Review
A review on the key issues for lithium-ion battery management in electric vehicleswhite christmas歌词
Languang Lu a ,Xuebing Han a ,Jianqiu Li a ,Jianfeng Hua b ,Minggao Ouyang a ,*
a State Key Laboratory of Automotive Safety and Energy,Tsinghua University,Beijing 100084,PR China b
Key Power Technology Corporation,Ltd,Beijing 100084,PR China
h i g h l i g h t s
<This paper brie fly reviews key technology of battery management system in EV.<The composition of battery management system is analyzed.
<The battery state estimation methods are summarized and compared.<The battery uniformity theory and equalization methods are reviewed.<The battery fault diagnosis methods are discusd.
a r t i c l e i n f o
Article history:
Received 22July 2012Received in revid form 24September 2012
依靠我英文歌曲Accepted 18October 2012
Available online 26November 2012Keywords:
Vehicle lithium-ion battery in electric vehicles
Battery management system Cell voltage measurement Battery states estimate
Battery uniformity and equalization Battery fault diagnosis
a b s t r a c t
Compared with other commonly ud batteries,lithium-ion batteries are featured by high energy density,high power density,long rvice life and environmental friendliness and thus have found wide application in the area of consumer electronics.However,lithium-ion batteries for vehicles have high capacity and large rial-parallel numbers,which,coupled with such problems as safety,durability,unif
ormity and cost,impos limitations on the wide application of lithium-ion batteries in the vehicle.The narrow area in which lithium-ion batteries operate with safety and reliability necessitates the effective control and management of battery management system.This prent paper,through the analysis of literature and in combination with our practical experience,gives a brief introduction to the composition of the battery management system (BMS)and its key issues such as battery cell voltage measurement,battery states estimation,battery uniformity and equalization,battery fault diagnosis and so on,in the hope of providing some inspirations to the design and rearch of the battery management system.
Ó2012Elvier B.V.All rights rerved.
1.Introduction
Compared with other commonly ud batteries,lithium-ion batteries are featured by high energy density,high power density,long life and environmental friendliness and thus have found wide application in the area of consumer electronics.However,auto-motive lithium-ion batteries have high capacity and large rial-parallel numbers,which,coupled with such problems as safety,durability,uniformity and cost,impos limitations on the wide application of lithium-ion batteries in the vehicle.Lithium-ion batteries must operate within the safe and reliable operating
area,which is restricted by temperature and voltage windows.Exceeding the restrictions of the windows will lead to rapid attenuation of battery performance and even result in safety problem.According to the instructions of most battery manufac-turers,the reliable operating temperatures required by a majority of current automotive lithium-ion batteries (graphite/LiMn 2O 4or by acronyms C/LMO,C/LiCo x Ni y Mn z O 2or C/NCM,C/LiFePO 4or C/LFP,C/LiNi 0.8Co 0.15Al 0.05O 2or C/NCA)are:discharging at À20to 55 C and charging at 0e 45 C and for lithium-ion battery with Li 4Ti 5O 12or LTO negative electrode,the minimum charge temper-ature can be À30 C.Usually,the operating voltage of lithium-ion batteries is between 1.5V and 4.2V (C/LCO,C/NCA,C/NCM and C/LMO about 2.5e 4.2V,LTO/LMO about 1.5e 2.7V and C/LFP about 2.0e 3.7V).As indicated in Fig.1,normally when the temperature is 90e 120 C,the SEI film will start exothermic decomposition [1e 3],
*Corresponding author.Tel.:þ861062792797;fax:þ861062789699.E-mail address:ouymg@tsinghua.edu (M.
