Battery Management system in Hybrid Vehicles

1. Battery Management
System
UNIT - II

2. Introduction
An electric vehicle generally contains the following major components: an electric motor, a
motor controller, a traction battery, a battery management system, a wiring system, a
vehicle body and a frame.
The battery management system is one of the most important components, especially
when using lithium-ion batteries.
The lithium-ion cell operating voltage, current and temperature must be maintained within
the “Safe Operation Area” (SOA) at all times.
To maintain the safe operation of these batteries, they require a protective device to be
built into each pack is called battery management system (BMS).
BMS make decisions on charge and discharge rates on the basis of load demands, cell
voltage, current, and temperature measurements, and estimated battery SOC, capacity,
impedance, etc. BMS is a part of complex and fast-acting power management system.

3. Battery Management System
Battery management system (BMS) is technology dedicated to the oversight of a battery pack, which is an
assembly of battery cells, electrically organized in a row x column matrix configuration to enable delivery of
targeted range of voltage and current for a duration of time against expected load scenarios.
The oversight that a BMS provides usually includes:
• Monitoring the battery
• Providing battery protection
• Estimating the battery’s operational state
• Continually optimizing battery performance
• Reporting operational status to external devices

4. How do battery management systems
work?
Battery management systems do not have a fixed or unique set of criteria that must be adopted. The
technology design scope and implemented features generally correlate with:
◦ The costs, complexity, and size of the battery pack
◦ Application of the battery and any safety, lifespan, and warranty concerns
◦ Certification requirements from various government regulations where costs and penalties are paramount if
inadequate functional safety measures are in place.
Battery pack protection management and capacity management being two essential features.
Battery pack protection management has two key arenas: electrical protection which implies not
allowing the battery to be damaged via usage outside its SOA, and thermal protection, which involves
passive and/or active temperature control to maintain or bring the pack into its SOA.

7. General Functions of BMS
Sensing and high-voltage control
Measure voltage, current, temperature, control contactor, pre-charge; ground-fault detection,
thermal management.
Protection
Over-charge, over-discharge, over-current, short circuit, extreme temperatures.
Interface
Range estimation, communications, data recording, reporting.
Performance management
State of charge (SOC) estimation, power limit computation, balance and equalize cells.
Diagnostics
Abuse detection, state of health (SOH) estimation, state of life (SOL) estimation.

8. BMS architecture
 A modular battery pack suggests a hierarchical master – slave BMS design.
 There is normally a single “master” unit for each pack.

9. BMS slave role
 Measure voltage of every cell within the module.
 Measure temperatures.
 Balance the energy stored in every cell within the module.
 Communicate this information to the master.

10. BMS master role
 Control contactors that connect battery to load.
 Monitor pack current, isolation.
 Communicate with BMS slaves.
 Control thermal-management.
 Communicate with host application controller.

11. BMS Architecture Types
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 Has one central BMS in the battery pack assembly.
 All the battery packages are connected to the central BMS directly.
 It is more compact, and it tends to be the most economical since
there is only one BMS.
 Since all the batteries are connected to the BMS directly, the BMS
needs a lot of ports to connect with all the battery packages. This
translates to lots of wires, cabling, connectors, etc. in large battery
packs, which complicates both troubleshooting and maintenance.

12. BMS Architecture Types
 Similar to a centralized implementation, the BMS is divided into
several duplicated modules, each with a dedicated bundle of wires
and connections to an adjacent assigned portion of a battery stack.
 Primary BMS module oversight whose function is to monitor the
status of the submodules and communicate with peripheral
equipment.
 Troubleshooting and maintenance is easier, and extension to larger
battery packs is straightforward.
 The downside is overall costs are slightly higher, and there may be
duplicated unused functionality depending on the application.

