Battery Management System (BMS)
Let’s start by talking about the evolution of battery usage in the past decades. It has been implemented in numerous devices and has revolutionized to reach better performance at the same time increase efficiency. The Royal Swedish Academy of Sciences awarded Nobel Prize in Chemistry, 2019 to John B. Goodenough, M. Stanley Whittingham, and Akira Yoshino for the development of lithium-ion batteries, batteries, which were developed in the ’90s. It has not lost its charm, rather the direct opposite has happened, the revolution it has created is tremendous and today from iPod shuffle to electric trucks everything carries lithium in its pocket. Recent customers of lithium are the electric vehicles, transportation running on the same principle as of a battery toy car.
Why EV manufacturers chose lithium batteries over other battery technology?
The beauty embodied in lithium battery packs is the energy they can carry in the required volume. The high energy density of lithium batteries makes it perfect for an electric vehicle, lighter and efficient. Charging is always a concern for any EV owner and with a faster charging rate of lithium, packs outshine other technologies. Lithium charges at a faster rate and reduces the refuelling time of the vehicle.
Moving on to the construction of lithium cell -the chemical structure, solid electrolyte, and stable encapsulation make it suitable for application prone to mechanical vibrations and impacts. The lithium cells are found performing at the same efficiency in both stationary and mobile applications. like every coin has two sides, Lithium brings a good amount of constraints and restrictions in its usage.
Why do we need BMS?
Thermal instability has always remained the greatest obstacle in a lithium cell’s design and application. The efficiency plummets considerably at higher operating temperatures. Though lithium brings higher energy density, faster-charging rate, low self-discharge, and longer life cycle but these factors are maintained only when it is provided by a manager. Overcharging is another prime concern with lithium batteries usage therefore charging needs to be closely & accurately monitored. memory effect is also a benefactor to its disadvantages.
So, what is meant by a manager? it definitely is not a physical entity, and here goes the answer: BMS. Lithium battery packs are comprised of multiple cells connected in series and parallel configuration, hence cell balancing is the important aspect for efficient power delivery. Imbalance cell causes internal loading of battery reducing the state of charge and increasing failure rate. A cell balancing mechanism is required for proper battery pack operation and higher cell life. A battery management system helps lithium batteries maintaining and enhancing their performance with protection against hostile conditions to prevent irreversible failure.
What is Battery Management System?
“To make a best lithium battery pack just manage its voltage, current & temperature and this is what a BMS does”
As the name is self-explanatory, a system designed to manage and control a battery unit for proper operation. Initial designs only incorporated protection components i.e. fuses, FET switches with analogue comparators sensing deviation, and pulling the complete battery unit down in case of abruptions. With due course of time, lithium batteries found their way into several electronic devices, and hence, simultaneously the need to design an efficient manager to take care of its working increased. A modern BMS comprises multiple blocks integrated to build a complete lithium battery unit:
- Power Delivery Unit (PDU): Comprises of FET switch and gate driver circuit which control the outgoing and incoming power in the battery. Cell balancing with individual current bypass FETs is also part of the PDU. Few specific PDU also manages regenerative braking power
- Sensing and measurement: Comprises of on-chip or external ADCs to compute cell voltages, junction temperature, and charge/discharge current. SAR-based ADC provides a high sampling rate and resolution. The modern BMS chip has SAR-ADC-based AFE integrated into it.
- Communication block: Comprises of CAN transceiver, RS232 transceiver for communicating with parallel battery packs or with the vehicle control unit
- Central processor: Manages the above blocks with an embedded algorithm deciding as per inputs and battery state. It also manages the external and internal communications of BMS.
What are the functions provided by BMS?
Battery Management System (BMS) performs three primary functions:
- It protects the battery pack from being over-charged (cell voltages going too high) or over-discharged (cell voltages going too low) thereby extending the life of the battery pack. It does this by constantly monitoring every cell in the battery pack and calculating exactly how much current can safely go in and come out of the battery pack without damaging it. These calculated current limits are then sent to the source and load (motor controller, power inverter, etc), which are responsible for respecting these limits.
- It calculates the State of Charge (the amount of energy remaining in the battery) by tracking how much energy goes in and out of the battery pack and by monitoring cell voltages.
- Itmonitors the health and safety of the battery pack by constantly checking for shorts, loose connections, breakdowns in wire insulation, and weak or defective battery cells that need to be replaced.
The secondary functions that the BMS performs:
- Balances all the cells in the battery pack by intelligently bleeding off excess energy from cells that are charged more than others. This provides the maximum amount of usable energy (capacity) from the battery pack since the pack is only as strong as the weakest cell.
- Monitors the temperature of the battery pack and controls a battery fan to regulate the temperature of the pack. Additionally, it constantly monitors the output of the fan to make sure it is working properly.
- Provides real-time information and values to other devices such as motor controllers, chargers, displays, and data loggers using several different methods.