Ouyang).Contents lists available at SciVer ScienceDirect
Journal of Power Sources
journal ho mep age:www./locate/jpo
wsour
0378-7753/$e e front matter Ó2012Elvier B.V.All rights rerved.dx.doi/10.1016/j.jpowsour.2012.10.060
Journal of Power Sources 226(2013)272e 288
but some electrolyte systems will decompo at a lower tempera-ture of about69 C[4].When the temperature exceeds120 C,the SEIfilm after decomposition is unable to protect negative carbon electrode from side reactions with the organic electrolyte and combustible gas would be produced[3].When the temperature is about130 C,the parator will start melting and shutting the cell down[5,6].When the temperature becomes higher,the positive material will start decomposition(LiCoO2will start decomposition at temperature of about150 C[7],LiNi0.8Co0.15Al0.05O2at about 160 C[8,9],LiNi x Co y Mn z O2at about210 C[8],LiMn2O4at about 265 C[1]and LiFePO4at about310 C[7])and produce oxygen. When the temperature is above200 C,the electrolyte will decompo and produce combustible gas[3],and it will have violent reaction with the oxygen produced by the decomposition of the positive electrode[9]and start to catchfire and lead to thermal runaway.To charge lithium-ion batteries below0 C will lead the metallic lithium to deposit on the carbon negative electrode surface and therefore reduce the cycle life of batteries[10].At an extremely low temperature,the cathode of batteries will break down,and result in short circuit[11].If the voltage is too low or the batteries are overdischarged,the pha change will lead the lattice to collap and therefore the performance of the batteries is influ-enced[12].Moreover,it will lead the negative copper collector to dissolve in the electrolyte(For this reaction,the thermodynamic equilibrium potential is0.521V vs.SHE(Standard Hydrogen Elec-trode)or
3.566V vs.Li/Liþunder standard condition).When the batteries are recharged,the copper dendrite will be formed at the negative electrode,which,conquently,will result in short circuits within the batteries[1,12].An extremely low voltage or over-discharge will also lead to the reduction of the electrolyte,produce combustible gas[12]and therefore po potential curity risks.An extremely high voltage or overcharge will lead the positive elec-trode to compo and therefore a great amount of heat is produced [12,13].It will also lead the metallic lithium to be deposited on the surface of negative electrode,which will accelerate the capacity fade,result in internal short circuits and safety problem[12],as well as the decomposition of the electrolyte(the common elec-trolyte will decompo if the voltage is higher than4.5V[12]).To solve tho problems,people try to develop new battery system that could be working under very bad situations,and on the other hand,the current commercial lithium-ion batteries must befitted with a management system,through which the lithium-ion batteries can be controlled and managed effectively,thus every single cell would be working under proper conditions that tho fault described above would not happen which means that every cell should be operated within the lithium-ion battery safety operating window shown in Fig.1.This prent paper,through the analysis of literature and in combination with our practical expe-rience,gives a brief introduction to the composition of the BMS and its key issues,including such issues as battery cell voltage measurement,battery states estimation,battery uniformity and equalizat
ion,battery fault diagnosis and so on,in the hope of providing some inspirations to the design and rearch of the battery management system.
2.Status of lithium-ion battery and battery management system(BMS)in EV
Many kinds of lithium-ion batteries are employed in electric vehicle(EV).The most widely ud power battery cells contain carbon anode(negative electrodes),and now the LTO anode is also developed fast for the kind of anodes would help to improve the battery dura-bilityandperformanceoffastcharging.Thepositiveelectrodematerial of the power battery could be LMO,LFP,NCM,NCA,etc.Some of the current EV and the employed batteries are listed in Table1[14e22].
Usually,the capacity and voltage of the battery cell ud in the EV are relatively small.Sofirst the single battery cells should be packed and integrated to a battery module,and the battery system in the EV often contains one or more module according to the requirement.The battery system usually consists of hundreds or thousands of single cells.To manage so many cells,the battery management system(BMS)is very important.
There is still no connsus of thefinal definition of BMS and what BMS do.According to Ref.[23,24],w
e adopt the wide view that BMS is any system that manages the battery.The system could be electronic systems,mechanical systems or any possible device and technology.The battery could be a single cell,battery module or battery pack,and it could be rechargeable or
non-rechargeable. Fig.1.Safety operating window for lithium ion battery(modified from[11]).
Table1
Some current EV and the employed lithium-ion batteries.去英国留学费用
Vehicle Battery
supplier Positive
electrode
Negative
electrode
Nissian Leaf EV Automotive Energy
Supply(Nissan NEC JV)
LMO C
Chevrolet Volt Compact Power
(subsidiary of LG Chem)
LMO C
Renault Fluence Automotive Energy
Supply(Nissan NEC JV)
LMO C
Tesla Roadster NCA C
Tesla Model S Panasonic Energy Nickel-type
BYD E6BYD LFP C
Subaru G4e Subaru LVP C
Honda Fit EV Toshiba Corporation NCM LTO
L.Lu et al./Journal of Power Sources226(2013)272e288273
The system could manage the battery by monitoring the battery,estimating the battery state,protecting the battery,reporting the data,balancing it,etc.
