lprds – cms – 2011

LPRDS – CMS – 2011 Per Cell Management Design

Presentation Outline•Introduction•Project Goals•One Board Per Pack•ESS Controller Board•System Communication•Mechanical Design•ATP / Requirements Analysis•Budget•Schedule

3-year Senior Design Project2009 Legacy Work

2010 Legacy Work

2011 Projected Work

Lafayette Photovoltaic Research and Development System (LPRDS)

LCD Display

SCADA Interface Box (SIB)Fit PC

System Status Display

Filter Inverter Box (FIB)

Switch Controller / Energy Management Unit(SC / EMU)

Energy Storage System (ESS)

Transformer

Energy Storage System (ESS)

LPRDS-CMS-2011•Finish a per-cell

balancing scheme for the 64-cell LiFePO4 battery pack.

•Complete design so that energy storage system is capable of being utilized by the LPRDS system.

Plan of Work•Develop a “Slave Board” (OBPP PCB) which

will balance during charge/discharge a pack of 4 cells

•Develop a “Master Board” (ESSCB PCB) which will control the functioning of the OBPPs to charge/discharge/bypass a particular cell.

•Develop a “Stand-alone” mode for the OBPP in which a pack and OBPP together do not need the master to make decisions for bypassing during charge/discharge.

Aggregate Battery Stack with OBPP PCBs

Energy Storage System Master Controller Board (ESSCB PCB)

Project Goals•Develop a One Board Per Pack PCB which

can handle the balancing of a 4-cell battery pack.

•Modify previous ESS Controller Board which can control individual OBPP packs for total pack charging/discharging.

•Develop method of visually demonstrating operation of ESS.

One Board Per Pack (OBPP)

One Board Per Pack :: Key Features•Individual cell balancing capabilities•Two Modes of Operation (Slave & Stand-

alone)•Boots in Stand-alone Mode•LEDs indicating operational state of pack•LEDs indicating operation of bypass•Scalability•Temperature Fail-Safe System

One Board Per Pack :: Design

One Board Per Pack :: Design•Resistive burn-off bypass solution•Independent redundant temperature safety

system (RTSS)•Individually addressable packs for master-

slave configuration•Stand-alone operation with charge state

controlled open collector output•Implements I2C communication in master-

slave configuration•*Current sensing capability

Cell Balancing Design•Breakdown of design trade-offs

▫Active vs. Passive Balancing▫Level of Integration▫Delegation between Controller and OBPP

boards▫Scalability▫Layout Space▫Cost▫Manufacturability▫Availability

Active Vs. Passive Balancing•Active: Using capacitive or inductive

loads to shuttle charge from higher charged cells to lower charged cells.▫Is more efficient from a power perspective▫Has scalability issues▫OBPP boards are larger and handle more

work▫Manufacturability issues

Active Vs. Passive Balancing•Passive: Bypasses cells and burns off the

excess charge from the cell.

▫Better large-stack scaling

▫Burn off can be significant

▫Controller board handles decision-making

Bypass Design•Grounding the floating reference

•Choosing a resistor value

•Choosing a suitable transistor

Bypass Design – Resistor Choice

Bypass Design

Bypass Design – Transistor Simulation

These numbers give a maximum power dissipation of 2.122 * 1.5 = 6.74W, which is about 35 degree temp rise using the thermal resistance of the resistor alone.

Bypass – Final Thoughts•Only the most recent simulations•Several different iterations of components

and control schemes•Final design can reasonably bypass 1/5 C

at full charge•Limitations of the bypass circuit heavily

influenced the balancing algorithm

Critical Monitoring•Battery Voltages•Temperature

▫On board and RTSS•Current

▫Direction and Amplitude•Open-Drain Output

▫Optional Automatic Control•Fuse

Critical Monitoring - Voltage

Critical Monitoring - Voltage•Difference Amp to buffer and isolate

battery voltages

•Monitors for voltage thresholds that indicate a full or empty state

•Balancing algorithm requires them

Critical Monitoring - Temperature•RTSS discussed later

•Voltage output temperature sensors for non-critical temperature monitoring

Critical Monitoring - Current•A relatively new addition

•Gives a way to independently judge whether the pack is charging or discharging

•Required for the balancing algorithm

Critical Monitoring – Output Pin•Based entirely on OBPP calculations

•Allows the user to have a charging circuit that is autonomous

•An open drain output from the microcontroller

Critical Monitoring - Fuse•Another new addition

•Will protect the CMS from currents above 25A

Digital I/O•Master/OBPP communications will be

over I2C

▫OBPP will have a 4 bit switch addressing

•OBPP will transfer from Standalone to Slave when I2C becomes active

•Master commands override OBPP automated tasks

Redundant Temperature Safety System (RTSS)• Independent functionality to shut down system when

temperature exceeds 65°C

• Connection to each OBPP using AD22105 “Low Voltage, Resistor Programmable Thermostatic Switch” Integrated Circuit▫(Setpoint accuracy = 2°C)