- Stores error codes and comprehensive diagnostic information to aid in fixing problems with the battery pack should any issues arise.
What are the basic components of a BMS?
Fuse: When a violent short circuit occurs, the battery cells need to be protected fast. Fuse is meant to be blown by the overvoltage control IC in case of overvoltages, driving it to the ground. The MCU can communicate the blown fuse’s condition, which is why the MCU power supply has to be before the fuse.
Current Sensing: Keeping a time reference and integrating the current over time, basically a sensor used to sense the current flow and report it to the MCU, we obtain the total energy entered or exited the battery, implementing a Coulomb counter. In other words, we can estimate the state of charge by using the following formula:
Thermistors: Temperature sensors, usually thermistors, are used both for temperature monitor and for safety intervention. It blows the fuse, when the temperature goes above the required rating without MCU intervention, leading to no time delay.
Balancer: Battery cells have given tolerances in their capacity and impedance. So, over cycles, a charge difference can accumulate among cells in series. If a weaker set of cells has less capacity, it will charge faster compared to others in series. The BMS has to therefore stop other cells from charging, or else the weaker cells will get overcharged.
Conversely, a cell can get discharged faster, risking that cells going under its minimum voltage. In this instance, a BMS without a balancer has to stop the power delivery earlier
In Figure 7, you can see a thermistor that controls the input of the overvoltage control IC. This artificially blows the SCP (the fuse shown in Figure 5) without MCU intervention.
In conclusion, a proper Battery Management System can lead to the optimal usage of a lithium-ion battery in any available electronic device. Its main features are utilized and amplified by BMS, also it acts as a catalyst for the work done by the battery by improving overall battery life and efficiency.
Выбор платы BMS
BMS (battery management system) — система управления батареи. Это устройство, без которого не обходится практически ни одна литиевая аккумуляторная батарея. BMS призвана защитить АКБ от различных негативных факторов и максимально продлить скок службы батареи. Сегодня постараемся разобраться во всех вопросах, возникающих при выборе BMS. В этой статье мы расскажем:
- для чего нужна BMS
- какие виды BMS существуют, и каким функционалом они обладают.
- как подобрать BMS под ваши задачи.
Функции BMS
Итак, все аккумуляторные батареи имеют свой рабочий диапазон напряжения, например:
для Li-ion АКБ в большинстве случаев это значение составляет от 2,7 до 4.2V на одну параллель. Существуют АКБ, состоящие как из одной параллели, так и из множества, соединенных последовательно. Например, Li-ion АКБ c номинальным напряжением 11.1V будет состоять и 3-х параллелей (3S), складываем рабочее напряжение всех параллелей, и получаем рабочее напряжение АКБ — от 8,1 до 12,6V. Если напряжение какой-либо параллели или напряжение всей АКБ выйдет за эти пределы, в лучшем случае батарею ждет необратимая деградация, а это в свою очередь значительно снизит емкость и срок службы батареи, в худшем — полный выход из строя АКБ, а также возможность пожара и взрыва.
Основная функция BMS — как раз-таки не давать выходить за пределы рабочих напряжений как всей АКБ в целом, так и каждой отдельной параллели. Так же платы BMS призваны решать еще ряд важный задач:
- Ограничение тока изащита от КЗ. BMS контролирует токи заряда и разряда. В случае, если сила тока выходит за определенные значения, BMS на некоторое время разрывает электрическую цепь. Служит это для защиты батареи от КЗ и чрезмерно высокой силы тока, поскольку каждая АКБ рассчитана на определенные нагрузки.
- Температурный контроль. Аккумуляторные батареи способны нагреваться во время своей работы, а как известно, температура выше определенных значений вредит АКБ. Например, у Li-ion батарей при нагреве свыше 60℃ наступает деградация, а при нагреве свыше 80-90℃ и вовсе появляется вероятность самовоспламенения. На многих BMS установлены температурные датчики, они следят за температурой АКБ, при превышении пороговых значений плата размыкает электрическую цепь, до тех пор, пока АКБ не остынет.
- Балансировка. Для того, чтобы АКБ работала нормально, и отдавала заявленную емкость, напряжение на всех параллелях должно быть одинаковым. Например, если одна из параллелей имеет большее напряжение, чем остальные, то при зарядке BMS будет отключать батарею именно по этой параллели, не давая ей перезарядится, в то время как остальные еще не успели зарядится. В итоге мы получаем АКБ, которую не получится зарядить до конца, соответственно и заявленную емкость она отдавать не будет. Что бы этого избежать во многих платах БМС присутствует режим балансировки. При балансировке напряжение на всех параллелях выравнивается, и АКБ в таком случае, может отдать всю свою энергию.
- защитные платы по напряжению и току.
- балансиры
- комплексные устройства, объединяющие в себе различный функционал.
- Возможность настраивать количество параллелей с которыми будет работать BMS.
