This document discusses the design of electric vehicle battery packs. It covers topics such as state of charge, implications of the SOC curve, measuring SOC accurately, what comprises a battery pack, building packs from cells by connecting them in series and parallel, modules and packs, electrical design considerations, insulation, costs, and insulation testing. The goal is to conceptualize battery pack design to optimize performance while ensuring safety.

1. ELECTRIC VEHICLE BATTERY PACK DESIGN AND INSULATION
Department of Electrical Engineering
NATIONAL INSTITUTE OF TECHNOLOGY DURGAPUR
(An Institute of National Importance under Ministry of Education, Government of India) 1
PROFESSOR & HEAD
HIGH VOLTAGE & INSULATION LABORATORY
DEPARTMENT OF ELECTRICAL ENGINEERING

2. Battary Pack Design
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3. State of Charge (SOC)
Percentage 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, as shown in figure
in Lead Acid battery12V battery varies from
11.7V to 12.85V .
48V battery varies from 46.5V to 51.5V
But not so in Li lon battery
Also even the proportionate to voltage
applicable when battery is neither charging
nor discharging (at rest for some time)
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5. Implications of SoC curve
Constant Current (CC) Charging at High rate (say 2C)
Only partially charge battery: possible only up to some low SoC (say 57%) Beyond that it will be a
Constant Voltage (CV) Charging, which is very low-current charging
High-rate charging only meaningful for large Battery
High-rate Charging & discharging also impacts life badly
Energy pumped into Cell between 3.5V and o 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 not a linear function of voltage
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6. 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 depend 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 initial Capacity is known
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7. What makes a Battery Pack?
Number of cells assembled to form a battery-pack for required voltage and capacity
Safety Issues
Cell Balancing
Careful electrical design
so that every cells get equally charged / dischargedCells:
Chemistry evolves continuously bringing costs down
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8. Building Packs from cells
Cell voltage typically 3.7V (usage voltage varying from 3.1V to 4.1V)
Cell Capacity is 3.4 Ah (cylindrical) to 50 Ah (prismatic/ pouch)
Requires cells connected in Series to get higher voltage: 14 cells in Series is 51.8V i.e 14*37
Required cells in Parallel for higher Capacity: 8 cells (50 Ah) in parallel gives 400 Ah i.e 8*50
Generally cells has to be connected in series and parallel to make a pack
mPnS implies m cells in parallel to form modules and then connecting n modules in series
nSmP implies n cells in series to form strings and then connecting m strings in parallel
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9. Building mSnP battery pack
Cells can be connected in series to form a
STRING
14 cells connected in series to form 48V
battery
20 cells in series for 72V battery
100 cells in series for 365V battery
200 cells in series for 730V
Battery Strings can be connected in parallel
to increase capacity
Two 14S2P strings with 15 Ah cells would be
of 48V 15 Ah capacity or 1.5 kWh capacity
Any capacity can be built
But a concern: if strings do not have exactly
same voltage (will mostly not be so), current
will flow from one string to another for
balancing
This will happen continuously while charging
or discharging and even when IDLE not good
for battery
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10. Building nPmS battery pack
Cells can be connected in parallel to form MODULES
Four 15 Ah cells give a module of 60 Ah
Eight 15 Ah cell give a module of 120 Ah
Sixteen 15 Ah cell give a module of 240 Ah
Modules can now be connected in series to make a battery of higher voltage
If we connect 14 modules of 60 Ah (four 15 Ah cells in parallel) in series, we get a pack of
4P14S
voltage is 51V and is 60 Ah Capacity = 51V x 60 Ah = 3.06 kWh
If we connect 14 modules of 120 Ah (eight 15 Ah cells in parallel) in series Pack is 8P14S and
capacity is 51V x 120 Ah or 6.12 kWh
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11. Cells to Module and Modules to Pack
Multiple Cells packed in parallel to form a Module
Cells selected so that they are of same voltage (balanced) Cells connected with a metal bar that conducts
electricity
Multiple Modules in series to form a battery Pack
Battery Management System (BMS) a 'must to get optimal performance Especially for Li Ion batteries
Cell equalization during Charging
Monitor voltages and temperature of each module and total pack current
If a module is over-charged (impacts life), equalize by
Passive balancing: bleed module with higher voltage through a resistor, so that it charges slower, or just drain it
Active balancing: stop charging module with higher voltage; instead, use its output to charge the rest of pack
(using a DC-DC converter)BMS could limit temperature of each module if active cooling is done
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12. Electrical Design
As high currents are involved, a conductor is not zero voltage drop Smallest resistance-difference
between two electrical paths may result into differential voltage-drops:
currents will flow more on one rather than the other creating imbalances
For example if current enters battery as shown by the arrow Electrical Path to cell marked"a" is
shorter than that for one marked “b”
Cell imbalance will be created between cells a and It happens more between modules,
As current from one cell in a module to other is more as compared to that from another cell A
pack with continuous imbalance will quickly deteriorate in Capacity.