• When any board exceeds the temperature limit, the switch within the safety loop is activated and the system shuts down.

Overall RTSS

•Does not work as stand-alone pack

•Must be connected to ESSCB Safety Loop

RTSS parts on OBPP

To other OBPPs

OBPP Connection to Safety Loop

to OBPPs

OBPP Thermal Analysis (Charging/Discharging)

Aluminum

CopperFR4 (Circuit board)

Lithium Iron Phosphate (Aluminum)

Acrylic Plastic

OBPP Thermal Analysis (Bypass Scenario)

Aluminum

CopperFR4 (Circuit board)

Lithium Iron Phosphate (Aluminum)

Acrylic Plastic

Stationary Analysis (1 cell heating)

Stationary Analysis (4 cells heating)

Stationary Analysis (Conductive slabs)

Stationary Analysis (Bypass scenario)

Time Dependent (1 cell)

Time dependent (Bypass Scenario)

OBPP Operational Verification•Bypass LEDs to indicate resistive

bypassing

•LEDs to indicate charge/discharge and mode of operationSolid –

ChargedBlink –

Charging

Solid – Discharged

Blink – Discharging

Solid – SlaveBlink – Stand-

aloneSolid – Bypassing

OBPP Additional Notes•Multiple levels of electrical isolation

▫Microcontroller/bypass loop

▫I2C on OBPP and Master board

▫RTSS isolated as well

OBPP Firmware•Stand-alone Mode

•Slave Mode

•Cell Balancing Algorithm

OBPP Firmware - Standalone•Begins after a reset or losing the I2C

clock signal

•Watches for voltage thresholds

•Cell balancing is enabled

•Waits for I2C connection

•First firmware development milestone

OBPP Firmware – Slave•Many of the same responsibilities

•If no explicit instructions from the master, very similar to Standalone

•Master commands are executed first and prioritized

OBPP FirmwareStand-alone Mode

Dis-chargin

g

Slave Mode

Charging

Check

Status

Bypass

Bypass

Sleep

Dis-chargin

gChargin

g

Check

Status

Bypass

Bypass

Sleep

•Type- Lithium Iron Phosphate (LiFePO4) •Nominal Voltage - 3.2 V •Capacity – 10 A-h

Cell Specifications

Cell Behavioral Simulation

Cell Behavioral Simulation

Average Slope (V/min) 0.00208

• Charging▫ If the voltage of any cell in a pack of 4 is greater than

any of the other 3 cells by more than 40mV, then that cell will go into bypass for 20 minutes.

▫ During charge, a green LED on the OBPP will blink▫ If the voltage of any cell exceeds 3.8V, then the pack will

be considered fully charged, and the CMS will notify the user to discontinue charging (this must happen regardless of whether the cell is in bypass or not)

▫ If the temperature of any cell exceeds 40° above ambient, then the CMS will notify the user to discontinue charging (this must happen regardless of whether the cell is in bypass or not)

Cell Balancing Algorithm (1 Cell)

• Discharging▫ If the voltage of any cell in a pack of 4 is less than any of

the other 3 cells by more than 40mV, then all other cells will go into bypass for 20 minutes.

