An Impedance-Based BMS

to Identify Bad Cells

Rengaswamy ‘Srini’ Srinivasan Bliss G. Carkhuff

Rengaswamy.srinivasan@jhuapl.edu (443) 841-8825

2

Impedance-Based Tinternal, Rinternal, SOC and SOH

Srinivasan et al., Electrochimica Acta, Vol. 56: 6198-6204, Year 2011

Methodology: • Perturbing the cell with a small-

amplitude ac current • Results in an ac voltage • Measure the amplitude and phase

shift • Compute the impedance • Impedance at any one frequency

between 40 Hz to 100 Hz measures anode temperature (Ta)

• Impedance at any one frequency between 10 Hz to 20 Hz measures cathode temperature (Tc)

• Ta and Tc can be estimated using phase shift values only

• High-frequency X-axis intercept corresponds to electrolyte resistance, Rs

• Impedance at low-frequency (<10 Hz) is sensitive to temperature and state of charge

0.0008 0.0012 0.0016 0.0020-0.0004

0.0000

0.0004

0.0008

0.0012

-Z'' /

Z' /

87% 80% 70% 60% 50% 40% 30% 20% 10%

Sensitive to SOC

and

Temperature

(SoC-Insensitive)

Temperature Sensitive

0.8 Hz 1 kHz

SOC

20 Hz

Note: This slide provides an overview of battery impedance and how we use it to measure anode temperature, cathode temperature, electrolyte resistance, state of charge, state of health, etc. For more details, see references below and in the next slide

3

Impedance-Based Tinternal, Rinternal, SOC and SOH

• R. Srinivasan et al., “Instantaneous Measurement of the Internal Temperature in Lithium-Ion Rechargeable Cells,” Electrochimica Acta, 2011, 56, 6198-6204.

• R. Srinivasan, “Monitoring Dynamic Thermal Behavior of the Carbon Anode in a Lithium-ion Cell Using a Four-probe Technique,” Journal of Power Sources, 2012, 198, 351-358.

• R. Srinivasan and B. G. Carkhuff, “Empirical analysis of contributing factors to heating in lithium-ion cells: Anode entropy versus internal resistance,” Journal of Power

Sources, 2013, 241, 560-566 • R. Srinivasan et al., “The Five Modes of Heat Generation in a Li-Ion Cell under

Discharge,” Journal of Power Sources, 2014, 262, 93-103. • R. Srinivasan and L. Srinivasan, “Graphitic carbon anode temperature excursions

reflect crystallographic phase transitions in lithium-ion cells,” Journal of Power

Sources, 2015, 293, 876-882. • R. Srinivasan et al., US Patent No. 7,544,294 B2, 30 June 2009, “Battery Health

Monitor.” • R. Srinivasan et al., US Patent No. 8,961,004 B2, 24 February 2015, “Battery Phase

Meter to Determine Internal Temperatures of Lithium-Ion Rechargeable Cells Under Charge and Discharge.” (Also see: (WO 2012/054473 A1 published 26 April, 2012; Europe: 2,630,687; Japan: 5,840,693.)

• R. Srinivasan and B. G. Carkhuff, US Patent No. 9,331,507 B2, 3 May 2016, “Control Apparatus and Method for Conducting Fast Battery Charge.”

4

Overview

• BMS based on current, voltage and surface

temperature monitors have struggled to provide safety Some have single-frequency (1 kHz) impedance sensor that

serves little or no purpose in providing cell or battery safety

• BMS based on multi-frequency impedance have a much better chance to ensure thermal safety and electrical efficiency

5

The 2009 Incident: BMS with Voltage, Current and Surface Temperature Monitors

What purpose did the voltmeter, ammeter and thermocouple serve?

Bat

tery

Vol

tage

(V) a

nd C

urre

nt (A

)

Temperature ( 0C

)

Time (hour)

Electrical and Thermal Data

6

What does a voltmeter tell you?

