8/17/2019 Battery pack modeling and BMS

1/24

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 bus

 DC-DC converter 

 Battery

 Management System ( BMS )

Control bus

V bat 

+

_

V  DC 

+

_

 Electric drive

 propulsioncomponents

Vehiclecontroller 

ncells

(+protection)in

series

Conventional Battery System

8/17/2019 Battery pack modeling and BMS

2/24

Packaging: Single Cylindrical Cells

2

8/17/2019 Battery pack modeling and BMS

3/24

Packaging: Prismatic Cells

3

8/17/2019 Battery pack modeling and BMS

4/24

Battery Packs

4

Nissan Leaf Ford C‐Max Energi

8/17/2019 Battery pack modeling and BMS

5/24

Temperature Effects: Cell Characteristics

5

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

8/17/2019 Battery pack modeling and BMS

6/24

Temperature effects: aging and cycle life

6

•   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

8/17/2019 Battery pack modeling and BMS

7/24

Battery system electrical circuit model

assumption: identical

 cells

7

8/17/2019 Battery pack modeling and BMS

8/24

Battery Simulink Model

8

ECEN5017Battery Model (D)

Mp = number of cells in parallel

Ns = number of cells in series

SOC

Open-circuit voltage

as a function of SOC

Series resistances

Diffusion

2

SOC

1

Vbat

positive Ibat

output

voltage

loop

negative Ibat

1-D T(u)

Voltage-vs-SOC

Characteristic

 >= 0

Switch

Rp/Mp

Series resistance

for Ibat>0

Rn/Mp

Series resistance

for Ibat

8/17/2019 Battery pack modeling and BMS

9/24

Battery Simulink Model

9

Scope

Repeating

Sequence

Stair 

Product

Ibat

Vbat

SOC

Li-ion battery

1

s

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

8/17/2019 Battery pack modeling and BMS

10/24

Battery waveforms (an example)

10

300

350

400

0 10 20 30 40 50 600

50

100

-10

0

10

0

2

4x 10

6

92.3%

8/17/2019 Battery pack modeling and BMS

11/24

Charging

11

8/17/2019 Battery pack modeling and BMS

12/24

CCCV Charging example

12

360

370

380

390

0 10 20 30 40 50 600

50

100

-10

0

10

8/17/2019 Battery pack modeling and BMS

13/24

Effects of  cell mismatches and the need for 

battery management

 system

 (BMS)

13

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

8/17/2019 Battery pack modeling and BMS

14/24

Cell versus system capacity [Ah]

14

Discharge, assuming equal 

starting SOC=100%

ii   SOC C Q min,1

iC C    minmin  

01

min  C Q

minC C system  

8/17/2019 Battery pack modeling and BMS

15/24

Charging and discharging a system with 

mismatched cells

15

System must sense individual cell 

voltages or

 (better)

 employ

 

individual cell SOC estimators

8/17/2019 Battery pack modeling and BMS

16/24

16

Discharge, assuming unequal 

starting SOC’s

iioi   SOC SOC C Q min,,  

minC C system  

System capacity with unbalanced SOC’s

iioisystem   SOC SOC C C Q min,,min  

0min,   iSOC 

8/17/2019 Battery pack modeling and BMS

17/24

Passive “top” balancing

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

8/17/2019 Battery pack modeling and BMS

18/24

Example: TI bq76PL536A

18

8/17/2019 Battery pack modeling and BMS

19/24

19

8/17/2019 Battery pack modeling and BMS

20/24

20

8/17/2019 Battery pack modeling and BMS

21/24

21

8/17/2019 Battery pack modeling and BMS

22/24

Active balancing

22

Example: switched‐capacitor charge shuttling

8/17/2019 Battery pack modeling and BMS

23/24

Active balancing architectures

23

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

8/17/2019 Battery pack modeling and BMS

24/24

Cell versus system capacity [Ah] with 

charge re

‐distribution

24

Discharge, assuming equal 

starting SOC=100%

isystem   C C