select component values
Subtask 3b – Simulate
Subtask 3c – Code an MSP430 to control the PWM
Task 4 – BQ76PL536 BMS system design
Implement a scalable battery management system using two BQ76PL536 boards to
monitor six series cells each.
Subtask 4a – Design a scalable PCB.
Subtask 4b – Select appropriate component values
Subtask 4c – Code an MSP430 to control the cell balancing.
Task 5 – Building and Testing
Build and test the systems designed in Tasks 2 through 4
Subtask 5a – Gather materials and components
Subtask 5b – Fabricate PCBs
Subtask 5c – Code the various MSP430’s
Subtask 5d – Test each system individually and together for complete
functionality.
Task 6 – Documentation and Demonstration
Provide End-Project Documentation and Project Reporting, as well as demonstrate the
project to faculty.
Subtask 5a – Write End-Project Documentation
Subtask 5b – Write Project Report
Subtask 5c – Develop a project poster
Subtask 5d – Weekly reporting
17
Resources and Schedules
Resources
Item
W/O Labor
With Labor
Parts and Materials:
a. Previous school sessions
$402.51
$402.51
b. Printed Circuit Boards
$50.00
$50.00
c. Discrete components
$100.00
$100.00
$0
$0
$50.00
$50.00
$150.00
$150.00
$752.51
$752.51
a. oscilloscope, function generator, digital multimeter
$0
$0
b. soldering equipment
$0
$0
$0
$0
d. Texas Instruments ICs
e. TI PFC boost converter
f. MSP430 programming board
Subtotal:
Test and Build equipment
Subtotal:
Labor at $20.00/hour:
a. Previous school sessions
$28,000
b. Hao Wang
$4,000
c. Pramit Tamrakar
$4,000
d. Matt Schulte
$4,000
e. Jimmy Skadal
$4,000
Subtotal:
$0
$44,000
Texas Instruments endowment:
($200)
($200)
Total:
$552.51
$88,752.51
18
Schedules
PROJECT TIMELINE
Tasks
August
Year: 2010
September October
November
December
January
February
Year 2011
March
April
May
Understand Problem
Research
PFC Boost Converter
design and modeling
Testing
Buck Converter
design and modeling
Testing
Battery Management System
bq76pl series circuit design
MPS430 programming
Testing
Complete System
Testing of scaled version
Scaling up low volt design
Testing of scaled version
Prototype Final Assembly
Final Testing
Final Prototype and Drive Car
Documentation
Project Design
Final Documentation
19
Closure Material
Project Team Information
Client’s Information
Adan R. Cervantes
Principle system Engineer
Email Address: [email protected]
3286 North Center Point Road, Marion, IA 52302
Phone: (319) 270-4357
www.element1system.com
Faculty advisor Information
Ayman A. Fayed
Assistant Professor
Email Address: [email protected]
Dept. of Electrical & Computer Engineering
Iowa State University
2117 Coover Hall Ames, IA 50011
Phone: (515) 294-6112 Fax: (515) 294-8432
http://home.eng.iastate.edu/~aafayed/
Student Team Information
Pramit Tamrakar
Major: Electrical Engineering
Email Address: [email protected]
918 NE Crestmoor Place Apt 306, Ankeny, IA 50021
Phone: 515-203-5291
Jimmy Skadal
Major: Electrical Engineering
Email Address: [email protected]
3819 Tripp St Unit 7, Ames, IA 50014
Phone: 563-320-6878
Matthew Schulte
Major: Electrical Engineering
Email Address: [email protected]
3223 Frederiksen Ct, Ames, IA 50010
Phone: 319-396-9959
Hao Wang
Major: Electrical Engineering
Email Address: [email protected]
230 Raphael Ave Unit 9, Ames, IA 50014-6711
Phone: 515-520-1787
20
Closing Summary
With almost every vehicle manufacturer striving to come out with an electric car,
the competition to develop a commercially viable solution is high. The world’s
dependence on oil makes transitioning to fully electric technology very difficult. The
ability to get away from such a need for oil comes by charging batteries instead of filling
up with gasoline or diesel. Battery technology has improved, but the ability to charge
those batteries is the major setback across the industry. Electric vehicles have to be easy,
safe, affordable, and have an ability to fulfill the needs of consumers. With a number of
electric vehicles and charging stations speculated to be on the market within the next few
years, this project is on the forefront of technology.
There are still many problems to solve when charging an electric vehicle and the
goals for this project are to find the best solutions for those problems. The use of
120VAC outlet would be ideal for home charging, but means that we must work with the
restrictions of home outlets. Home charging is a very important problem to solve for this
project.
Scaling down problems and focusing on solving each step is going to be essential
to successfully building this charger. Scaling down allows the team to focus on
developing a working solution without dealing with dangerous voltage levels during
conception. Our project goal is to successfully implement a lithium-ion battery charger
prototype for the electric vehicle owned by Adan Cervantes and Element 1 Systems.
21
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22
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23
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d_3_charging>.
24
Appendix A
Below is information about charging a lithium-ion battery from the TI data sheet SLAA287
25