An Impedance-Based BMS to Identify Bad Cells Rengaswamy Srini Srinivasan Bliss G. Carkhuff Rengaswamy.srinivasan@jhuapl.edu (443) 841-8825
Impedance-Based T internal, R internal, SOC and SOH 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 -Z'' / 0.0012 0.0008 0.0004 0.0000-0.0004 SOC 87% 80% 70% 60% 50% 40% 30% 20% 10% 1 khz 20 Hz (SoC-Insensitive) Temperature Sensitive 0.0008 0.0012 0.0016 0.0020 Z' / Sensitive to SOC and Temperature 0.8 Hz Methodology: Srinivasan et al., Electrochimica Acta, Vol. 56: 6198-6204, Year 2011 Perturbing the cell with a smallamplitude 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 (T a ) Impedance at any one frequency between 10 Hz to 20 Hz measures cathode temperature (T c ) T a and T c can be estimated using phase shift values only High-frequency X-axis intercept corresponds to electrolyte resistance, R s Impedance at low-frequency (<10 Hz) is sensitive to temperature and state of charge 2
Impedance-Based T internal, R internal, SOC and SOH 3 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.
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 4
The 2009 Incident: BMS with Voltage, Current and Surface Temperature Monitors Electrical and Thermal Data Battery Voltage (V) and Current (A) Time (hour) Temperature ( 0 C) What purpose did the voltmeter, ammeter and thermocouple serve? 5
What does a voltmeter tell you? 6 Cell Voltage (V) 4.5 4.0 3.5 3.0 50% SOC 3.65771 V 3.65449 V 3.65234 V 30% SOC 3.42783 V 3.42246 V 3.42031 V ±0.0027 V 10% SOC 3.20439 V Cell 40A Cell 40B Cell 40C 3.21621 V ±0.00387 V 3.21084 V ±0.00592 V -50 0 50 100 150 200 Time (minute) 83% SOC 4.19160 V 4.13779 V 4.19590 V New 3-cell battery ±0.00377 V Cell voltages in a new battery are nearly identical
What does a voltmeter tell you? 4.5 99% SOC 4.18193 V 4.17871 V 4.17764 V ±0.00223 Cell Voltage (V) 4.0 3.5 3.0 30% SOC 3.47188 V 3.46865 V 3.45684 V ±0.00792 <5% SOC 2.88643 V 2.86709 V 2.76826 V ±0.06338 2.5-200 0 200 400 600 800 1000 Time (minute) Cell 11 Cell 30A Cell 30 Cycle-aged 3-cell battery Cell voltages in a cycled battery are nearly identical 7
8 What does a voltmeter tell you? Cell Voltage (V) 4.50 4.25 4.00 3.75 3.50 Cell Voltage (V) 4.5 4.0 3.5 3.0 2.5 Cell 1 Cell 2 Cell 3 Bad Cell Cell 5 Cell 6 83% SOC 4.1775 V 4.1860 V 4.1849 V 4.1850 V 4.1892 V 4.1913 V ±4.73 mv 2.0-200 0 200 400 600 800 1000 Time (minute) 3.25 180 200 220 240 260 280 Time (minute) Cell 1 Cell 2 Cell 3 Bad Cell Cell 5 Cell 6 Bad Cell: Twice over-discharged to 0.01 V 30% SOC 3.4706 V 3.4674 V 3.4706 V 3.4642 V 3.4674 V 3.4749 V ±3.68 mv Cell voltages in a battery with a bad cell are nearly identical
What does a voltmeter tell you? Nothing much, unless you drain the battery Cell Voltage (V) 4.5 4.0 3.5 3.0 2.5 Cell Voltage (V) 4.5 4.0 3.5 3.0 2.5 Cell 1 Cell 2 Cell 3 Bad Cell Cell 5 Cell 6 83% SOC 4.1775 V 4.1860 V 4.1849 V 4.1850 V 4.1892 V 4.1913 V ±4.73 mv 30% SOC 3.4706 V 3.4674 V 3.4706 V 3.4642 V 3.4674 V 3.4749 V ±3.68 mv Cell 1 Cell 2 Cell 3 Bad Cell Cell 5 Cell 6 10% SOC 3.3239 V 3.3229 V 3.3250 V 3.2963 V 3.3112 V 3.3229 V ±11.34 mv 9 2.0-200 0 200 400 600 800 1000 Time (minute) 2.0 Low Voltage Cutoff = 6x2.7 V 180 200 220 240 260 280 300 320 Time (minute) Cell voltages in a new battery are nearly identical until the battery is fully discharged