13. BMS Architecture Types
 Conceptually similar to the modular topology, however, in this case,
the slaves are more restricted to just relaying measurement
information, and the master is dedicated to computation and
control, as well as external communication.
 So, while like the modular types, the costs may be lower since the
functionality of the slaves tends to be simpler, with likely less
overhead and fewer unused features.

14. BMS Architecture Types
Distributed BMS
 Considerably different from the other topologies, where the electronic hardware and
software are encapsulated in modules that interface to the cells via bundles of
attached wiring.
 A distributed BMS incorporates all the electronic hardware on a control board placed
directly on the cell or module that is being monitored. This alleviates the bulk of the
cabling to a few sensor wires and communication wires between adjacent BMS
modules.
 Consequently, each BMS is more self-contained, and handles computations and
communications as required. However, despite this apparent simplicity, this
integrated form does make troubleshooting and maintenance potentially problematic,
as it resides deep inside a shield module assembly.
 Costs also tend to be higher as there are more BMSs in the overall battery pack
structure.

15. Charging with a balancing BMS
controlling the charger

16. Discharging with a BMS controlling the
load: (a, b) discharging; and (c)
discharging stops when any one cell drops
to the bottom cutoff voltage.
Battery Charging Management

17. Cell balancing technique based on state of charge
of Li-ion battery

18. Measurement
The first function of a sophisticated, digital BMS is to gather data about a BMS (a simple, analog
BMS does not include this function). These measurements are:
• Cell voltage (and possibly pack voltage);
• Typically, cell temperature, or at least battery temperature;
• Most often, pack current.
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19. Voltage
A sophisticated, digital BMS measures the voltage of each and every cell in series. It may also
measure the total pack voltage, though that is not necessary, as that value can be calculated by
adding the individual cell voltages.
A distributed BMS may measure directly the voltage across a cell. (Normally the cell board is
powered by the cell itself, as it measures its voltage.)
Otherwise, the BMS may measure the voltage of various taps in a battery, and calculates a cell’s
voltage as the voltage difference between two taps.
Or, the BMS may take two measurements simultaneously of the two taps on either side of a cell,
and calculate the difference as the cell’s voltage.
The voltage is sampled by an analog multiplexer, and the reading is taken by an analog to digital
(A/D) converter (which may be on the same IC), which then passes the value to a processor
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21. Temperature
Temperature measurement of the pack or, better yet, of individual cells is useful for a few
reasons:
• Li-Ion cells must not be discharged if outside a certain temperature range, and
not be charged outside an even tighter range, which is a concern in applications that are
not temperature controlled, such as mobile applications.
• Should a cell become particularly hot due to internal problems (the cell is bad or
is being abused) or external ones (poorly done power connection, localized heat source),
it is best to warn the system than to wait for catastrophic failure.
• In a distributed BMS, it is very easy to include a sensor on each cell board, not
only sense its cell’s temperature, but also to detect whether a balancing load is working.
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23. Current
Knowledge of the battery current allows a BMS to perform additional functions, which, while not
essential, are expected to be offered by a professional product.
These are, in order of likelihood that a particular BMS will implement that function:
• Prevent the cells in the battery from being operated outside their SOA in terms of continuous
current (analog BMSs that measure battery current usually implement just this one point).
• Use an integral of the current as part of the DOD calculation, to implement a fuel gauge
function.
• Simply report the battery current.
• Prevent the cells in the battery from being operated outside their SOA in terms of both peak
and continuous current.
• Calculate the cells’ internal DC resistance.
• Use the battery current, together with the calculated internal DC resistance, to do IR
compensation of the cells’ terminal voltage.
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•There are mainly two ways of measuring high currents:
• Current shunt: a very low-resistance, high-precision resistor;
• Hall effect current sensor.
 A current shunt is simply a high precision, low value, high power resistor.
 The pack current is routed through the shunt, which results in a voltage drop across it
proportional to the current.
 That voltage across the sense connections can be amplified and measured to derive the pack
current.
 A shunt sensor introduces some energy losses.
 A shunt current sensor produces a tiny signal (on the order of millivolts full
scale). The BMS must provide an amplifier, and any wiring between them
must be protected from electrical noise interference, typically by using
shielded, twisted pair wiring.

