l8: battery management system - värmeöverföring · 2019-04-11 · l8: battery management system...

L8: Battery Management System

L8: 9-APR-2019

Lund University / LTH / IEA / AR / MVKF25 / 2019-04-09 2

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


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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dC

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losses

outputinputlossesgenerationonaccumulati

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Battery : store energy and use it ;)

CH

AR

GE

DIS

CH

AR

GE

Voltage [V]

Energy [Ah]

Current [A]


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


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


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


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


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)


Battery modelling B

Heat generation

model

Cell Electrical

model

Cell Thermalmodel

VtVoc

SoC

I* ϑa

Q

ϑ

• SimulinkSimPowerSystem

• Generic dynamicmodel

• Pre characterisedcharging/dischargingcharacteristics


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


Step response


Frequency response


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”


Principles, definitions, realizationsLEFT:

Negative electrode

RIGHT:Positive electrode

Ox

Redln0 nF

RTEEEE leftrightcell


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


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


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


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


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


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


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


Electrochemical impedance spectroscopy

• Frequency response of battery

• Detect changes of the interfacial properties of the electrode – charge transfer impedance (R||C)


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


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


Battery failure

• Safety=thermal stability →Failure mechanisms– External/internal – internal short circuits

– Mechanical, electrical, thermal – abusive conditions


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


Energy and power demands• Optimal performance and lifetime capacity

– Power demand vs energy capacity

– Historic use and outlook

– Energy capacity and thermal capability


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


Cell balancing

• Voltage equalization, which is to fill up energyand maximize capacityand life by ”removing”unbalanced weak links

• Active/passive –taking/wasting energy


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


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


BMS control sequence

• Intelligent batteries due to base functions of a battery management system

http://mocha-java.uccs.edu/ideate/courses.html


BMS Failure recognition

http://www.mpoweruk.com/bms.htm


BMS implementation


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


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


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


Some future trends by Bosch


Useful links

• mpoweruk.com

• Batteryuniversity.com

• liionbms.com/php/cells.php

l8: battery management system - värmeöverföring · 2019-04-11 · l8: battery management system...

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