-50 0 50 100 150 200

3.0

3.5

4.0

4.5

Cel

l Vol

tage

(V)

Time (minute)

Cell 40A Cell 40B Cell 40C

50% SOC3.65771 V

3.65449 V

3.65234 V

3.42783 V

3.42246 V

3.42031 V

30% SOC

10% SOC3.20439 V

3.21621 V

3.21084 V

83% SOC4.19160 V

4.13779 V

4.19590 V

New 3-cell battery

±0.0027 V

±0.00387 V ±0.00592 V

±0.00377 V

Cell voltages in a new battery are nearly identical

7

What does a voltmeter tell you?

-200 0 200 400 600 800 10002.5

3.0

3.5

4.0

4.5C

ell V

olta

ge (V

)

Time (minute)

Cell 11 Cell 30A Cell 30

99% SOC4.18193 V4.17871 V4.17764 V

30% SOC3.47188 V3.46865 V3.45684 V

2.88643 V2.86709 V2.76826 V

<5% SOC

Cycle-aged 3-cell battery

±0.00223

±0.00792

±0.06338

Cell voltages in a cycled battery are nearly identical

8

What does a voltmeter tell you?

180 200 220 240 260 2803.25

3.50

3.75

4.00

4.25

4.50C

ell V

olta

ge (V

)

Time (minute)

Cell 1 Cell 2 Cell 3 Bad Cell Cell 5 Cell 6

83% SOC4.1775 V4.1860 V4.1849 V4.1850 V4.1892 V4.1913 V

30% SOC3.4706 V3.4674 V3.4706 V3.4642 V3.4674 V3.4749 V-200 0 200 400 600 800 1000

2.0

2.5

3.0

3.5

4.0

4.5

Cel

l Vol

tage

(V)

Time (minute)

Cell 1 Cell 2 Cell 3 Bad Cell Cell 5 Cell 6

±3.68 mV

±4.73 mV

Bad Cell: Twice over-discharged to 0.01 V

Cell voltages in a battery with a bad cell are nearly identical

9

What does a voltmeter tell you? Nothing much, unless you drain the battery

180 200 220 240 260 280 300 3202.0

2.5

3.0

3.5

4.0

4.5C

ell V

olta

ge (V

)

Time (minute)

Cell 1 Cell 2 Cell 3 Bad Cell Cell 5 Cell 6

83% SOC4.1775 V4.1860 V4.1849 V4.1850 V4.1892 V4.1913 V

30% SOC3.4706 V3.4674 V3.4706 V3.4642 V3.4674 V3.4749 V

10% SOC3.3239 V3.3229 V3.3250 V3.2963 V3.3112 V3.3229 V

-200 0 200 400 600 800 10002.0

2.5

3.0

3.5

4.0

4.5

Cel

l Vol

tage

(V)

Time (minute)

Cell 1 Cell 2 Cell 3 Bad Cell Cell 5 Cell 6

±3.68 mV

±4.73 mV

Low Voltage Cutoff = 6x2.7 V

±11.34 mV

Cell voltages in a new battery are nearly identical – until the battery is fully discharged

10

What does Surface-Mounted Sensor Identifies?

550 600 650 700 750 80021

22

23

24

25

Bad Normal

Sur

face

Tem

pera

ture

(0 C)

Time (minute)

16

18

20

22

24

26

Bat

tery

Vol

tage

(V)∆T = 1 ºC

Cell Temperatures in a battery with a bad cell are nearly identical

11

Protecting Armors for Battery Safety?

VOLTMETER THERMOCOUPLE

AMMETER

Archaic instruments do not help protect Li-ion batteries

12

The 2016 Incident: Had Battery Voltage and PackTemperature Monitors

http://gizmodo.com/this-is-why-you-should-take-lithium-ion-battery-fires-v-1788281947

in 3 seconds

Archaic instruments did not help protect this Li-ion battery

13

What does a thermocouple tell you?