- Возможность настраивать пороги напряжений на параллелях, это позволяет плате работать с разными типами аккумуляторов.
- Возможность настраивать порог максимально допустимых
- Следить за такими параметрами работы АКБ, как, общее напряжение, напряжение на каждой параллели, токи заряда/разряда, температура.
- Контроль процесса зарядки и разрядки аккумулятора, счёт циклов
- Регистрация состояния всех компонентов
- Снятие параметров температуры, напряжения, сопротивления, тока заряда
- Распределение и балансировка токов между разными компонентами
- Защита подключения к нагрузке и отключения
- BMS не даёт какой-либо одной параллели перезарядиться и обгонять другие ветви. Если бы этой функции не было, то АКБ невозможно было бы зарядить полностью, и тогда заявленную ёмкость отдавать в полной мере она не сможет. Балансировка производит стимуляцию заряда на «отстающих » частях батареи, выравнивает напряжение на всех параллелях и обеспечивает корректную работу АКБ.
- Активный BMS контролирует, чтобы после полного заряда одной параллели она отключалась от питания, и заряд перенаправлялся на следующие, еще не зарядившиеся полностью ячейки.
- Пассивный балансир состоит из аналоговых комплектующих с более надежной точностью, для его работы не нужно внешнее питание. Через резисторы с помощью малых токов устройство снижает напряжение на заряженных элементах, направляя избыток на ячейки с недостатком заряда.
- “ Battery Management Systems, Volume I & II ” by Gregory L. Plett along with his course.
- “ Advances in Battery Technologies for Electric Vehicles” (Woodhead Publishing Series in Energy: Number 80)
- “ ELECTRIC VEHICLE BATTERY SYSTEMS ” by Sandeep Dhameja
- “ Battery Reference Book” 3e by T.R Crompton.
- Batteries and battery packs are made up from groups of cells wired in series, parallel or a combination of them.
- Nominal Voltage is a representative voltage that depends on the combination of the active chamicals used. It dosent relate to the voltage under load, its more of an average or typical voltage.
- C rate is a relative measure of the cell electrical current eg. A 20 Ah cell should be able to deliver 20 A (“1C”) for 1 h or 2 A (“C/10”) for about 10 h (but, the relationship is not strictly linear)
- A cell stores energy in electrochemical form, which it can later release to do work, The energy release rate is the cell’s instantaneous power (mW, W, or kW)
- xEVs: Electric vehicles come in different categories and are abbreviated as xEVs
- Hybrid-electric vehicles (HEVs) These vehicles have drive provided by an electric motor and one other source ( like petrol engine ). The battery pack in these systems store small amount of energy and are used only for power boost when the vehicle must accelerate, or as a power sink when the vehicle must decelerate. They essentially have zero all-electric vehicle range and are never plugged in to recharge their battery pack; instead, the gasoline engine recharges the battery when extra power is available. An example HEV is the Toyota Prius.
- Plug-in hybrid-electric vehicles (PHEVs). These vehicles are similar to HEVs but have a somewhat larger battery pack and motor. They can operate in electric-only mode under some operating conditions, typically at lower speeds such as for residential or city driving
- Extended-Range Electric Vehicle (E- REV ): Larger battery than PHEV allows some all-electric range under full-load conditions.
- Electric Vehicle ( EV ), a.k.a. Battery-Electric Vehicle ( BEV ): Battery provides only motive power.
- Protect human safety of device’s operator:
- Detect unsafe operating conditions and respond
- Protect cells of battery from damage in abuse/failure cases
- Prolong life of battery (normal operating cases)
- Maintain battery in a state in which it can fulfil its functional design requirements
- Inform the application controller how to make the best use of the pack right now (e.g., by providing power limits), control charger, etc.
- During discharge, it gives up electrons to external circuit, is oxidized ( OIL )
- During charge, accepts electrons from external circuit, is reduced
- During discharge, accepts electrons from circuit, is reduced
- During charge, gives up electrons to external circuit, is oxidized
- During discharge, Li exits the surface of the negative-electrode particles, gives up an electron, becoming Li in the electrolyte
- Li diffuses outward from center of negative-electrode particles to equalize concentrations, replenishing Li at particle surface (over time)
- Meanwhile, electron travels through external circuit to positive electrode
- Li joins with the electron, and Li enters positive-electrode particles at their surface
- Li diffuses into positive-electrode particles to equalize concentration (over time)
- Presently, essentially all commercial lithium-ion cells use some form of graphite (C6 ) for the negative-electrode material
- Graphite has graphene layers of C6 structures that are tightly bonded These layers are loosely stacked and there is room for lithium to intercalate between them
- Lithium titanate oxide (Li4 Ti5 O12 , LTO ) is an alternative negative-electrode material
- Disadvantage: high open-circuit potential (making cell voltage low) Advantage: nearly indestructible — very long life
- Using graphite, one can store up to one Li per six C atoms; using silicon, one can (in principle) store four Li per every Si atom!