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13. To Sumup
We have looked at Conceptual Design of Battery Pack so far Not looked carefully at Mechanical
Design (bulging, pressure, vibration hurts life), Thermal design (high temperature hurts) and
BMS design (getting the best out of cells and Safety)
Even the electrical Design was minimally dealt with Cell selection is critical in price and
performance
Cell performance in terms of life-cycles and impact of charging-discharging rate, operation
temperature, depth of discharge on life-cycles is important
But Pack-design also impacts battery life-cycle Cell-imbalance has highest impact
Pressure on cells, Vibration, cell temperature difference, differential currents impact life-cycles
Inaccurate determination of SoH and SoC impacts vehicle performance
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14. Battery Pack Capital Costs
Costs depends on Capacity (kWh) or number of cells used and Cost per Cell
Cost per cell depends upon chemistry and life-cycles (when used in standard conditions) DoD,
rate of charge discharge, usage temperature affects the useable life-cycles
Battery Pack cost not dependent on Voltage used: 1 kWh battery at 12V or 24V or 48V or 380V
will typically cost the same
Battery Capital Costs: Cell costs + BMS costs + packaging costs + Cooling costs
Cell costs directly proportional to capacity:
oBMS and packaging goes up only slightly with capacity
oCooling costs depends upon nature of cooling (naturally cooled, air-cooled, liquid cooled)
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15. Electrical design of battery pack
ISSUES TO BE ADDRESSED
A. Least resistance to current flow.
B. Temperature rise due to current flow.
C. Short-circuit current stresses and
protection.
D. EMI noise suppression.
E. Joining methods and performance.
WHY BUSBAR DESIGN IN IMPORTANT?
An improperly designed busbar can:
Impair system operation.
Result in poor efficiency.
Overheat self & nearby components.
Pose safety & reliability issues.
Incur structural damage.
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16. Conductor material selection
Conductor material → critical for electrical performance, mechanical rigidity & functional safety.-
Thermal considerations → system ventilation to remove excess heat due to joule heating.
Characteristics of busbar materials:
1.Low electrical and thermal resistance.
2.High mechanical strength in tension,compression and shear.
3.High resistance to fatigue failure.
4.Low electrical resistance of surface films.
5.Ease of fabrication.
6.High resistance to corrosion
7.Competitive first cost and high eventual recovery value.- best available material. The next alternative is aluminum
Aluminum has 61% lower conductivity than copper. But, resulting increase in busbar dimensions dissipate more heat.
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17. Design criteria
For any electrical conductor, ohm's law states that:
Conductor Potential Drop (V) = Current flowing though it(I) * Conductor Resistance(R)
The resistance of the conductor is dependent on the material property & physical dimensions:
i. Electrical Resistvitiy(p).
ii. Conductor Length(L)
iii. Conductor Resistance(R)
iv. Conductor Cross Section(A)
Also, the resistivity (p) of the conductor material is a function of temperature:
The current carrying capacity of a busbar is limited by its maximum acceptable temperature.
Simply, the physical relation can be equated as:Heat generated by joule heating is heat dissipated to ambient.
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18. Short Circuit Scenarios
High heat generation during short-circuit higher temperature. The rate of temperature rise
during short circuit scenario can be obtained
The specific heat & resistivity dependent on temperature.
Copper resistivity increases by 60% as temperature rises from 20°cto 300°c.
In case of no protection, busbar melts. Heats up other components nearby
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19. Voltage Drop in Sensor Harness
For robust battery pack operation, voltage,
current & temperature monitoring is essential.
The harness provide a electrical pathway for
measuring the parameters.
The harness being electrical conductor, voltage
drop present across every ends.
Critical when the harness measures at both
nearer & farthest places.
The longer conductor higher voltage drop &
hence, incorrect measurement.
Sensing system is calibrated using:
1. The software (multiplier & gain factor).
2. The harness (equal resistance to all points)
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20. Current Equalization in Parallel Path
When a Current path splits to many parallel
path, the path resistance would vary due to
variations in length & cross section of the
Conductor.•
In shown representation, paths 1 & 4
experience higher resistance than paths 2 & 3,
due to the additional distance the current
flows.
When the parallel paths must experience
equal current flow, equalize the resistance of
each path. Providing higher cross section to
longer paths by achieving same resistance to
that of shorter path.
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21. Summary - Electrical Design
Busbars should be designed with:
1. Average current density of 5 A/mm2
2. Sufficient cooling by natural convection.
3. Easy manufacturability & assembly.
4. Cost effective material solutions.
Electrical Systems should have:
1. Minimized voltage drop wherever possible.
2. Sufficient electrical insulation to prevent
accident & mitigation.