▫ During discharge, a Red LED on the OBPP will blink▫ If the voltage of any cell drops below 2.8V, then the pack

will be considered fully discharged, and the CMS will notify the user to discontinue discharging (this must happen regardless of whether the cell is in bypass or not)

▫ If the temperature of any cell exceeds 40° above ambient, then the CMS will notify the user to discontinue discharging (this must happen regardless of whether the cell is in bypass or not)


•OFF▫If the CMS is in the OFF state, either a

Solid Red LED will indicate that the pack is fully discharged, or a Solid Green LED will indicate that the pack is fully charged

▫If the CMS is in the OFF state, no cells will be in bypass

▫If the CMS is in the OFF state, all time differentials will be set to zero


•Bypass▫If a cell is in bypass, a Solid Red LED in

parallel with the Bypass resistor will be lit



Check Status ChargingBlink Green LED

Bypass CellBypass LED

Blink Green LED

OFFSolid Red/Green

DischargingBlink Red LED

Bypass CellsNot in This StateBlink Green LED

Time < 20 min

Always

Time < 20 min

Temp > 60°C || V < 2.8V

V < (V of any Cell – 40mV)

ELSE

V > (40mV + V of any Cell) || V > 3.5V

ELSE

isCharging

isDischarging

Reset || Change in Status

Temp > 60°C || V > 3.8V

Any Other Cell is in OFF State

3/9/2011Cell Balancing Algorithm

State DiagramJustin Bunnell

LPRDS-CMS-2011

Temp > 60°C || V > 3.8V

Temp > 60°C || V < 2.8V

Cell Balancing Simulations

Power dissipation across power resistor

Time

Pow

er D

issi

patio

n (W

)

Cell Balancing Algorithm Pros•Cell Balancing within 10 charge/discharge

cycles•Ability to be done in Standalone Mode•Relative Simplicity•Strict conditions to keep cell within safe

ranges•Bypass current does not scale at same

rate as charge current

Cell Balancing Algorithm Cons•Cell Characteristic Differences•State of Health of Cell•High State Of Charge Mismatch•Power Losses to Bypass Resistor

(especially during discharge cycle)•Losing balancing time by limiting

maximum temperature (limit to bypass resistance)

•Minimum charge and discharge currents

ESS Controller Board

ESS Controller Board … redesigned

NC

NC

PIC 18F4525

HV Lines

12/5 V Supplies

Safety

RS-485

I2C

Temp Safety Loop

NC

To ABS(Aggregate

Battery Stack)

ESS Controller Board :: Key Features

•Fuel Gauge Algorithm (FGA)•I2C Interface Communication with OBPP•I2C Interface LCD Screen•4 LEDs indicating state of CMS•Current Sensing•RS-485 Communication with SCADA•Redundant Temperature Safety System

(RTSS)

ESS Control BoardPRELIMINARY DESIGN

ESS Control Board• Primary Functions:

▫Transmit CMS information (Voltage, Temperature, Current) to SCADA system

▫Monitor current Fuel Gauge Algorithm

▫High Voltage Indicator▫CMS Display (LED’s and/or LCD)▫Safety Loop▫Override OBPP’s if necessary

ESSCB Continued…•Re-use PIC18F4525

• Re-use code from last year• Re-use power sources, sensors, terminals,

LED’s, etc from last year• Re-use safety loop

•Communication• RS-485 Interface with SCADA system (SPI)• I2C Interface with OBPP’s and LCD• For the PIC I2C and SPI share the same line

TI I2C I/O Expander

ESSCB Continued…•Fuel Gauge Algorithm

• Coulomb counting Use current sensor to measure charge in and

out of cells Reset to full capacity at full voltage threshold

ESSCB Continued…•Display

• Several LED’s: Charging, Discharging, Fault, 30V Indicator

• LCD Display I2C interface System Reset System Power

ESS Bill of MaterialsPart number Description Price Quantity Subtotal

CFA533-YYH-KC LCD Panel/ Keypad $54.84 1 $54.84 LCD Cable $5.00 1 $5.00

PIC18F4525 Microntroller $5.60 1 $5.60

ADUM2250 Opto $6.00 3 $18.00

LM2901 Comparator $1.20 1 $1.20

HLMP-1790-A0002 LED-Green $0.62 6 $3.72

HXS 20-NP Current Sensor $14.00 1 $14.00M57184N-715B Voltage Regulator $7.81 1 $7.81

LM2936 Voltage Regulator $1.93 1 $1.93

555-1058-ND Voltage Regulator $12.10 1 $12.10

PCB $66.00 1 $66.00

6N135 Optoisolator $0.73 1 $0.73SN75240P EDS Protection $1.15 1 $1.15

BS170 Mosfet $0.23 1 $0.23

tca9554a I/O Expander $2.84 1` $2.84

Caps, Resistors, Connectors $15.00 1 $15.00

TOTAL: $210.15

LPRDS Software Architecture 2010

SCADA Communication

SCADA Communication• Add additional parameters for query

• Increase polling times/ polling delay

• Poll ESS ESS Poll OBPP OBPP Respond ESS Respond to OBPP

System Communication

RPI EMU ESS SIB FitPC

1 162345678

1514131211109RS-485

SCADA Communication (half-duplex & daisy-chained)