BMS in vehicles is comprid of kinds of nsors,actuators,controllers which have various algorithms and signal wires.Three main tasks of the BMS in vehicles are as follows [25].
To protect the cells and battery packs from being damaged. To make the batteries operate within the proper voltage and temperature interval,guarantee the safety and prolong their rvice life as long as possible.
To maintain the batteries to operate in a state that the batteries could ful fill the vehicles ’requirements.
And the automotive power batteries must also meet relevant standards or speci fications [26e 32].
The basic framework of hardware from BMS in vehicle is shown in Fig.2.
TheBMSwouldhaveinputssuchas:maincircuitcurrentnsorand voltage nsor to measure the main c
urrent and voltage;temperature nsors to measure the temperature of the cells,the temperature outside the batterybox,andmaybealsothe temperature atthe battery coolant inlet and outlet;general analog inputs like accelerate pedal nsorand brake pedal nsor;and general digital inputs like Start key ON/OFF signals,charging allow/banned switch,etc.
The BMS would have outputs such as:thermal management module like fan and electric heater to do the cooling control and heating control;balancing module like capacitor þswitch array and dissipation resistance to do the battery equalization;voltage safety management like main circuit contactor,battery module contactor;general digital outputs like charging indicator,failure alarm;and communication module.And also the BMS would have the internal power supply module and global clock module.And it may have the charging system and man e machine interface module.The electromagnetic compatibility should also be guar-anteed.The bad working environment of electric motor cars requires BMS to posss good anti-electromagnetic interference capacity and nd out low levels of radiation as well.
The software of BMS would cover the functions.(1).Battery parameters detection
This includes total voltage,total current and individual cell voltage detection (to prevent overcharging,overdischarging and
chairman
antipole),temperature detection,smoke detection,insulation detection,collision detection,impedance detection and so on.(2).Estimation of battery states
This includes state of charge (SOC)or depth of discharge (DOD),state of health (SOH)and state of function (SOF).SOC or DOD of batteries is estimated according to such conditions as working current,temperature and voltage.SOH is estimated according to the extent of abu and performance degradation of batteries.SOF is estimated according to SOC,SOH and operating environment of batteries.
(3).On-board diagnosis (OBD)
The faults include nsor fault,actuator fault,network fault,battery fault,overvoltage (overcharge),undervoltage (over-discharge),overcurrent,ultra high temperature,ultra low tempera-ture,loo connection,exceeding combustible gas concentration,insulation fault,uniformity fault,over-fast temperature ri andso on.(4).Battery safety control and alarm
This includes thermal system control and high voltage safety control.When the faults are diagnod,the vehicle control unit or the charger will be informed through the network and they are required to handle the faults (when a certain threshold value is exceeded,BMS can also cut-off the b
attery power supply)to prevent damage to batteries or injuries to people caud by high temperature,low temperature,overcharge,overdischarge,over-current,electric leakage and so on.(5).Charge control
letter of guaranteeOn the basis of the properties of its own batteries and the power level of the charger,BMS could control the charger to charge the batteries.
(6).Battery equalization
According to the information of each cell,BMS adopts such equalization methods as equalizing charging,dissipative equaliza-tion or non-dissipative equalization to make the SOC between cells as consistent as possible.
Digital core
Charging System Temperature nsors array
Bus
Voltage/current measurement
General analogue inputs Cell voltage measurement
General digital
inputs Internal power supply module
qq英文名字Man-machine interface module Communication
module
Thermal management module
Balancing control module
High voltage safety control
Global clock module General digital
outputs
Accelerating pedals nsor 1Brake pedal nsor 1Brake pedal nsor 2Start key ON/OFF signals
Economy/sport switch
Charging allow/banned switch Battery box outside temperature Battery box coolant inlet temperature Main circuit positive contactor Battery module contactor
Main circuit fu
main circuit current nsor Cooling control Heating control
Balanced cell lect Active/passive balancing Calibration channel
The controller CAN channel 1Balancing working indicator SOH indicator Charging indicator
Overload overvoltage alarm Battery box cover state switch Insulation measurement main circuit voltaget nsor Battery box coolant outlet temperature
Battery cell temperatures
Battery cell voltages Battery module voltage Accelerating pedals nsor 2
Balancing allow/banned switch
Main circuit negative contactor The controller CAN channel 2
Failure alarm
Fig.2.Basic framework of software and hardware of BMS in vehicle.