- Unfortunately, while volume change for a charge/discharge cycle for graphite is around 10 %, it is around 400 % for silicon
- Therefore, silicon electrodes tend to fracture quickly and have short lives
- Possible workarounds: mix graphite with silicon, or build small forests of silicon nanowires with space in-between to allow for expansion
- In 1980, John B. Goodenough discovered that Lix CoO2 ( LCO ) was a viable material for lithium intercalation
- Li intercalates between the layers of CoO6 octahedra
- LCO has layers, somewhat like graphite, so it is often called a “layered cathode”
- Cobalt is rare, toxic, and expensive;
- Only about half its theoretic capacity is useable (“x” can be min 0.5), else cell ages rapidly
- In 1983, Goodenough and Thackery proposed LixMn2O4 ( LMO ) as an alternate intercalation material: Mn sits in the octahedral sites, Li in the tetrahedral
- This material has a cubic “spinel” structure. It allows 3D diffusion (vs. 2D for layered and 1D for olivine)
- LMO is cheaper and safer than LCO , but can have short lifetime due to the manganese dissolving into the electrolyte under some conditions
- Additives can be added to help prevent this, but this “art” is presently well guarded by trade secrets
- In 1997, Goodenough proposed olivine-style phosphates as a third major category of positive-electrode material LixFePO4 ( LFP ) is the most common in this family
- This material is low cost, and low toxicity, but also has low energy density due to a low open-circuit potential and low specific energy due to heaviness of Fe
- 1D structure tends to have high resistance, which can be overcome in part by using very small particles and including conductive additives
- Layered cathodes ( LCO , NMC , NCA ) can use only around half their theoretic capacity
- Olivine cathodes ( LFP ) have low voltage (and very little state information in their voltage)
- Spinel cathodes ( LMO ) are inexpensive and non-toxic, but can degrade rapidly
- Protects safety of the operator of the host application; detects unsafe operating conditions and responds
- Protects cells of battery from damage in abuse/failure cases
- Prolongs life of battery (normal operating cases)
- Maintains battery in a state in which it can fulfill its functional design requirements
- Informs the host-application control computer how to make the best use of the pack right now (e.g., power limits), control charger, etc.
- A modular battery pack suggests a hierarchical master–slave BMS design as well
- One “slave” BMS unit is associated with each module
- Module’s cells welded/bolted to slave PCB , minimizing wiring and wiring losses
- Slave has electronics for voltage measurement, cell balancing
- Master measures pack current, controls contactors
- Communicates with slaves via daisy-chain or star architecture
- Master/slave communication uses few (e.g. two) wires—minimizes wiring-harness nightmare
BMS slave role¶
BMS slave needs to:
- Measure voltage of every cell within the module
- Measure temperatures Ideally of every cell, but in many packs some temperatures are estimated, especially if the pack has cells in parallel
- Balance the energy stored in every cell within the module this is needed as cells have different efficiencies, self-discharge rates, etc.
- Communicate this information to the master
- Is slave design reusable?
- Often “yes”, assuming electronics flexible in terms of number of cells monitored, physical size matches different applications While electronics design may be reusable in most cases, may need to redesign PCB footprint to fit individual applications For high volumes there may be overall cost savings in developing a specific slave optimized for a given module
BMS master role¶
BMS master needs to
- Control contactors that connect battery to load
- Monitor pack current, isolation
- Communicate with BMS slaves Communicate with host- application controller
- Control thermal-management
- Is master design reusable?