I2C Communication (half-duplex)

Mechanical Design

Pack Indicators & Heatsink Possibilities

Heatsink

CELL 1 BYPASS CELL 2 BYPASS

CELL 3 BYPASS CELL 4 BYPASS

CHARGE

DISCHARGE

MODE

Negative

Terminal

Positive Termina

l

Nylon Stando

ff

2-Position Terminal

Block

Female Wire

Connector

Male Wire

Connector

Female Plug

Wire harness 1 (packs 1-8)

Wire harness 2 (packs 9-16)

Physical Dimensions

117 mm

107 mm

53 mm

15 mm

160 mm

21 mm

8 mm

Energy StorageManual Battery Disconnect

Status of ESS

DANGER

Acceptance Test Plan (ATP)• Modified the requirements of the system

▫Agreed upon by Professor Nadovich• Testing at the highest level: full CMS• All requirements not verified at top level:

▫Low-level Testing (QA Audit)▫Analysis (Technical Memos)

• Requirements are checked off on the Acceptance Test Report (ATR) as they are met

• ATR is based on the ATP

Expected Tests• ATP Test 001

• Demonstrates per cell battery management• Charge every cell to maximum capacity• Stand alone operation • Operate for at least 24 hours autonomously

• QA Test 001• Prevent over-charge or over-discharge

• QA Test 002• Verifies operation of SCADA system

• QA Test 003• 30V Indicator LED

Enhanced Requirement Analysis•Breakdown of the ATP•Matches each of the requirements with its

respective top-level or low-level test

ATP T001 QA Audit R002-4 QA Audit R002-6 QA Audit R002b-10

R002-2 X

R002-3 X

R002-4 X

R002-5 X

R002-6 X

R002b-2 X

R002b-10 X

R002b-13 X

GPR006-4 X

Brief Maintainability Analysis•Recommended Spare Parts: fuses,

connectors, wires, full boards•Troubleshooting scenarios in User’s

Manual using parts in Maintenance Manual▫How to replace a blown fuse▫Reset buttons on system boards▫Reprogram OBPP/ESS microcontrollers

Brief Manufacturability Analysis•All components listed on Bill of Materials can

be purchased from at least two independent suppliers.

•Critical components are identified and tolerances of these components are considered.▫RTSS resistor to set activation temperature▫Voltage threshold for cell balancing algorithm▫Resistors to manage the bypass loop▫Components for fuel gauge algorithm -> NOT

critical (only used for general measurements)

Reliability Analysis• Accomplishments

▫ Simplified schematic of OBPP board to be used for analysis

▫ MTBF of each isolated component

• Upcoming tasks▫MTBF for Temperature

Sensor ▫Determine failure criteria▫Calculate overall MTBF

ATMEGA 16

257 Series Blade Fuse

LM2936

+12V

+5V

TC1023

+5V

SIMPLIFIED OBPP CIRCUIT BOARD

TLC2254

TLC2254

TLC2254

TLC2254

6N135(Opto-

Isolator)

HV

FusePower

OP-Amps

Voltage Regulato

r

Temp. Sensor

μprocessorOptoisolator

Bypass mechanisms

(resistor + BJT)

Budget

ESSOBPPWiringHardwareRemaining Budget:

$1915.85

$418.86$257.70

$208.60

$198.99

ESSOBPP (scaled)WiringHardwareRemaining Budget:

Budget – With 14 Added OBPPs

$2061.60

$418.86

$111.95

$198.99$208.60

Schedule•We made several complete design

changes which caused us to stray from the initial schedule.

•Initial schedule was incredibly vigorous and less reasonable.

•Current schedule is more reasonable, but we have still fallen behind due to redesigns of the OBPP and fine-tuning our stand-alone operation.

Most of schedule slip occurred because design took longer than expected.

Questions?•Thank you for your attention.

lprds – cms – 2011

Documents

pack obpp

slave board obpp pcb

cell battery pack

pack pcb

previous ess controller

lprds system

master board esscb pcb

cell lifepo4 battery