L.Lu et al./Journal of Power Sources 226(2013)272e 288
274
(7).Thermal management
According to the temperature distribution within the battery pack and the requirements of charge or discharge,BMS decides whether to start heating or cooling as well as heating power and cooling power.
(8).Networking
Since it is not convenient to disasmble BMS in a vehicle and meantime the vehicle is required to have network functions,it is desirable to conduct on-line calibrating and monitoring,automatic code g
eneration and on-line program downloading(program update without disasmbling the ca)for BMS without dis-asmbling the ca.Usually the network CAN(Controller Area Network)is adopted.
(9).Information storage
BMS is ud to store key data,such as SOC,SOH,accumulated charge and discharge Ah numbers,fault code,uniformity and so on.
The real BMS in the vehicle may only have parts of the hardware and software which is mentioned above.There should be at least one cell voltage nsor and one temperature nsor for each battery cell.For a battery system with only scores of cells,there may be only one BMS controller or even the BMS function would be integrated in the main controller of the vehicle.And for the battery system with hundreds of cells,there may be one master controller and veral slave controllers which only manage one battery module. For each battery module with dozens of cells,there could be some module circuit contactor and balancing module,and the slave controller would manage the battery module like measuring the voltage and current,controlling the contactor,equalizing the cells and communicating with the master controller.The master controller would do the battery state estimation,fault diagnosis, thermal management,etc.according to the data reported by the slave controllers.
Nowadays,BMS has become a focus that is developed by various vehicle companies,colleges and universities[33e39].Currently, quite a few companies have developed corresponding BMS prod-ucts,such as the products developed by Beijing Key Power Tech-nology Co.,Ltd[40],Harbin Guantuo Power Equipment Co.Ltd[41], Anhui Ligoo New Energy Technology Co.Ltd[42],Huizhou Epower Electronic Co.Ltd[43],American Elithion Corporation[44], Australian EV power[45]and British REAPSystems[46],etc.
3.Key issues of BMS
Although BMS has many functional modules,this prent paper only analyzes and summarizes its key issues.At prent,the key issues or difficulties of BMS are preci measurement of cell voltage,estimation of battery states,battery uniformity and equalization,and battery fault diagnosis.
3.1.Cell voltage measurement(CVM)
The major difficulties of CVM lie in:(1)the battery packs of electric motor cars have hundreds of cells connected in ries and thus there are many channels to measure the voltage.As there is accumulated potential when the cell voltage is measured and the accumulated potential of each cell is different from that of another,which makes it impossible to have unified compensation or eliminatio
n methods,certain difficulties ari in the design of circuit measurement.(2)Voltage measurement requires high precision(especially for C/LiFePO4battery).Estimation of SOC and other battery states impos high requirements on cell voltage precision.Here we take the C/LFP and LTO/NCM type batteries as example.Fig.3shows the open circuit voltage(OCV)of batteries C/LiFePO4and LTO/NCM as well as corresponding SOC variation per mV voltage.From thisfigure,we canfind that the slope of OCV curve of LTO/NCM is relatively steep and the maximal corresponding SOC rate of change per mV voltage is lower than 0.4%in most range(except SOC60e70%).Therefore,if the measurement precision of cell voltage is10mV,then the SOC error obtained through OCV estimation method is lower than4%. Accordingly,for LTO/NCM battery,the measurement precision of cell voltage needs to be smaller than10mV.But the slope of OCV curve of C/LiFePO4is relatively gentle and the maximal corre-sponding SOC rate of change per mV voltage reaches4%in most range(except SOC<40%and65e80%).Therefore,the collection precision of cell voltage has a high requirement,reaching around 1mV.At prent,most collection precision of cell voltage reaches only5mV.
In literature[47]and[48]the voltage measurement methods of batteries cells and fuel cells stacks are,respectively,summarized. The methods include resistance voltage divider method,optical couplin
g isolation amplifier method,discrete transistor method [49],distributed measurement method[50],optical coupling relay method[51],and so on.Currently,the voltage and temperature sampling of cells has formed chip industrialization and Table2 compares the performance of chips ud in most BMS.
3.2.Battery states estimation
Battery states include SOC,SOH and SOF and their relationship is shown in Fig.4.SOH is determined by rvice life prediction and fault diagnosis output together.SOF is determined by SOC,SOH and the fault states.SOF takes into consideration the influence of aging factor,SOC range,temperature range and fault level.