TSurface response-time is slow

0

2

4

6

8

10

12

-60.00 -30.00 0.00 30.00 60.004

6

8

10

12Te

mpe

ratu

re (

0 C)

Cur

rent

(A)

Discharge Time (second)

Surface Temperature

Discharge Current

14

What does a thermocouple tell you?

0

1

2

3

4

5

0 2000 4000 6000 8000

16

20

24

28

32

36

40

44

Tem

pera

ture

( 0 C

)

Charge Time (second)

Vol

tage

(V),

Cur

rent

(A) o

r Cap

acity

(Ah)

Voltage

Current Capacity

Current

Surface

Temperature

Tsurface is <32 ºC, indicating everything is fine!

2.5-Ah Cell is being overcharged

2.5 Ah

15

What does a thermocouple tell you?

Tsurface is <40 ºC therefore safe?

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.810

20

30

40

50

60

70

Srini; 10/14/2015

Tem

pera

ture

(0 C)

Time (hour)

Discharge Charge(CC part)

9

10

11

12

13

Bat

tery

Vol

tage

(V)

Rate of discharge and charge = 1C

Surface

Temperature

Voltage

16

A Practical Impedance-Based BMS

• Ageing due to cycle life and calendar life • Cell screening and matching

Before and after constructing a battery

• Identifying internal defects caused by over-discharge,

over-charge, etc.

• Thermal safety

• …

Non-Invasive and Autonomous

17

16 18 20 22 24 26 280

2

4

6

1.6 kHz

- Im

agin

ary

(milli

-ohm

)

Real (milli-ohm)

Cell #1 Cell # 2 Cell # 3

70 Hz0.082 Hz

(A) New

16 18 20 22 24 26 280

2

4

6

1.2 kHz

- Im

agin

ary

(milli

-ohm

)

Real (milli-ohm)

Cell # 4 Cell # 5 Cell # 6 Cell # 7 Cell # 8 Cell # 9

70 Hz

0.082 Hz

(B) Aged

16 18 20 22 24 26 28 300

2

4

6

1.2 kHz

0.082 Hz70 Hz

1.2 kHz

- Im

agin

ary

(milli

-ohm

)

Real (milli-ohm)

Cell # 4 Cell # 5 Cell # 6 Cell # 7 Cell # 8 Cell # 9 Over-discharged (twice to 0.01 V)

70 Hz

0.082 Hz

(C) One Over-discharged

0 40000 80000 120000 160000 2000000

40000

80000

120000

160000

200000

1.2 kHz

- Im

agin

ary

(milli

-ohm

)

Real (milli-ohm)

Cell # 4 Cell # 5 Cell # 6 Cell # 7 Cell # 8 Cell # 9 Over-discharged (twice to 0.01 V) Over-charged (twice to 4.6 V)

(C) One Overcharged

Sensitivity: Measurements based on bench-top impedance meter (Solartron)

18

BMS 16-Cell

Multiplexer

Miniaturized Impedance-Based BMS

BMS for 16 cells, 55-Ah battery • Frequency range: 2 Hz to 1 kHz • Anode and cathode temperatures • Anode, cathode and electrolyte resistance • SOH • Cell Voltage

19

Perspectives of Impedance-Based BMS

Far simpler than using thermocouple for Tsurface

20

Identify Matched Cells within Seconds

The three cells in the battery are matched within ±0.5 %

16 18 20 22 24 26 280

2

4

6

1.6 kHz

- Im

agin

ary

(milli

-ohm

)

Real (milli-ohm)

Cell #1 Cell # 2 Cell # 3

70 Hz0.082 Hz

New

0 50 100 150 200

3360

3380

3400

3420

3440

3460

Cell #1 = 3406 9

Cell #2 = 3381 9

BM

S O

utpu

t (A

rbitr

ary

Uni

t)

Time (second)

