Challenges in battery pack design for electric vehicles with special emphasis on

state-of-charge (SOC) estimation

by Dr. Angelique Janse van Rensburg, North-West University

Overview

• Introduction

• Battery packs

• State-of-charge (SOC)

• Electromotive force (EMF)

• Electrolyte concentration

• Conclusion

https://cleantechnica.com/2017/04/11/panasonic-hints-beyond-lithium-technology-ev-battery-improvements/

http://lygte-info.dk/info/batteryDisassembly5000mAh%20UK.html

Electric vehicle technology

• 2 million EVs on road (2017)

• 0.2% of light-duty passenger vehicles

• Adoption barrier: purchase cost

– Improvements in mass production

– Tax exemptions

• Major adoption barrier for BEV

• ‘Fuel gauging’

• Accurate SOC information for consumer

• Battery management system (BMS) performance + lifetime

• Batteries currently in EVs ≈ 100 GWh

• Managed safely + reliably for performance

Introduction⇛ Battery Pack ⇛ SOC ⇛ EMF ⇛ Concentration ⇛ Conclusion

RANGE

Battery energy storage in electric vehicles

• Battery pack = entire battery energy storage system

http://www.hybridcars.com/chevy-bolt-evs-battery-is-as-big-as-a-teslas/

wiring

Introduction⇛ Battery Pack ⇛ SOC ⇛ EMF ⇛ Concentration ⇛ Conclusion

sensors

switches

ICs

BMS

TMS

holders

tubing

connectors

casings

MSCELLS

Measured voltage of 18650 lithium-ion cell

Introduction⇛ Battery Pack ⇛ SOC⇛ EMF ⇛ Concentration ⇛ Conclusion

100%

0%

100%

intR

Capacity and state-of-charge (SOC)

Introduction⇛ Battery Pack ⇛ SOC⇛ EMF ⇛ Concentration ⇛ Conclusion

• How long can ion transport and electron transfer continue to sustain observed current within voltage range?

= depletion active material / ion concentration, passivation

Observed current from main reaction

Introduction⇛ Battery Pack ⇛ SOC⇛ EMF ⇛ Concentration ⇛ Conclusion

Discharging:

Electrochemical double-layer

REDOX

Maximum work possible by a cell

Introduction ⇛ Battery Pack ⇛ SOC ⇛ EMF⇛ Concentration ⇛ Conclusion

EMF – SOC

The Nernst equation

Introduction ⇛ Battery Pack ⇛ SOC ⇛ EMF⇛ Concentration ⇛ Conclusion

EMF – Concentration

( )Q f m

concentrationm

• Validated model in COMSOL Multiphysics:

Concentration in lead-acid cell during discharge

Introduction ⇛ Battery Pack ⇛ SOC ⇛ EMF ⇛ Concentration⇛ Conclusion

Work, electromotive force and concentration

Introduction ⇛ Battery Pack ⇛ SOC ⇛ EMF ⇛ Concentration⇛ Conclusion

maxw nFE

inttE V I R

ln ( )RT

E E f mnF

, at resttE V E

at restm

during operationw

Measurements/estimates:Equations:

Mapping concentration to work done

Introduction ⇛ Battery Pack ⇛ SOC ⇛ EMF ⇛ Concentration⇛ Conclusion

( )E m

Electrolyte concentration for EMF characterization

• Experimental result for VRLA cell with AGM separator:

Introduction ⇛ Battery Pack ⇛ SOC ⇛ EMF ⇛ Concentration⇛ Conclusion

Final thoughts

Introduction ⇛ Battery Pack ⇛ SOC ⇛ EMF ⇛ Concentration ⇛ Conclusion

Final thoughts

• Understanding of smallest unit

• Meaning behind capacity

• Improve SOC estimation

• Address challenges increase safety, reliability and performance

• Advances in fundamental research

• Accelerate EV adoption

Power of battery energy storage

Enabling EV as disruptive technology

Introduction ⇛ Battery Pack ⇛ SOC ⇛ EMF ⇛ Concentration ⇛ Conclusion

Questions?

Dr. Angelique Janse van RensburgSenior Lecturer in Computer & Electronic Engineering

Angelique.JanseVanRensburg@nwu.ac.za

Unit for Energy and Technology SystemsFaculty of Engineering, North-West University

Potchefstroom, South Africa