- More difficult than for slave designs Master needs to be more flexible; for example, Number and type of contactors it controls Types of current sensors used Ways it connects to charger, thermal management system
1a. Battery-pack sensing: Voltage¶
All cell voltages are measured in a lithium-ion pack
- Indicator of relative balance of cells
- Input to most SOC and SOH estimation algorithms
- Safety: overcharging a lithium-ion cell can lead to “thermal runaway,” so we cannot skip measuring any voltages
Some methods for analog-to-digital conversion¶
At the most basic level, voltage is measured using an analog-o-digital converter
There are several common ADC architectures; for example,
- A direct-conversion or flash ADC uses a bank of comparators and fixed reference voltages, outputs code of closest reference (fast, expensive)
- Successive approximation compares input to output from DAC and uses feedback to modify DAC signal, resolving input to desired accuracy (slow, inexpensive)
- Delta-sigma uses oversampled 1-bit flash ADC to encode difference ( ) between approximation and input, sums ( ) differences and filters to give final high-resolution result at desired slower sample rate (very popular)
- The resolution of an ADC is the smallest change in the input signal that can be measured; it is also the step size between consecutive ADC output codes (typically 16bit adc has a resolution of 76uV)
- Resolution for a M bit ADC is the full scale voltage difference divided by 2 to the power of M
Accuracy of an ADC ¶
The accuracy of an ADC has to do with the absolute difference between the reported value and the true value:difference may be due to several sources
- Quantization error (unmeasurable value between)
- Offset error (constant difference between ideal and measured value over whole measurement range)
- Gain error (difference between slope of ideal and measured value over whole measurement range, expressed as %)
- Nonlinear error (deviation between actual and ideal step widths, expressed as ADC counts)
Chipsets¶
Special chipsets are made to aid high-voltage BMS design
- Low-cost “dumb” measurement chips used in modules, proximate to cells; high-cost computational processing in distant master unit
- Special chips implement difficult task of highly accurate A2D voltage sensing with high common-mode rejection and fast response in high- EMI , high-heat, high-vibration environments
- Can often be placed in parallel for redundant fault-tolerant designs. Multiple vendors make chipsets (e.g., Analog Devices, Maxim, Texas Instruments)
Example chipset: LTC6811 ¶
example ( LTC6811 ) designed by Analog Devices (formerly Linear Technology)
- Monitors up to 12 cells in series in a module, 100s of cells in series in pack
- Has built-in isolated communications between daisy-chained parts
- Supports internal or external cell- equalization circuitry
- Powered by module itself, or externally
- Measures up to five temperatures (more with some external circuitry)
Selecting a chipset¶
Points to be considered in a design:
- How many cells can each IC monitor?
- How many cells total can be monitored?
- Does it support passive/active balancing?
- What is the measurement accuracy?
- How many temperature measurements can be made?
- How many wires to communicate from IC to IC ?
- What is chipset availability and cost, per cell?
1b. Battery-pack sensing: Temperature¶
Battery cell operational characteristics and cell degradation rates are very strong functions of temperature
- Don’t charge at low temperature; control thermal management systems to keep temperature in “safe” region
- Unexpected temperature changes can indicate cell failure or impending safety concern
- Ideally, we measure each cell’s internal temperature; but, With accurate pack thermal model, can place sensors external to one or more cells per module and calibrate internal temperature
Measurement devices:¶
- Thermocouple (uses amplifier and ADC )
- Thermistor (uses voltage divider circuit)
Voltage-divider + thermistor example
1c. Battery-pack sensing: Current¶
Battery pack electrical current must be measured to monitor safety, log abuse, and inform SOC and SOH algorithms
Shunt current sensor (Kelvin four-wire)
Hall-effect current sensing
Both methods have advantages and disadvantages and both are in common use in BMS today
1d: High-voltage contactor control¶
When not in use, the battery pack internal high-voltage bus is completely disconnected from the load at both terminals
- Dis/connecting pack at both terminals requires two high-current capable relays or “contactors”
- A low-voltage/low-current signal activates the contactor, closing an internal switch that connects its main terminals
- As load is often capacitive, if both contactors were closed simultaneously, enormous current would flow instantly, potentially welding the contactors closed or blowing a fuse
- So, a third “pre-charge” contactor is used
1e. Isolation sensing¶
Isolation sensing detects presence of a ground fault
- Primary concern is safety: Is it safe to touch a battery terminal and chassis ground at the same time?
- Battery “should” be completely isolated from chassis ground, so “should” be no problem
- FMVSS says isolation is sufficient if less than 2 mA of current will flow when connecting chassis ground to either the positive or negative terminal of the battery pack via a direct short
For fault on low side
Isolation is deemed sufficient if Ri>Vb /0.002 or R1 > 500Vb
1f. Thermal control¶
Important to keep battery-pack cells at a “comfortable”temperature to ensure safety and to extend life
- Also important to keep cells at a uniform temperature for consistent aging (also reduces need for many temperature sensors)
- Present commercial systems use either air or liquid systems, and some reports indicate that range and life are negatively impacted by air systems
- Active heating and cooling while vehicle plugged in can extend life, shorten charge times
How can a BMS protect the user and battery pack?¶
BMS requirement 2: Protection¶
BMS must provide monitoring and control to protect:
- Cells from out-of-tolerance ambient operating conditions
- User from consequences of battery failures
High-energy storage batteries can be very dangerous:
- If energy is released in an uncontrolled way (short circuit, physical damage), can have catastrophic consequences
- In a short circuit, hundreds of amperes can develop in microseconds; protection circuitry must act quickly state
What to protect against¶
Different applications and different cell chemistries require different degrees of protection
- Failure in a lithium-ion cell can be very serious: explosion/fire
- Protection is indispensable in automotive environment
- Protection must address following undesirable events or conditions:
- Excessive current during charging or discharging
- Short circuit
- Over voltage and under voltage
- High ambient temperature, overheating
- Loss of isolation
- Abuse
Overcurrent/overtemperature protection¶
When possible, fallback protection paths should be implemented
- Red = cell-manufacturer specified region where cells will most likely be subject to permanent damage
- Anywhere else “okay” but need a margin of error
- Generally design to limit cell’s operating conditions to smaller “safe” region, shown here in green
- Safety devices are then specified to constrain cells to safe region
- White = safety margin
Overvoltage/overtemperature protection¶
Similar for voltage limits:
But, each protection device added into main current path increases battery impedance, reducing power delivered to load Examples of protection devices include:
- Thermal fuse: Opens contactor when the temperature exceeds limit.
