Today: Battery pack modeling and BMS

• Battery pack: Stack battery cells in parallel and series• BMS: Monitor battery state (voltage, SOC, SOH), perform cell balancing, communicate with vehicle controller

1

DC busDC-DC

converter

BatteryManagementSystem (BMS)

Control bus

Vbat

+

_

VDC

+

_

Electric drivepropulsioncomponents

Vehiclecontroller

ncells

(+protection)in

series

Conventional Battery System

Packaging: Single Cylindrical Cells

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Packaging: Prismatic Cells

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Battery Packs

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Nissan LeafFord C‐Max Energi

Temperature Effects: Cell Characteristics

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Cnom = 20 Ah cell tested at 0.5C charge/discharge rate

Reference: S. Chackoa, Y. M. Chunga, Thermal modeling of Li‐ion polymer battery for electric vehicle drive cycles, Journal of Power Sources, Volume 213, 1 September 2012, Pages 296–303

Temperature effects: aging and cycle life

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• Crystal formation around the electrodes, reduces the effective surface area

• Passivation: growth of a resistive  layer that impedes the chemical reactions

• Corrosion consumes some of the active chemicals

Aging leads to:• Increased series resistance, reduced power rating• Reduced capacity

End of life: drop to 80% of the original power or capacity rating

All of aging processes are accelerated with increased temperature

Battery system electrical circuit modelassumption: identical cells

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Battery Simulink Model

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ECEN5017Battery Model (D)

Mp = number of cells in parallelNs = number of cells in series

SOC

Open-circuit voltageas a function of SOC

Series resistances

Diffusion

2SOC

1Vbat

positive Ibat

output voltage

loop

negative Ibat

1-D T(u)

Voltage-vs-SOCCharacteristic

>= 0

Switch

Rp/Mp

Series resistance for Ibat>0

Rn/Mp

Series resistance for Ibat<0

R1

R1 (diffusion)

100

PercentConversion

Ns

Number ofcells in series

1s

IntegratorLimited

Ibat Vh

Hysteresis

1

tau1.s+1Diffusionlow-pass

etaC

Coulombefficiency

-1/(Mp*Cnom*3600)

Cnom(capacity in Ah)

1Ibat

SOCVz VOC_cell

Vsp

Vsn

Vr

Vcell

Vh

Ibat

Vbat

R1/Mp

Battery Simulink Model

9

Scope

RepeatingSequence

Stair

Product

Ibat

Vbat

SOC

Li-ion battery

1s

Integrator

-1

Gain

Ibat

Vbat

SOC

Ebat

Example:  Mp = 2, Ns = 100

Ro+ = Ro‐ = 10 m, SOC(0) = 50%, Cnom = 5 Ah, VM = 20 mV, tH = 50 s, R1 = 10 m, 1 = 100 s

Battery waveforms (an example)

10

300

350

400

0 10 20 30 40 50 600

50

100

-10

0

10

0

2

4x 106

92.3%

Charging

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CCCV Charging example

12

360

370

380

390

0 10 20 30 40 50 600

50

100

-10

0

10

Effects of cell mismatches and the need for battery management system (BMS)

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Reference: Davide Andrea, Battery Management Systems for Large Lithium‐IonBattery Packs, Artech House 2011, http://book.liionbms.com/

Mismatches in cell characteristics• Capacity• Leakage (self‐discharge) current• Other parameters: series resistance, …Mismatches in cell SOC’s during operation

Animations at: http://liionbms.com/balance/index.html

Cell versus system capacity [Ah]

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Discharge, assuming equal starting SOC=100%

ii SOCCQ min,1

iCC minmin

01min CQ

minCCsystem

Charging and discharging a system with mismatched cells

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System must sense individual cell voltages or (better) employ individual cell SOC estimators

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Discharge, assuming unequal starting SOC’s

iioi SOCSOCCQ min,,

minCCsystem

System capacity with unbalanced SOC’s

iioisystem SOCSOCCCQ min,,min

0min, iSOC

Passive “top” balancing

17Animated example: http://liionbms.com/balance/index.html

Example: TI bq76PL536A

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19

20

21

Active balancing

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Example: switched‐capacitor charge shuttling

Active balancing architectures

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http://liionbms.com/php/wp_passive_active_balancing.php

Cell versus system capacity [Ah] with charge re‐distribution

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Discharge, assuming equal starting SOC=100%

isystem CC