What does Surface-Mounted Sensor Identifies? 25 26 Surface Temperature ( 0 C) 24 23 22 Bad Normal T = 1 ºC 24 22 20 18 Battery Voltage (V) 10 21 16 550 600 650 700 750 800 Time (minute) Cell Temperatures in a battery with a bad cell are nearly identical
Protecting Armors for Battery Safety? AMMETER VOLTMETER THERMOCOUPLE 11 Archaic instruments do not help protect Li-ion batteries
The 2016 Incident: Had Battery Voltage and PackTemperature Monitors in 3 seconds http://gizmodo.com/this-is-why-you-should-take-lithium-ion-battery-fires-v-1788281947 12 Archaic instruments did not help protect this Li-ion battery
What does a thermocouple tell you? 12 12 Temperature ( 0 C) 10 8 6 Discharge Current Surface Temperature 10 8 6 4 2 Current (A) 13 0 4-60.00-30.00 0.00 30.00 60.00 Discharge Time (second) T Surface response-time is slow
What does a thermocouple tell you? 14 Temperature ( 0 C) 44 40 36 32 28 24 20 16 2.5-Ah Cell is being overcharged Surface Temperature Current Capacity 0 2000 4000 6000 8000 Charge Time (second) Voltage Current 2.5 Ah T surface is <32 ºC, indicating everything is fine! 5 4 3 2 1 0 Voltage (V), Current (A) or Capacity (Ah)
What does a thermocouple tell you? 70 Rate of discharge and charge = 1C 13 60 12 Temperature ( 0 C) 50 40 30 Surface Temperature Voltage 11 10 Battery Voltage (V) Discharge 20 Charge (CC part) 9 10-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Time (hour) Srini; 10/14/2015 T surface is <40 ºC therefore safe? 15
A Practical Impedance-Based BMS Non-Invasive and Autonomous 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 16
Sensitivity: Measurements based on bench-top impedance meter (Solartron) - Imaginary (milli-ohm) 6 4 2 Cell #1 Cell # 2 Cell # 3 70 Hz (A) New 0.082 Hz - Imaginary (milli-ohm) 6 4 2 Cell # 4 Cell # 5 Cell # 6 Cell # 7 Cell # 8 Cell # 9 70 Hz (B) Aged 0.082 Hz 1.6 khz 0 16 18 20 22 24 26 28 Real (milli-ohm) 1.2 khz 0 16 18 20 22 24 26 28 Real (milli-ohm) - Imaginary (milli-ohm) 200000 160000 120000 80000 40000 1.2 khz (C) One Overcharged 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) - Imaginary (milli-ohm) 6 4 2 Cell # 4 Cell # 5 Cell # 6 Cell # 7 Cell # 8 Cell # 9 Over-discharged (twice to 0.01 V) 70 Hz 70 Hz (C) One Over-discharged 0.082 Hz 0.082 Hz 17 0 0 40000 80000 120000 160000 200000 Real (milli-ohm) 1.2 khz 1.2 khz 0 16 18 20 22 24 26 28 30 Real (milli-ohm)
Miniaturized Impedance-Based BMS BMS 16-Cell Multiplexer 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 18
Perspectives of Impedance-Based BMS Far simpler than using thermocouple for T surface 19
Identify Matched Cells within Seconds BMS Output (Arbitrary Unit) 3460 3440 3420 3400 3380 3360 New Cell #3 = 3414 15 Cell #1 = 3406 9 Cell #2 = 3381 9 - Imaginary (milli-ohm) 6 4 2 Cell #1 Cell # 2 Cell # 3 70 Hz 0.082 Hz 0 50 100 150 200 Time (second) 1.6 khz 0 16 18 20 22 24 26 28 Real (milli-ohm) The three cells in the battery are matched within ±0.5 % 20
