improved soh estimation through coulomb counting position... · • in this case it must be...

University of MarylandCopyright © 2011 CALCE

1Center for Advanced Life Cycle Engineering

Improved SOH Estimation through Coulomb Counting

Nick Williard, Wei He, Michael Osterman, Michael Pecht

Position Statement for BMS workshop at the 2011 PHM Society Conference

Montreal, Quebec

Objective: To demonstrate a novel method of estimating state of health within the framework of the coulomb counting technique

Center for Advanced Life Cycle EngineeringUniversity of MarylandCollege Park, MD 20742

http://www.calce.umd.edu



• The Center for Advanced Life Cycle Engineering (CALCE) formally started in 1984, as a NSF Center of Excellence in systems reliability.

• One of the world’s most advanced and comprehensive testing and failure analysis laboratories

• Funded at .$6M by over 150 of the world’s leading companies

• Supported by over 120 faculty, visiting scientists and research assistants

• Received NSF innovation award in 2009

CALCE Overview



State Of Health

0

0.4

0.8

1.2

0 200 400 600 800 1000

Cap

acity

(Ah)

Cycle Number

• State of Health (SOH) refers to the general decline in battery performance with usage or aging.

• SOH can be characterized by capacity fade, power fade, or increase in internal resistance.

• Monitoring SOH is critical for performing condition based maintenance and for mitigating failure.

Decline in Capacity vs. Cycle Number

• In an electric vehicle, a SOH indicator would be analogous to a “check engine” light, which informs the user that maintenance or battery replacement is required when some degradation threshold is crossed.

When using capacity as the metric for degradation, we could define SOH as :

100%

Where the SOH at cycle c is equal to the capacity Q at cycle c over the capacity at the beginning of life, expressed as a percentage



Determining SOH Electrochemical Impedance

Spectroscopy (EIS) Internal DC Resistance Coulomb Counting

Parameters from EIS are fit to an equivalent circuit model. Estimations can be improved though filtering methods such as particle filter and Kalman filter.

)()(

12

12

IIVVR

Internal DC resistance can be measured by applying 2 current pulses at I1 and I2 and then measuring the voltage change at each pulse. This feature has been shown to have a linear relationship to capacity

Coulomb counting refers to the continuous monitoring of current that inters and leaves the battery. By integrating current with time, the capacity can be calculated. Whenever the battery is fully discharged to it’s cut-off voltage the maximum capacity can be calculated and compared with the maximum capacity at the beginning of life then:

100%



Effect of Partial Discharges on Coulomb Counting Capacity Measurements

-0.6-0.58-0.56-0.54-0.52-0.5

3.853.9

3.954

4.054.1

4.15

0 0.2 0.4 0.6

Cur

rent

(A)

Volta

ge (V

)

Time (hours)

-0.6-0.58-0.56-0.54-0.52-0.5

3.23.43.63.8

44.2

0 0.5 1 1.5 2

Cur

rent

(A)

Volta

ge (V

)

Time (hours)

Discharge was cut off at 3.4V

Discharge was cut off at 3.9V

Capacity = 1.06Ah

Capacity = 0.24Ah

Idt (Ah)Capacity

If a particular discharge is cut-off midway though it’s operation the, observed capacity will be lower due to the decreased about of time spent in operation. This decrease in capacity is not due too degradation mechanisms but rather the time spent in operation

CurrentVoltage



Shallow Charging Example

0

0.2

0.4

0.6

0.8

1

0 400 800 1200

Cap

acity

(Ah)

Cycles

• The plot below shows the measured capacity of a battery that under went shallow charging for 900 cycles.

• The charge profile was then changed to cycle from a completely charged state to a completely discharged state.

• It can be seen that little degradation occurred during the shallow charging cycles.

Cycled between 2.7 and 3.7V

Cycled between 2.7 and 4.2V



Nature of Capacity Fade Assumptions•Discharging under a constant condition will produce a smooth continuous trend in capacity fade

•Different conditions lead to different rates in capacity fade •When all other conditions are constant, changes in capacity can be attributed solely to degradation Rate of degradation = when all discharge conditions are held constant

dcdQ

Cap

acity

Cycles

Idealized trends in capacity fade

Observed capacity measurements

Equivalent Capacity

Increasing cut-off Voltage



For Discharge Profiles That Do Not Reach Vdischarge

• When a capacity is recorded that does not reach the end cut-off voltage it can not be used to infer SOH.

• In this case it must be compared to a previous capacity value that was calculated at a similar cut-off voltage.

• When comparing capacities from similar cut-off voltages, the effective loss of capacity can be attributed to declining SOH rather than depth of discharge.

• When two discharges are recorded that have similar cut-off voltages the equivalent capacity can be calculated by:

Where: c is the cycle number, Qsimilar are the capacity values that were recorded at similar cut-off voltages, and csimilar are the cycle numbers associated with the similar capacity values .



