L8: Battery Management System
L8: 9-APR-2019
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Outlook
• Energy management– Battery management system
– Electric and Thermal management
• Battery states – Charge (SoC), Function (SoF), Health (SoH)
– Performance deterioration and battery degradation
• Battery characteristics and models
• Charging and discharging
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Guide
• Battery characteristicsand model (Ch 1)
– Cell components– Electrochemical energy
conversion– Performance
characteristics– Electrochemical analysis
methods
• Battery control and management (Ch 6)
– Energy management– State functions
• Battery usage and degradation (Ch 7)
– Degradation mechanisms– Degradation of Li-ion cells– Degradation analysis
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dt
dC
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d
thambth
losses
outputinputlossesgenerationonaccumulati
1
Battery : store energy and use it ;)
CH
AR
GE
DIS
CH
AR
GE
Voltage [V]
Energy [Ah]
Current [A]
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Battery overview
• Parameters (of primary interest)– Voltage, E [V]
– Capacity, Q [Ah]
• Models
• E model approaches– Mathematical: chemical process kinetics & Markov
process based stochastic model
– Electrochemical: physics based set of coupled partial differential equations connecting laws related to chemical concentration and electric current flow
– Equivalent circuit: Electric circuit consisting of RCE
Ageing model
Thermal model
Electrical model
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Voltage and Capacity
• Electric operation domain– Voltage range – Vmin-Vmax
– State of Charge – SOC
• Depth of discharge DOD=100%-SOC
– Voltage range – min-max
• Open circuit voltage OCV(SOC, )
• Internal resistance R(SOC,I,) results voltage drop and power losses
Voltage [V]
Capacity [Ah]
Vmax
VminQmin
Qmax
%100
nomQ
tQtSOC
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More voltage and more capacity
• Series → increase voltage
• Cells in parallel → increase capacity → unequal voltage drives current
• The largest safely drawn charge is the one that is stored in the weakest cell
• Purpose of BMS– Indentify state of charge SOC
– Maximize capacity
– Provide safe function
Voltage [V]
Capacity [Ah]
BM
S
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Battery control and BMS• Energy management
– Electric operation range –energy balancing for better usage
– Thermal operation range –Keep temperature & use little energy for operation
• Battery cell-module-pack development and control is supported buy models
– From models in physics (FEM) towards datasheet and equivalent circuit modeling – from component physics towards system realisation
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Battery modeling A• Simple approach @ limited data,
parameters are independent of SOC, current (rate) and temperature
• Cell voltage U=Eo-RoI where Ro is internal resistance and Eo is open circuit voltage (OCV)
• Heating power Ploss=(RoI)2 only Ohmic losses
• Transient temperature rise =PlossRh(1-e-t/R
hC
h)
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Battery modelling B
Heat generation
model
Cell Electrical
model
Cell Thermalmodel
VtVoc
SoC
I* ϑa
Q
ϑ
• SimulinkSimPowerSystem
• Generic dynamicmodel
• Pre characterisedcharging/dischargingcharacteristics
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Electric equivalent circuit
• Electrolyte resistance – causes resistive voltage drop at current flow
• Diffusion and surface reaction – results the voltage transient(s) at current step
R1
C1
R2
C2
R0
E=Uoc(Vsoc)
Ccap
Rsdc
Ibat Ibat
Vsoc
Battery Lifetime
Voltage-Current Characteristics
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Step response
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Frequency response
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Electrochemical force and cell
• Chemical reaction = two half-reactions: oxidation+reduction=redox
– Side reactions due to thermal loads, pressure?
• Active, electrodes, non-active, the rest including electrolyte, components
B. Averill, P. Eldredge, “General Chemistry: Principles, Patterns and Applications”
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Principles, definitions, realizationsLEFT:
Negative electrode
RIGHT:Positive electrode
Ox
Redln0 nF
RTEEEE leftrightcell
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Keywords to previous slide
• Electrochemical energy conversion
– Ch → E: Galvanic, Oxidation → loss of electrons → discharging
– E → Ch: Electrolytic, Reduction → gain of electrons → charging
• Nernst equation
• Electrode domain – μm-scale
• From left to right = negative electrode positive electrode and boundaries for different domains in between
CH 1 : The electrochemical cell
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Energy and power
• Specific energy originates from material chemistry
– Capacity capability
• Specific power is related to material physics and production
– Internal power losses and thermal constrains –durability and safety
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Cell voltage
cell
negcctposcct
negposcell
iR
EEE
• Activation polarization –charge transfer (ηct) from electrode surface
• Concentration polarization –caused by concentration (ηc) differences between electrode and electrolyte due to ionic conductivity and transport properties
• Ohmic polarization – IR drop proportional to current
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Voltage hysteresis
cha
disE
cha
disQ
t
t
dttItV
dttItV
Q
Q
dttIQ
tSOCtSOC
0
10
• Charge-Discharge profile
• Delay in chemical and electrochemical reactions, causes difference between charging and discharging voltages
• Voltage hysteresis, ΔE, may increase with charge and discharge rate
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Charge and Discharge rates
• A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity.
