battery pack modeling and bms

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


2/24

Packaging: Single Cylindrical Cells

2


3/24

Packaging: Prismatic Cells

3


4/24

Battery Packs

4

Nissan Leaf Ford C‐Max Energi


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


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


7/24

Battery system electrical circuit model

assumption: identical

cells

7


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


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


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%


11/24

Charging

11


12/24

CCCV Charging example

12

360

370

380

390

0 10 20 30 40 50 600

50

100

-10

0

10


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


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


15/24

Charging and discharging a system with

mismatched cells

15

System must sense individual cell

voltages or

(better)

employ

individual cell SOC estimators


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


17/24

Passive “top” balancing

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


18/24

Example: TI bq76PL536A

18


19/24

19


20/24

20


21/24

21


22/24

Active balancing

22

Example: switched‐capacitor charge shuttling


23/24

Active balancing architectures

23

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


24/24

Cell versus system capacity [Ah] with

charge re

‐distribution

24

Discharge, assuming equal

starting SOC=100%

isystem C C

battery pack modeling and bms

Documents