3.2.1.SOC estimation algorithm
There is not afinal generally accepted definition of SOC.Here we take this view that the state of charge(SOC)means the ratio of the remaining charge of the battery and the total charge while the battery is fully charged at the same specific standard condition[52]. And the SOC is often expresd in percent,100%means fully charged and0%means fully discharged.
There is no doubt of this definition for a single battery cell,but for the battery module(or a battery pack,since the battery pack is consisted of veral modules,so to calculate the SOC of the battery pack from the SOC of the battery modules is just like the way tofind the SOC of the battery module from the SOC of the single cells.It is the same for the other states variables),the situation is a bit complicated.Battery module which is connected by veral cells in parallel could be considered as a single cell with high capacity and the SOC could be just estimated just like the single cell,since the lf-balancing characteristic of the parallel connection.
Though under the ries connection condition,the SOC of the battery module could also be estimated just like the single cell,but consider the battery uniformity,it would be better to considered in detail.Assume that the capacity and SOC of each single cell in the battery module are known.If there is a very efficiency and non-lossy balancing device,then the SOC of the battery module is: SOC M¼
P
SOC i C i
P
i
(1)
where SOC M means the SOC of the battery module,the SOC i means the SOC of the i th battery cell and the C i means the capacity of the i th battery cell.If the balancing device is not so efficiency,the real
L.Lu et al./Journal of Power Sources226(2013)272e288275英语广场
SOC of the battery module is related to the real performance of this balancing device.
If there is no balancing device or with dissipation balancing device,there would be some waste capacity which could not be ud like shown in Fig.5while there exists variations between the cells in the battery module.
Thus,the capacity of the battery module is:
C ¼min ðSOC i C i Þþmin ðð1ÀSOC i ÞC i Þ(2)
And the remaining available capacity of the battery module is:
C R ¼min ðSOC i C i Þ
(3)
Thus,the SOC of the battery module is:
SOC M ¼
min ðSOC i C i Þ
min ðSOC i C i Þþmin ðð1ÀSOC i ÞC i Þ
(4)
Anyway,with the preci estimation of the cell SOC and the uniformity of the battery modules,the SOC of the module could be calculated.The most challenging work is how to estimate the cell SOC for the BMS in the vehicles.
There are many methods to estimate the SOC in electrical chemistry laboratory like coulometric titration technique [53].But it is quite challenging to estimate the SOC of commercial batteries without
destruction of the battery or interruption of the battery power supply,especially the on-line estimation in vehicle.Currently there has been intensive study on SOC estimation algorithm would be introduced as follows.(1)Discharge test method
The most reliable method to determine the battery SOC is the discharge test with controlled ,speci fied discharge
Table 2
Statistics of battery management and equalization chips.Company and product name Analog Devices Co.AD7802Linear Technology Co.LTC6802
Texas Instrument Co.bq76PL536Atmel
Co.ATA6870Maxim
Co.MAX11068Voltage measurement channels 6126612Temperature measurement channels
62222Max chips in daisy chain 2036>161631Max cells in rial
120432>9696372Max voltage of daisy chain (V)380>1000
china girlN/A N/A N/A AD resolution (Bit)1212141212AD conversion time 1m s 1.08ms 6m s N/A 10m s Equalization
Yes
Yes
Yes
Yes
Yes
Operating temperature range ( C)
À40to 105À40to 85À40to 85À40to 85À40to 105Standby current (m A)46012101Input voltage range (V)
7.5e 30
10e 50
6e 36
skr什么意思中文6e 30
6e 70
a
OCV curves of LTO/NCM cell
b
SOC variation per mV voltage of LTO/NCM
cell
c
OCV curves of C/LiFeP O 4cell
时代周刊年度人物
d
SOC variation per mV voltage of C/LiFePO 4
cell
0.20.40.60.8122.22.42.6
2.8SOC
U O C / V
0.20.4
0.60.81
02e-34e-36e-38e-3SOC
d S O C /d O C V / m V -1
0.2
0.4
0.6
0.8
1
2.62.8
3.03.23.4
3.6SOC
U O C / V
0.20.4
0.60.81
02e-2
4e-2
6e-2
SOC
d S O C /d O C V / m V -1
Fig.3.OCV curves and the SOC variation per mV voltage (measured under 25 C,and rest time 3h).
L.Lu et al./Journal of Power Sources 226(2013)272e 288
276