• A 1C rate means that the discharge current will discharge the entire battery in 1 hour.
• Practical capacity is defined as the current density passing through the cell until the cut-off voltage is reached
• How C-rate affect cell performance
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Capacity
Ageing model
Electrical model
current Thermal model
power
temperature voltage
DoD SoH
• Specific capacity [Ah/kg] of used electrochemical active material
• Capacity fade due to loss of recyclable Lithium and SEI build up
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Cyclic voltammetry
• Galvanostatic cycling – voltage response at constant current – study the cell capacity and degradation
• Potentiostatic cycling – holding voltage constant and decline the current
• Cyclic voltametry for electrode reaction response at linearly changed voltage resulting current peaks
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Electrochemical impedance spectroscopy
• Frequency response of battery
• Detect changes of the interfacial properties of the electrode – charge transfer impedance (R||C)
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EIS battery testing• Electrochemical dynamic
response– Respons is related to ion-
current/diffusion rate in the cell
– Slower response for weakerbatteries
• Characterization– LF dubbed diffusion
– MF charge transfer
– HF migration
• Batteries with faded capacity suffer from low charge transfer and slow active Li-ion diffusion.
http://batteryuniversity.com/learn/article/testing_lithium_based_batteries
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Battery performance degradation
• Degradation – deterioration of useful capacity and power capabilities
• Identification of physical and chemical processes behind degradation mechanisms . Origins related to technology and usage.
• SoH – state of health remaining capacity due to ageing
http://epg.eng.ox.ac.uk/content/degradation-lithium-ion-batteries
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Battery failure
• Safety=thermal stability →Failure mechanisms– External/internal – internal short circuits
– Mechanical, electrical, thermal – abusive conditions
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Thermal runaway• Failure propagation from
cell to module and pack • Rapid temperature increase
– Most likely due to internal spontaneous short circuits due to impurities (that can grow during time as side effect of chemical reactions)
• Avoid thermal runaway– Overcharge/discharge protection
activated by over pressure– Current interrupt device (CID)– Positive temperature coefficient
(PTC) – Separator specified for PTC &
CID, layered separators for reducing internal short circuits
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Energy and power demands• Optimal performance and lifetime capacity
– Power demand vs energy capacity
– Historic use and outlook
– Energy capacity and thermal capability
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Charge and discharge control
• maintain the voltage limits while respecting the currentand temperature limits
• LOW Constant current charging followed by voltageand temperature control
• HIGH current for constant voltage charging
• Combined CV+CC
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Cell balancing
• Voltage equalization, which is to fill up energyand maximize capacityand life by ”removing”unbalanced weak links
• Active/passive –taking/wasting energy
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BMS
• Cell protection, charge control, demandmanagement, SoC and SoH determination, cell balancing, authenticationand identification, communication – are some objectives for BMS
http://www.mdpi.com/1996-1073/4/11/1840/htm
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BMS development
• Overall functional safety is better match to global FPGA than to local micro processor units
– parallelism for performance with fail-safe logic
https://www.altera.com/solutions/industry/automotive/applications/electric-vehicles/battery-management-system.html
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BMS control sequence
• Intelligent batteries due to base functions of a battery management system
http://mocha-java.uccs.edu/ideate/courses.html
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BMS Failure recognition
http://www.mpoweruk.com/bms.htm
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BMS implementation
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BMS architectures for xEVs
• Communication, reliability and accuracy
• Practical attachment, number of components and connections
• Few architectures with different features in connections and communication
http://www.electronicproducts.com/Power_Products/Batteries_and_Fuel_Cells/Battery_management_architectures_fo
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Multicell Battery Stack Monitor
• Component name LTC6802-1, Up to 12 cells, 13 ms measurement interval, up to 1000V, passive cell balancing
http://www.linear.com/product/LTC6802-1
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BMS sensor module
MM9Z1 638 4-Cell Lithium Battery BMS unit
•battery stack monitor IC can measure a number of cell voltages and provide for the discharge of individual cells to bring them into balance with the rest of the stack
http://www.nxp.com/products/automotive-products/energy-power-management/can-transceivers/reference-design-mm
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Some future trends by Bosch
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Useful links
• mpoweruk.com
• Batteryuniversity.com
• liionbms.com/php/cells.php