Equivalent Capacity (Case Study)• A battery underwent charge/discharge cycling at a constant discharge current • Every cycle the battery was charged to it’s fully charged state • Every 15 cycles the discharge cut-off voltage was randomly changed

• The change in cut-off voltage resulted in variations in the observed capacity for each cycle

• This test simulates the situation where a user decides to re-charge their battery before it has reached a completely discharged state

Cut-off Voltage



Results

• The red points show the observed capacity for each cycle

• The blue points show the equivalent capacity for each cycle (estimation of Qmax)

• The green points are cycles that were fully discharged indicating the true value of Qmax

• The percent error between the equivalent capacity and the next observed Qmax is shown with respect to the number of cycles between recalibration. The model was able to run for over 600 cycles with less than a 10% error of the maximum capacity

-10

-5

0

5

10

0 200 400 600 800% E

rror

Of Q

equi

vale

ntto

Qm

ax

# of cycles between recalibration



Conclusions

• By calculating an equivalent capacity, the maximum capacity can be estimated for partial discharges allowing for SOH to be updated for every cycle by:

• Frequent SOH estimations using equivalent capacity can improve maintenance strategies and mitigate failure.

100%



A special thanks to our research sponsors!• Alcatel-Lucent• Aero Contol Systes• Agilent Technologies• American Competitiveness Inst.• Amkor• Arbitron• Arcelik• ASC Capacitors• ASE• Astronautics• Atlantic Inertial Systems• AVIC• AVI-Inc• Axsys Engineering• BAE Systems• Benchmark Electronics• Boeing• Branson Ultrasonics• Brooks Instruments• Buehler• Capricorn Pharma• Cascade Engineering • CAPE – China• Celestical International• Channel One International• Cisco Systems, Inc.• Crane Aerospace & Electronics• Curtiss-Wright Corp• CDI• De Brauw Blackstone Westbroek• Dell Computer Corp.• DMEA• Dow Solar• DRS EW Network Systems, Inc.• EIT, Inc.• Embedded Computing & Power• EMCORE Corporation• EADS IW France• EMC

• Emerson Advanced Design Ctr• Emerson Appliance Controls• Emerson Appliance Solutions• Emerson Network Power• Emerson Process Management• Engent, Inc.• Ericsson AB• Essex Corporation• Ethicon Endo-Surgery, Inc.• Exponent, Inc.• Fairchild Controls Corp.• Filtronic Comtek• GE, GE Healthcare• General Dynamics, AIS & Land Sys.• General Motors• Guideline• Hamlin Electronics Europe• Hamilton Sundstrand• Harris Corp• Henkel Technologies• Honda• Honeywell• Howrey, LLP• IBM• Intel• Instituto Nokia de Technologia• Juniper Networks• Johnson and Johnson• Johns Hopkins University• Kimball Electronics• L-3 Communication Systems• LaBarge, Inc• Lansmont Corporation • Laird Technologies • LG, Korea• Liebert Power and Cooling• Lockheed Martin Aerospace• Lutron Electronics• Maxion Technologies, Inc.

• Microsoft• MIT Lincoln Laboratory• Motorola• Mobile Digital Systems, Inc.• NASA• National Oilwell Varco• NetApp• nCode International• Nokia Siemens• Nortel Networks• NOK AG• Northrop Grumman• NTSB• NXP Semiconductors• Ortho-Clinical Diagnostics• Park Advanced Product Dev. • Penn State University• PEO Integrated Warfare• Petra Solar • Philips• Philips Lighting• Pole Zero Corporation• Pressure Biosciences• Oracle• Qualmark• Quanterion Solutions Inc• Quinby & Rundle Law• Raytheon Company• Rendell Sales Company• Research in Motion• Resin Designs LLC• RNT, Inc.• Roadtrack• Rolls Royce• Rockwell Automation• Rockwell Collins• Saab Avitronics• Samsung Mechtronics• Samsung Memory

• S.C. Johnson Wax• Sandia National Labs• SanDisk• Schlumberger• Schweitzer Engineering Labs • Selex-SAS• Sensors for Medicine and Science• SiliconExpert• Silicon Power• Space Systems Loral• SolarEdge Technologies• Starkey Laboratories, Inc• Symbol Technologies, Inc• SymCom• Team Corp• Tech Film• Tekelec• Teradyne• Textron Systems• The Bergquist Company• The M&T Company• The University of Michigan• Tin Technology Inc.• TÜBİTAK Space Technologies• U.K. Ministry of Defence• U.S. Air Force Research Lab• U.S. AMSAA• U.S. ARL• U.S. NSWC, NAVAIR• U.S. Army Picatinney/UTRS• U.S. Army RDECOM/ARDEC• Vectron International, LLC• Vestas Wind System AS• Virginia Tech• Weil, Gotshal & Manges LLP• WesternGeco AS• Whirlpool Corporation• WiSpry, Inc.• Woodward Governor

improved soh estimation through coulomb counting position... · • in this case it must be...

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