25. Hall Effect Sensor
A Hall effect sensor is placed inside
the magnetic field produced by a
cable that carries the pack current,
and it produces a voltage that is
proportional to that current; that
voltage can be measured directly.
High-current Hall effect sensors are
modules shaped like a ring, through
whose opening a cable carrying the
pack current is routed. Low-current
Hall effect sensors are ICs with two
power terminals, through which the
current is routed.
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26. State of Charge
In general, the SOC of a battery is defined as the ratio of its current capacity (𝑄(𝑡)) to the
nominal capacity (𝑄(n)). The nominal capacity is given by the manufacturer and represents the
maximum amount of charge that can be stored in the battery.

27. State of Charge (SoC)
Percent of total charge at which the battery is currently
at
70% SoC implies that battery is 30% empty and 70% full
Open circuit Battery voltage directly proportional to its
SoC in a lead acid battery
12 V batteries varies from 11.7V to 12.85V
48V battery varies from 46.5V to 51.5V
But not so in Li ion battery
Also even the proportionate to voltage applicable when
battery is neither charging or discharging (at rest for some
time)

28. Li-Ion Battery: Voltage vs SOC
Charging in range 3.0V to 4.2V Discharging till 2.75V

29. Implications of SoC curves
Constant Current (CC) Charging at High rate (say 2C)
Only partially charge battery: possible only up to some low SoC (say57%)
Beyond that it will be a Constant Voltage (CV) Charging, which is very low-current charging
High-rate charging only meaning for large battery
High-rate charging also impacts life badly
High-rate discharge also hurts battery life
Energy pumped into Cell between 3.5V and 4.2V when
Slow-charged
For fast charge, it is between 3.9V and 4.2V
Slow-discharge energy is between 3.4V and 4.1V
SoC is not a linear function of Voltage

30. How does one measure SoC accurately?
Voltage method
Obtain the Open Circuit Cell Voltage (OCV) Vs SoC accurately in lab at very low
charging rate (C/25 going to C/100) for different temperatures
 Does OCV Vs SoC curve depends on SoH: not clear – conflicting opinions amongst researchers
SoC is a non-linear function of open-circuit voltage, only when battery is fully
at rest (very slow charge or discharge is ok)
Coulomb counting: Very accurate but dependent on accurate SoH
and precision of current measurement
◦ Measuring the current (total Coulombs) flowing in and out of battery: gives
one a change in SoC if SoH as well as the Capacity is known

31. SoC estimation using Coulomb Count
(cont)
Coulomb Counting requires correct starting point (initial SoC)
oWhat is SoC was in beginning? Mostly a reset to 100% is done after full charge cycle
Coulomb count between two instant will indeed be a good measure of energy
added or removed from a battery but will represent SoC only to the extent that
initial SoC was good!
Change in SoC (∆SoC) = Charge pumped in or out of battery/ (Capacity * SoH)
oNeed to be converted to percentage
oWhere charge pumped in and out is Coulomb Count * electron change or
integration of current over time: if computed charge is IN in the ∆SoC is positive,
else it is negative.

32. SoC estimation using Coulomb Count
(cont)
SoC new = SoC old + ∆SoC
oRequires SoH to be correctly known as ∆SoC is dependent on SoH.
Will repeated charging and discharging reduce accuracy as error build-up?
oA repeated partial charge and discharge (without a 100% reset cycle) builds up
the accumulation errors in SoC
oThe extent of error directly depends on the errors in current measurement
device and the degradation of battery SoH

33. SoC estimation using Kalman Filter
Technique
The complete SOC estimation is divided into three tasks. The first task is to estimate a predetermined SOC using the AHC
(Ampere Hour Counting) . The second task is to estimate the model voltage by using the selected battery model. The final task is
to update the Kalman gain, in which the model voltage and measured voltage are compared, and the voltage error is used to
modify the Kalman gain. The estimated SOC can be generated with the help of the updated gain based on the KF family
algorithm.