- Conventional fuse: May not act quickly enough
- Active fault detection: BMS monitoring for fault conditions
Fault detection/tolerance¶
- Another aspect of protection is detecting, withstanding, and (when possible) rectifying faults
- State-of-art BMS use processors having dual CPUs that execute the same instructions at slightly different times on different cores, then compare results
- Slaves often can detect most cell faults without intervention of the main processor
- Cell over/under voltage, over/under temperature, redundant sensing, etc.
- Serious slave faults should be able to shut pack down without using master microprocessor
Standards¶
Different applications have different standards for safety
- Passenger cars having maximum gross vehicle mass up to 3500 kg fall under ISO26262 :2011
- Electric motorcycles fall under ISO / PAS 19695:2015 (similar to ISO26262 )
- Larger trucks over 3500 kg, such as Ford F250, 350, Chevy Silverado 2500, as well as semis, buses, etc…) fall under IEC61508
- While these safety standards have the same goals they are different in application
- Use different “safety integrity levels” (SILs), evaluated in different ways
- Very difficult to design to all these standards simultaneously Standards are complex—require study of their own to understand how to comply most likely 40% of the code written for BMS is to ensure these standards.
3 How must a BMS interface with other system components?¶
3a. Communication via CAN bus¶
- Control Area Network ( CAN ) bus is industry ISO standard foron-board vehicle communications
- Designed to provide robust communications in the very harsh automotive operating environments with high levels of electrical noise
- Two-wire serial bus designed to network intelligent sensors and actuators; can operate at two rates:
- High speed (e.g., 1M Baud): Used for critical operations such as engine management, vehicle stability, motion control
- Low speed (e.g., 100 kBaud): Simple switching and control of lighting, windows, mirror adjustments, and instrument displays (etc.)
Format of CAN -bus packet¶
The protocol defines the following:
- Method of addressing the devices connected to the bus
- Data format (the “message”)
- Transmission speed, priority settings, and sequence
- Error detection and handling
- Control signals
A typical packet(frame) in CAN bus looks like :
Abbreviations : SOF — Start of frame; RTR — Ready to receive; CRC — cyclic redundancy check; ACK -acknowledgement bit
Data frames are transmitted sequentially over the bus.
3b. Charger control¶
Battery packs are charged in two ways:
- Random: Charge delivered in unpredictable patterns; e.g., regenerative braking and are Control by providing inverter power limits
- Plug-in: For EV / PHEV /E- REV
- Control charger current, voltage, balancing
- Often CP / CV ; more exotic methods possible (in the research phase)
- Heating systems may be required to increase charge efficiency and to increase battery longevity
What limits fast charging?¶
Passenger vehicles require approximately 200–300 Wh / mile
- For 300 mile range, 60–90 kWh capacity, charge in 3 min (typical petrol refill time) . requires a rate of up to 1.8 MW !
- Domestic 15 A 110 V ( US ) or 1 5 kW “level 1” service charges pack in 40–60 h
- Domestic 30 A 220 V ( US — high power sockets) or 6 6 kW “level 2” service charges pack in 10–15 h
- DC “level 3” (CHAdeMO) fast charging, 500 V, 125 A can provide up to 80 % charge in 30 min
- Tesla “level 3” fast charging for model S can provide 50 % charge in 20 min So, limit is usually the electrical service, not the battery pack
3c. Log book function¶
- For warranty and diagnostic purposes, BMS must store a log of atypical/abuse events
- Abuse type: out of tolerance, voltage, current, temperature
- Duration and magnitude of abuse
Can also store diagnostic information regarding
- Number of charge/discharge cycles completed
- SOH estimates at the beginning of each driving cycle And much more…
- Data stored in nonvolatile (e.g., FLASH ) memory and downloaded when required
3d. Range estimation¶
How far can I drive before available energy is depleted? Heavily influenced by environmental factors:
- What are the vehicle characteristics?
- How is the vehicle being driven (gently/aggressively)?
- Are there a lot of hills, a lot of wind?
- Is it warm or cold out?
- At present, it appears that each OEM will have their own range algorithms
BMS requirement 4: Performance management¶
Battery applications need to know two battery quantities:
- How much energy is available in the battery pack
- How much power is available in the immediate future
Knowing energy is most important for applications such as EV : Tells me how far I can drive Knowing power is most important for applications such as HEV : Tells me whether I can accelerate or accept braking charge
Both are important for applications such as E- REV / PHEV
Why must we estimate energy, power?¶
We Can’t measure available energy or available power directly. Instead, must estimate these values.