BMS Identifies Mismatching in Aged Cells BMS Output (Arbitrary Unit) 2680 2640 2600 2560 Cell #1 = 2668 8 Cell #6 = 2637 9 Cell #2 and #3 = 2600 8 Cell #4 = 2550 9 - Imaginary (milli-ohm) 6 4 2 Imepdace data shows the cells are not matched Cell # 4 Cell # 5 Cell # 6 Cell # 7 Cell # 8 Cell # 9 70 Hz Calendar Aged (6 months) 0.082 Hz 2520 Cell #5 = 2541 9 0 200 400 600 Time (second) 1.2 khz 0 16 18 20 22 24 26 28 Real (milli-ohm) In the 6-cell battery, calendar ageing drove the cell apart by ±2% 21
BMS Output of the Five Calendar-Aged Cells Five Calendar-Aged, but Otherwise Normal Cells 2600 26 BMS Output (Arbitrary Unit) 2550 2500 2450 2400 24 Battery Voltage (V) 22 20 18 2350 16 550 600 650 700 750 800 Time (minutes) 22
One of the Six Cells was over-discharged: The BMS identifies the bad cell 2520 Normal Cell 26 2520 Bad Cell 26 BMS Output (Arbitrary Unit) 2500 24 2480 22 2460 20 2440 18 Battery Voltage (V) BMS Output (Arbitrary Unit) 2500 2480 2460 2440 24 22 20 18 Battery Voltage (V) 2420 550 16 600 650 700 750 800 Time (minute) 2420 550 16 600 650 700 750 800 Time (minute) 23
Why anode temperature is more important? Samsung SDI F26 (2.6-Ah Li-ion) behavior during overcharging 44 5 Temperature ( 0 C) 40 36 32 28 24 20 16 Voltage Anode Temperature Current Capacity Gas release Surface Temperature Current 0 2000 4000 6000 8000 4 3 2 1 0 Voltage (V), Current (A) or Capacity (Ah) Charge Time (second) 24 T surface misses overcharging completely, but not T anode Srinivasan et al., Journal of Power Sources, 241, pp. 560-566; Year: 2013
Continuous Discharge-Charge at 1C Rate T internal can be much hotter than T surface 70 13 Temperature ( 0 C) 60 50 40 30 T internal T surface 12 11 10 Battery Voltage (V) Discharge 20 Charge (CC part) 9 10-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Time (hour) Srini; 10/14/2015 T internal >> T surface when the current is continuous 25
Srinivasan et al., Journal of Power Sources, 262, pp. 93-103; Year: 2014 Surface temperature can be misleading 26 Temperature ( 0 C) 12 10 8 6 Surface Anode Cathode 0 4-60.00-30.00 0.00 30.00 60.00 Discharge Time (second) T Surface response time is slower 12 10 8 6 4 2 Current (A)
Fast Charge through Feedback-Controlled Closed-Loop 27 Charging current profile is semi-autonomously determined by the charger: based on the user-set limits on T internal and Cell Voltage Temperature ( 0 C) 36 32 28 24 20 Current T internal T surface 0 20 40 60 80 100 Charge Time (minute) Voltage 10 8 6 4 2 0 Voltage (V) or Current (A) Example shown: Conventional charging time is 150 minutes Fast charger-enabled time is 95 minutes
Impedance-Based SOH Estimation Electrolyte Resistance (milli ohm) 28 (A) 26 24 22 20 18 0 200 400 600 800 1000 1200 1400 Nomalized Ah-Capacity (Ah) 1.02 1.00 (B) 0.98 0.96 0.94 0.92 0.90 0.88 0 200 400 600 800 1000 1200 1400 Cycle Number Cycle Number 28