34. General SOC system architecture

36. Self –discharge of battery
Self discharge defines the rate at which the battery looses its energy while on self.
If the results in 30% self
discharge per month,
discard the battery

37. How do we estimate the cycle/lifetime
remaining in a battery?
Not very accurately, but we can estimate!!
oReferred to as age/health of a battery tell us “State of Health (SoH)”
o Represents the amount by which battery has deteriorated due to irreversible physical and chemical changes
Periodically completely discharge and then charge the battery (track open-circuit voltage) and then
again discharge slowly and carry out the coulomb count
o Give several hours rest after full charge
o Indicates maximum charge that the battery can hold currently
o Compare it with past date: Gives an estimate of SoH
Alternate method: Internal resistance
oAs battery electrode deteriorates, its capacity to deliver current also reduces.
o Internal resistance of a cell indicates the capability to deliver current
o Difference between internal resistance of fresh and used cell, helps in estimating SoH

38. State of Health
If SOC is the indicator of remaining battery charge, the state of health (SOH) is the indicator of
remaining full battery capacity, compared to capacity at beginning of life (BOL).
Predicting the remaining life is a challenge due to the complexity of performance degradation.
Therefore, it is essential to monitor performance concerning long-term changes in the battery,
since the SOH is path dependent.
SOH is not a physical quantity, but it depends on and can be represented by several physical
parameters: e.g. the number of charge‒discharge cycles, capacity and power fade, increase in
impedance or internal resistance.
where: Qr—rated capacity and
Qm—current maximum available capacity of the battery,
which is measured under rated conditions

39. The ageing factors can arise from vehicle usage, and battery and cell design, and main ageing factors are
temperature, SOC range, energy, and power conditions, as well as time, both usage and calendar time.
The ageing rate depends on operational conditions encountered over battery life.
Changes in the usage conditions (i.e. the ageing conditions) may accelerate or decelerate the degradation
mechanisms, and even initiate new ones.

40. Voltage Vs Capacity Plots
Voltage vs. Capacity plots give crude
estimation of battery degradation. As a
battery is cycled, the charge decreases,
indicating a loss of energy.

42. State of Energy
SOE is defined as the ratio of the battery residual energy under specific operating
conditions, e.g., varying load and temperature, over the total battery available
energy.
where SOE(t) and SOE(t0) are the SOE values at the time t and the initial moment (t0),
respectively, EN represents the nominal energy amount, and P(τ) denotes the power at the time
τ.

43. An equal charge throughput at different SOC levels results in
different energy amounts.

44. State of Energy
Compared with SOC, the research on SOE estimation is relatively less. In reality, it is
more practical to obtain accurate SOE as it is directly related to energy consumption,
thus determining the driving range of an electric vehicle.
An inaccurate SOE estimation will negatively affect user experiences. For instance, in
the case of electric vehicles, overestimation will lead to vehicle breakdowns on the way
while underestimation cannot maximize battery utilization.
One of the most commonly existed SOE estimation methods is the power integral
method, which measures the battery usage energy directly. However, it is difficult for
online SOE estimation due to the error accumulation and complex calibration.

46. State of Energy
A common SOE estimation method is the power integration approach , which is able to
effectively restrain the computational burden.
However, cumbered by its open-loop nature, this type of approach inevitably results in
accumulated errors subject to uncertain noises, limited sensor resolution, and measurement
imperfection.
As an improved solution, suitable characteristic mappings, such as the correlation among the
discharge power, remaining energy, and SOE, are conducted .
Although these mapping-based methods exhibit performance improvements relative to the
power integration method, expensive and time-consuming calibration and characterization tests
are needed.