- To estimate energy, we must know (atleast) all cell states-of-charge Zk and capacities Qk
- To estimate power, we must know (atleast) all cell states-of-charge Zk and internal resistances Rk
But, cannot directly measure these parameters either! Therefore, must estimate SOC , SOH
Available inputs include all cell voltages, pack current, and temperatures of cells or modules.
There are both good and poor methods to produce estimates: Poor methods are generally simpler to understand, code, and validate, but yield less-accurate results
Impacts of a poor estimator can be:
- Abrupt corrections when voltage or current limits exceeded,
- leading to customer perception of poor drivability, or
- Overcharge or overdischarge, which damages cells, or
- Compensating for uncertainty by overdesigning pack All of these have costs in dollars, weight and/or volume
A good way to do SOC estimate is to implement Kalman filter
Whats left to explore ?¶
- Article 2: How to model cells, needed by algorithms
- Article 3: Advanced methods for SOC estimation
- Article 4: SOH estimation
- Article 5: Balancing and power-limits estimation
Comments
So what do you think? Did I miss something? Is any part unclear? Leave your comments below.
Виды BMS
Под понятие BMS попадают сразу несколько видов устройств, которые служат для нормального функционирования АКБ. Их можно разделить на несколько категорий:
Чаще всего используются именно комплексные устройства.Они обладают защитой по напряжению и току, защитой от КЗ, и имеют температурный датчик. Так же они способны балансировать АКБ небольшими токам — до 50-100mA, как правило, этого хватает для качественно собранной батареи.
Такие платы бывают симметричными и несимметричными. При использовании симметричной BMS заряд и разряд батареи можно осуществлять через один и тот же разъем, в случае с несимметричной платой, необходимо использование двух разъемов — зарядного и разрядного.
Сегодня на рынке существуют и более «прокаченные» варианты плат BMS — Smart BMS.
Как правило, такие платы имеют множество температурных датчиков, это необходимо для лучшего контроля температуры АКБ. Так же к таким платам возможно подключение специального дисплея, на котором будет отображаться основная информация о состоянии АКБ. Ну и самым главным преимуществом smart BMS является возможность подключится к ней по bluetooth с помощью смартфона. При помощи специальных приложений открывается возможность наблюдать и изменять огромное количество параметров, отвечающих за работу АКБ. Вот основные из них:
токов заряда и разряда.
Так же следует поговорить о балансирах. Они бывают двух типов — активные и пассивные.
BMS плата — что это?
Современные продвинутые модели литиевых аккумуляторов для электроники дополняются системами управления батарей, которые упрощают контроль и настройку параметров работы. Эта статья объяснит, что означает BMS плата, какие функции она выполняет и в каких случаях необходима.
Что такое БМС в аккумуляторах
BMS контроллер — это электронная плата, расшифровка которой «battery management system» переводится как «система управления батареи». Этот элемент предназначен для защиты аккумулятора и увеличения её срока эксплуатации. Более дорогие модели имеют расширенный функционал, оснащены дисплеями и делают доступной настройку рабочих параметров для оптимизации процесса.
Необходимость применения контроллера вытекает из того, что любая АКБ рассчитана на вполне конкретный рабочий диапазон напряжения. Если хотя бы одна параллель в батарее в ходе эксплуатации превысит предел нагрузки, то запустится необратимая деградация. В самом лёгком случае это приводит к заметной потере ёмкости и длительности срока службы, если не повезёт — может произойти поломка и даже возгорание элемента. Плата BMS измеряет параметры работы и предотвращает аварийные ситуации.
Понятие платы управления включает в себя любые микросхемы, цель которых — защитить и выстроить корректную работу АКБ. Сюда относятся как самые упрощенные платы балансировки или защиты, так и сложные микроконтроллеры с дисплеем для отображения данных и возможностью тонкой настройки параметров.
Что делает система BMS
Управляющая плата собирает данные и регулирует работу аккумулятора, чтобы спасти его от короткого замыкания, перегрева, перезаряда и перегрузки. В задачи системы управления входят:
Более сложные устройства с расширенным функционалом имеют возможность обеспечивать и другие задачи. Например, интеллектуально-вычислительную функцию. Такие платы сами рассчитывают предел допустимого тока заряда, сопротивление компонентов, определяют количество энергии на входе и выходе, а также ведут счет «пробега » электро накопителя. Обычно такие продвинутые модификации умеют передавать данные на ПК или приложение в смартфоне.
Устройство работы и функции BMS
Пользуясь преимуществами управляющей платы, можно добиться от аккумулятора максимальной производительности и защитить его в непредвиденных экстренных случаях, предотвратить поломку. Накопители с БМС служат дольше и реже нуждаются в ремонте. Как именно плата регулирует работу и принимает решения, описано ниже:
-
Измерение тока на заряде и разряде, предотвращение КЗ.