3. Mechanically rigid.
Busbars joining methods preferred with:
1. Highest mechanical strength
2. Minimal contact resistance.
3. Easy weldability.
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22. Battery pack resistance
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23. Cont…
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24. Battery pack insulation
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25. Battery casing
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27. Battery pack insulation
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Insulation resistance gives an indication of the leakage current owing through a circuit. In an
electric vehicle, the insulation resistance of the high voltage bus with respect to ground depicts
the safety state of the HV bus and is an indicator of the electric vehicle's safety performance
A low insulation resistance level may result in exposing passengers to dangerous voltage levels.
The insulation resistance must always remain within the limits specied by international
standards.
 According to ISO 6469-1:2009 [22], the insulation resistance divided by its maximum working
voltage should be maintained at least greater than 100 ohm/V for DC circuits and 500 ohm/V for
combined AC and DC circuits but having a value greater than 500 ohm/V is preferred to ensure
safety of vehicle users. This requirement is for the entire circuit including all components;
therefore each component is required to have a higher insulation resistance

28. Insulation degradation
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Typically, the insulation of electrical equipment is made up of dierent components carefully chosen
to withstand stresses such as electrical, thermal, mechanical and environmental to which it is
subjected.
This degradation often referred to as aging of the electrical insulation is a process that causes
irreversible changes in the properties of the insulation due to interaction of one or more factors of
inuence (stresses)
Causes of insulation failure:
1. Mechanical stresses: Mechanical stresses resulting from shock or vibration during operation
2. Electrical stresses: Increased electrical stresses occur in the form of over voltages, or as an
effect of partial discharges in the insulating material.
3. Thermal stress: heat due to joules heating and charging discharging heat generated by the
battery internal resistances and also dur too ambient temp.
4. Environmental stress: These stresses could be in the form of moisture, humidity,
radiation, chemicals, dirt and oils

29. Cases of insulation failure
Punctured cell pouch
Abrasion on pouches after vibration test
Bus bar misalignment after vibration test
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30. Some insulation test
Partial discharge test: In this standard, the purpose of this test is to verify that no partial discharge are
maintainedin the solid insulation and no surface discharges occur in the surfaces at the highest of the
following values: recurring peak voltage, peak value of the steady state voltage, peak value of the long
term temporary overvoltage. It is also used for localization of partial discharges when they occur.
The impulse voltage withstand test:The purpose of this test is verication of clearances; this test is
used to assess the stresses caused by transient overvoltages for clearances and solid insulation. This
test was not performed because it could lead to break down of the battery insulation. It is required that
the test is carried under normal lab conditions; temperature 15 􀀀 35 C, air pressure 86-106 kPa at sea
level, relative humidity 25-75 %. For a DC voltage source of 400 V such as the battery the test impulse
voltage recommended for this test was 2500 V with a 1.2/50 us waveform.
The insulation resistance test:This standard species that the insulation resistance test on a battery
used in an electric vehicle should be performed after conditioning the battery in accordance with the
Standard for Environmental Testing { Part 2-30: Tests { Test Db: Damp Heat, Cyclic (12h + 12h Cycle),
IEC 60068-2-30 using the following parameters: a) Variant 1; b) At maximum temperature of 552 C
(1313 F); and c) 6 cycles. It is also species that the measuring instrument used for insulation resistance
test should have an internal resistance above 10 M ohm.
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31. Cont…
Dielectric voltage withstand test: The purpose of this test as stated in this standard is to
evaluate the electrical spacing and insulation at hazardous voltage circuits of the electric energy
storage assembly (batteries).The device under test is to subjected to preconditioning at constant
temperature of 302 C for 24 hour duration followed by conditioning at temperature of 232 C for 48
hours, humidity 93 %5 and atmospheric pressure 86 kPa to 106 kPa.
The purpose of this test as stated in this standard is to evaluate the electrical spacing and
insulation at hazardous voltage circuits of the electric energy storage assembly (batteries).The
device under test is to subjected to preconditioning at constant temperature of 302 C for 24 hour
duration followed by conditioning at temperature of 232 C for 48 hours, humidity 93 %5 and
atmospheric pressure 86 kPa to 106 kPa.
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32. Reference
Electrical Insulation in a 400 V Battery Module for Hybrid Vehicles MOHAMMAD H. MEMARI,
VICTORIA J. NAKANWAGI, 2014.
Report: 'A Guidance Document on Accelerating Electric Mobility in India
https://wri-
india.org/sites/default/files/Accelerating%20electric%20mobility%20in%20India_WRI%20India_
CBEEVIITM.pdf
NITI Aayog Report: Zero Emission Vehicle(ZEV): Towards a policy Framework
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