47. State of Power
State of Power (SOP) generally refers to the available of power that a battery can supply to or
absorb from the vehicle powertrain over a time horizon.
Battery SOP can be viewed as a product of the threshold current and the corresponding voltage,
while various operational constraints should be explicitly considered and respected.
Assume that the battery power is positive for discharging and negative for charging, a general
definition of SOP is expressed by

48. State of Power
SOP estimation methods can be primarily classified into two groups, as illustrated

49. Thermal Runaway
Thermal runaway is a phenomenon in which the lithium-ion cell enters an
uncontrollable, self-heating state. Thermal runaway can result in extremely high
temperatures, violent cell venting, smoke and fire.
What causes thermal runaway?
Faults in a lithium-ion cell can result in a thermal runaway. These faults can be caused
by internal failure or external conditions.
One example of such internal failure is an internal short circuit. In a lithium-ion cell, the
cathode and anode electrodes are physically separated by a component called the
separator. Defects in the cell that compromise the separator’s integrity can cause an
internal short circuit condition that can result in thermal runaway. This is especially likely
in cells of poor quality.

50. Thermal Runaway
External, off-nominal conditions can also cause thermal runaway.
Examples of off-nominal conditions include:
•Overcharge: Can be due to incompatibility between cell and charger, or poorly designed
battery management system (BMS)
•Multiple over-discharges followed by charge: Discharging the cell or battery below
the cell manufacturer-recommended lower voltage threshold multiple times, then
charging the cell
•External short circuit
•High- and low-temperature environments

52. Thermal Runaway

54. Pre-charge Circuit
Pre-charge circuits are often used in electric vehicles (EVs) such as battery management
systems, onboard chargers, and in industrial applications such as power supplies and power
distribution units.
In EVs, controllers with high capacitive loads regulate motors.
High voltage (HV) positive and negative contactors are used in this system to act as an
emergency disconnect when the motor regulator fails.
Without a pre-charge circuit, welding can occur within the contactor as it closes and there
could be a brief arc resulting in pitting.

56. Contactor Control
A contactor is an electrically-controlled switch used for switching an electrical
power circuit.
A contactor is typically controlled by a circuit which has a much lower power
level than the switched circuit, such as a 24-volt coil electromagnet controlling a
230-volt motor switch.
The main contactor’s function is to open under a potential short circuit load
(up to 3,000A or more) making the motor drive electronics as safe as possible,
both in the general running of the vehicle and in a crash situation, where a
potentially damaging, high current short circuit, might occur, resulting in a fire.

57. Contactor Control
Rincon Power SPST normally-open Form-X (double
make) hermetically sealed DC contactor cut-away
animation showing main movable contacts and AUX
feedback plunger.

62. Aging of Lithium Cells
As a general definition, the aging of lithium-ion cells describes the
fact that due to the degradation of the individual cell components,
the cell’s chemical and electrical properties change with time and
usage in relation to the initial and brand-new condition.
Degradation includes aging mechanisms that are chemical based
(oxidation and reduction processes including the deposition of
reaction products and release of gaseous side-products) or
mechanical based (volumetric and structural changes)

64. Ultra-Capacitors
Ultracapacitor which is also known as a supercapacitor is an electrical device that
stores charge in large amounts.
It is called ultra because it has a higher capacitance value than regular capacitors.
These capacitors have low voltage limits and they have become a better choice over the
common capacitors. This is because they offer higher power density, consume less
power, and are safe and easy to operate.
An ultracapacitor operates between the limit of an ordinary capacitor and a battery.
Although, the device has just begun to gain population in the industry. They are suitable
for applications from efficient large-scale energy storage to a very small portable
devices.
This is because of their energy density, short charging cycle, and wide range of
operating temperatures.
Finally, ultracapacitors are defined as electronic devices that are used to store
extremely large amounts of electric charge.

65. Ultra-Capacitors
Supercapacitors have three main
characteristics which include charge time,
specific power, and cycle life, and safety.