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

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 Project 2009 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)

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

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.

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

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

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 I 2 C 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 * 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

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 Copper FR4 (Circuit board) Lithium Iron Phosphate (Aluminum) Acrylic Plastic

OBPP Thermal Analysis (Bypass Scenario) Aluminum Copper FR4 (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 operation Solid – Charged Blink – Charging Solid – Discharged Blink – Discharging Solid – Slave Blink – Stand-alone Solid – 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 Firmware Stand- alone Mode Dis- charging Slave Mode Charging Check Status Bypass Sleep Dis- charging Charging Check Status Bypass Sleep

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

Cell Behavioral Simulation

Average Slope (V/min)

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) Cell Balancing Algorithm (1 Cell)

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 Cell Balancing Algorithm (1 Cell)

Bypass ▫If a cell is in bypass, a Solid Red LED in parallel with the Bypass resistor will be lit Cell Balancing Algorithm (1 Cell)

Cell Balancing Simulations

Power dissipation across power resistor Time Power Dissipation (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

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

ESS Controller Board

ESS Controller Board … redesigned NC PIC 18F4525 HV Lines 12/5 V Supplies Safety RS-485 I2CI2C Temp Safety Loop NC To ABS (Aggregate Battery Stack)

ESS Controller Board :: Key Features Fuel Gauge Algorithm (FGA) I 2 C Interface Communication with OBPP I 2 C Interface LCD Screen 4 LEDs indicating state of CMS Current Sensing RS-485 Communication with SCADA Redundant Temperature Safety System (RTSS)

ESS Control Board PRELIMINARY 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) I 2 C Interface with OBPP’s and LCD For the PIC I 2 C and SPI share the same line  TI I 2 C 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  I 2 C interface  System Reset  System Power

ESS Bill of Materials Part number DescriptionPriceQuantitySubtotal CFA533-YYH-KC LCD Panel/ Keypad$ LCD Cable$5.001 PIC18F4525Microntroller$5.601 ADUM2250Opto$6.003$18.00 LM2901Comparator$1.201 HLMP-1790-A0002LED-Green$0.626$3.72 HXS 20-NPCurrent Sensor$ M57184N-715B Voltage Regulator$7.811 LM2936Voltage Regulator$ NDVoltage Regulator$ PCB$ N135Optoisolator$0.731 SN75240P EDS Protection$1.151 BS170Mosfet$0.231 tca9554aI/O Expander$2.841`$2.84 Caps, Resistors, Connectors $ TOTAL:$210.15

LPRDS Software Architecture 2010 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 RPIEMUESSSIBFitPC RS-485 SCADA Communication (half-duplex & daisy-chained) I2C Communication (half-duplex)

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

Mechanical Design

Pack Indicators & Heatsink Possibilities Heatsink CELL 1 BYPASSCELL 2 BYPASS CELL 3 BYPASS CELL 4 BYPASS CHARGEDISCHARGE MODE

Negative Terminal Positive Terminal Nylon Standoff 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 Storage Manual Battery Disconnect Status of ESS DANGER

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

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 V Indicator LED

Enhanced Requirement Analysis Breakdown of the ATP Matches each of the requirements with its respective top-level or low-level test ATP T001QA Audit R002-4QA Audit R002-6QA Audit R002b-10 R002-2X R002-3X R002-4 X R002-5X R002-6 X R002b-2 X R002b-10 X R002b-13 X GPR006-4X

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 Fuse Power OP-Amps Voltage Regulator Temp. Sensor μprocessor Optoisolator Bypass mechanisms (resistor + BJT)

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

Budget $ $ $ $ $198.99

Budget – With 14 Added OBPPs $ $ $ $ $208.60

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