BMS контролирует, чтобы ток не выходил за пределы максимального и минимального значения. Если это происходит, контроллер размыкает ненадолго цепь.
Производители Battery Management System заботятся о том, чтобы защитить микросхему от загрязнений и влаги, поэтому зачастую эти платы имеют особое защитное покрытие.
BMS как балансир аккумулятора
В роли балансира микросхема управления следит за тем, чтобы все ячейки батареи заряжались синхронно и до одинакового уровня. БМС могут быть активными или пассивными:
Для каких аккумуляторов нужна плата BMS
В определенной степени плата контроля BMS будет полезна практически для любой литиевой АКБ. Например, литий-ионные батареи при всём своём многообразии преимуществ печально известны высоким риском возгорания при перегреве, как раз такую ситуацию плата БМС способна заранее выявить и предотвратить. В этом и состоит причина, для чего нужна BMS плата на самокате или электровелосипеде.
Ещё актуальнее этот элемент для LiFePO4 — литий-железо-фосфатных аккумуляторов. В сравнении с Li-ion, такие батареи безопасны; кроме того, у них больше производительность и стабильнее работа. Существенный недостаток состоит в том, что устройство восприимчиво к избыточному заряду или, наоборот, предельному разряду ниже уровня 10%. Установив систему контроля, удается обезопасить девайс от порчи как отдельных ячеек, так и всей АКБ и её поломки.
Неравномерность заряда на разных участках АКБ не так сильно грозит в случае параллельного соединения ячеек. Однако при сборке с последовательным подключением ровного распределения заряда добиться без BMS невозможно. Одни ячейки будут недозаряжаться, другие — получать избыточный заряд. Хроническое и регулярное повторение ситуации приводит к резкому ухудшению эксплуатационных качеств батареи.
Специфика выбора БМС-плат
Сейчас можно найти аккумуляторные батареи уже с интегрированным блоком контроля. Если же вы собираете комплектующие отдельно, удостоверьтесь, что микросхема BMS подходит для вашего аккумулятора. Разные модели плат рассчитаны под конкретный тип АКБ, исключением являются только Smart-модификации, которые могут настраиваться на совместимость с разными литиевыми батареями.
Важно также знать количество параллелей, или ячеек батареи: к примеру, платы с пометками «10 cells» или «10S » предназначены на обслуживание батареи с 12 элементами.
Ранее мы выпускали материал о том, как выбрать плату BMS. Если у вас возникли вопросы, обращайтесь к нам: мы бесплатно проконсультируем в выборе, эксплуатации и уходе за любыми компонентами аккумуляторной батареи.
Battery Management System for an xEV — Part 1 Basic BMS requiremnets
This is part 1 of a series on Battery management system design.
The data presented here was collected as a part of study based Internship at Kaynes Technology, Mysore.
Most of the data here is my notes on the following resources:
Basic terminologies¶
What must a BMS do?¶
The primary functions of a BMS are to:
Basic working of an electrochemical cell¶
The function of the negative electrode¶
In an electrochemical cell, the negative electrode is often a metal or an alloy or hydrogen (lead metal or paste for PbA)
The function of the positive electrode¶
In an electrochemical cell, the positive electrode is often a metallic oxide, sulfide, or oxygen (lead oxide for PbA)
Li-ion working (The process of intercalation)¶
The active electrode materials are coated on both sides of metallic foils which act as the current collectors conducting the current into and out of the cell
Negative electrodes for lithium-ion cells¶
Alternate negative-electrode material LTO ¶
Future negative-electrode material silicon¶
Positive electrodes for lithium-ion cells¶
Lithium cobalt oxide ( LCO )¶
NCM (a.k.a. NMC ) is a blend of Ni, Co, and Mn, which retains the layered structure, and has properties from all three constituent metals; NCA is a blend of Ni, Co, and Al (used in tesla batteries)
Spinel cathodes¶
Olivine cathodes¶
In summary¶
What are the primary functions of a BMS ?¶
A BMS has the following priorities:
1. Sensing and high-voltage control¶
Measure voltage, current,temperature; control contactor, pre-charge; ground-fault detection,thermal management
2. Protection against¶
Over-charge, over-discharge, over-current, short circuit, extreme temperatures
3. Interface¶
Range estimation, communications, data recording, reporting
4. Performance management¶
State-of-charge ( SOC ) estimation,power-limit computation,balance/equalize cells
5. Diagnostics¶
Abuse detection, state-of-health ( SOH ) estimation, state-of-life ( SOL ) estimation
The issue of cost¶
There is a cost associated with battery management, so not all applications implement all features
Modular design of BMS ¶
Design extreme 1: Parallel-cell modules ( PCM ) Design extreme 2: Series-cell modules ( SCM )