Cell #3 = 3414 15

21

BMS Identifies Mismatching in Aged Cells

In the 6-cell battery, calendar ageing drove the cell apart by ±2%

0 200 400 6002520

2560

2600

2640

2680

Cell #5 = 2541 9

Cell #4 = 2550 9

Cell #6 = 2637 9

Cell #2 and #3 = 2600 8

Cell #1 = 2668 8

BM

S O

utpu

t (A

rbitr

ary

Uni

t)

Time (second)16 18 20 22 24 26 28

0

2

4

6

1.2 kHz

- Im

agin

ary

(milli

-ohm

)

Real (milli-ohm)

Cell # 4 Cell # 5 Cell # 6 Cell # 7 Cell # 8 Cell # 9

70 Hz

0.082 Hz

Imepdace data shows the cells are not matched

Calendar Aged (6 months)

22

BMS Output of the Five Calendar-Aged Cells

Five Calendar-Aged, but Otherwise Normal Cells

550 600 650 700 750 8002350

2400

2450

2500

2550

2600B

MS

Out

put (

Arb

itrar

y U

nit)

Time (minutes)

16

18

20

22

24

26

Bat

tery

Vol

tage

(V)

23

One of the Six Cells was over-discharged: The BMS identifies the bad cell

600 650 700 750 8005502420

2440

2460

2480

2500

2520

BM

S O

utpu

t (A

rbitr

ary

Uni

t)

Time (minute)

16

18

20

22

24

26

Bat

tery

Vol

tage

(V)

600 650 700 750 8005502420

2440

2460

2480

2500

2520

BM

S O

utpu

t (A

rbitr

ary

Uni

t)

Time (minute)

16

18

20

22

24

26

Bat

tery

Vol

tage

(V)

Normal Cell Bad Cell

24

Why anode temperature is more important?

Gas release

0

1

2

3

4

5

0 2000 4000 6000 8000

16

20

24

28

32

36

40

44

Tem

pera

ture

( 0 C

)

Charge Time (second)

Vol

tage

(V),

Cur

rent

(A) o

r Cap

acity

(Ah)

Voltage

Current Capacity

Current

Anode Temperature

Surface

Temperature

Samsung SDI F26 (2.6-Ah Li-ion) behavior during overcharging

Srinivasan et al., Journal of Power Sources, 241, pp. 560-566; Year: 2013

Tsurface misses overcharging completely, but not Tanode

25

Continuous Discharge-Charge at 1C Rate

Tinternal >> Tsurface when the current is continuous

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.810

20

30

40

50

60

70

Srini; 10/14/2015

Tem

pera

ture

(0 C)

Time (hour)

Discharge Charge(CC part)

9

10

11

12

13

Bat

tery

Vol

tage

(V)

Tinternal can be much hotter than Tsurface

Tinternal

Tsurface

26

Surface temperature can be misleading

0

2

4

6

8

10

12

-60.00 -30.00 0.00 30.00 60.004

6

8

10

12 Surface

Tem

pera

ture

( 0 C

)

Cur

rent

(A)

Anode Cathode

Discharge Time (second)

TSurface response time is slower

Srinivasan et al., Journal of Power Sources, 262, pp. 93-103; Year: 2014

27

Fast Charge through Feedback-Controlled Closed-Loop

0 20 40 60 80 10020

24

28

32

36Te

mpe

ratu

re (0 C

)

Charge Time (minute)

Current

Tsurface

Voltage

Tinternal

0

2

4

6

8

10

Vol

tage

(V) o

r Cur

rent

(A)

• Charging current profile is semi-autonomously determined by the charger: based on the user-set limits on Tinternal and Cell Voltage

Example shown: Conventional charging time is 150 minutes Fast charger-enabled time is 95 minutes

28

0 200 400 600 800 1000 1200 140018

20

22

24

26

28

Ele

ctro

lyte

Res

ista

nce

(milli

ohm

)

Cycle Number

(A)

0 200 400 600 800 1000 1200 14000.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

Nom

aliz

ed A

h-C

apac

ity (A

h)Cycle Number

(B)

Impedance-Based SOH Estimation