Project 1 Mechanical design of battery pack

Mechanical Design of Battery Pack

Introduction

As we know that the electric vehicle market is gradually increasing. With the growth of an electric vehicle, the growth and interest in Lithium-ion cell are also increasing. The energy source for an electric vehicle is stored in cylindrical cells.

Lithium-ion cells are capable of delivering very high power. Lithium-ion cells are used in aerospace, automotive electric vehicle, mobile devices, medical devices and other electronic devices which is based on these cells. The battery pack is installed in lower-floor of the electric vehicle so that weight may distribute equally and also the centre of gravity also maintain properly.

The high voltage battery pack is one of the most important components of any electric vehicle. The traction battery of an electric vehicle must achieve the following thing:

• Provide maximum traction motor torque
• Vehicle range in sense of kilometre
• To achieve the proper life cycle of battery

The battery pack of an electric vehicle is generally having a series or parallel combination. Large battery pack have multiple cell modules, in which many numbers of cells are kept together to build a large battery pack. Basically, a series connection is used to compound the voltage whereas the parallel connection is for compound current capacity. A battery cell consists of five major components that are electrodes – anode and cathode, separators, terminals, electrolyte and enclosure.

Mechanical Design of Battery Pack

Safety and reliability are two major things for large battery packs. While mechanical designing of large battery pack we should follow some technical criteria so, that the battery may not fail.

• Mechanical vibration should decrease
• Controlling on high impact force
• Better thermal runway for cooling
• Serviceability and recyclability

For the designing of 18 kWh battery pack here, I use a base model of ANR26650M1B Lithium-Ion Cell. ANR26650M1B cell is consist of lithium iron phosphate chemistry (LiFePO4) and it is best for use in a wide range of temperatures and conditions. The ANR26650M1B Lithium Ion cell may achieve the following features:

• Stable chemistry
• Faster charging
• Consistent output
• Excellent cycle life
• Superior cost performance

Technical Specification ANR26650M1B

NOMINAL RATINGS
Voltage	3.3 V
Capacity 23°C Typ (Min)	2.6 Ah (2.5)
Energy 23°C	8.25 Wh
Specific Power	2600 W/Kg
Impedance (1KHz AC) Typ	6 mΩ
Cycle Life at 1C/1C, 100% DOD	>4000 Cycle
DISCHARGING
Max Continuous Discharge Current	50 A
Max Pulse Discharge Current (10s)	120 A
Minimum Voltage / HPPC Pulse	2 V / 1.6 V
Temperature	-30°C to 55°C
CHARGING
Recommended Charge Current	3 A
Max Continuous Charge Current	10 A
Max Pulse Charge Current (10s)	20 A
Float Voltage	3.45 V
Recommended Charge V & Cut-off current	3.6 V, tapper to 125Ma
Temperature Range (reduce charging current to 250Ma When Under 0°C)	0°C to 55°C
STORAGE
Storage Temperature	-40°C to 60°C
MECHANICAL
Diameter	25.96 +/- 0.5 mm
Length	65.15 +/- 0.5 mm
Mass	76 g +/- 1.0 g

Dimension

Applications

• Energy storage
• Uninterruptible Power Supplies
• Communication technologies
• Aerospace
• Electrified mobility devices
• Industrial equipment
• Medical devices
• Toys

Calculation as per assumptions: 18kWh Battery Pack

The mathematical calculation for designing an 18kWh battery pack, basically 18kWh is energy require to achieve the vehicle its proper range. It is dependingupon the voltage (V) parameter and ampere-hour or capacity (Ah).

Here I used to design a high voltage battery pack for an electric vehicle to achieve its range up to 250 km. For this calculation, I assumed 145.2 V a nominal voltage for series connection.

General required value depending upon a single cell:

Diameter = 25.96 / 26 mm

0.026 m

Length = 15 / 65 mm

0.065 m

Mass = 76 g

0.076 kg

Single-cell Voltage = 3.3 V

Capacity = 6 Ah

Max Continuous Charge Current (C . rate) = 10 A

Max Pulse Charge Current (C-rate) = 20 A

Mathematical Battery Calculation

Single Cell Calculation

= πr2h

= π x 0.013² x 0.065

= 3.45104 x 10-5

= 3.14 (0.026)² x 0.065

= 0.03451m³

D = Diameter of cell

L = Length of cell

Basically, the value of the volume of the cell is used to find out the total volume of the battery pack.

• Single Cell Energy (Wh)

= Cell Capacity (Ah) × Cell Voltage (V)

= 2.6 × 3.3

= 8.58 Wh

The value of a single cell is used to calculate the total energy of the battery pack.

• Single Cell Energy Density

Volumetric energy (Wh/m³ ) = Single Cell Energy / Volume of each cell

= 8.58 Wh / 0.03451 m³

= 248.62Wh/m³

Gravimetric energy (Wh/kg) = Single Cell Energy / Single Cell Mass

= 8.58 Wh / 0.076 kg

= 112. 89Wh/kg

Calculation of High Voltage Battery Pack

For design and calculation of battery pack here, I assume a vehicle to achieve 250 km in 18 kWh battery pack. Also, I assume 145.2 nominal voltage for series connection.

So,

Vehicle Range = 250 KM (assumed)

Battery Pack capacity = 18.121 kWh or 18120.96 Wh (For better and accurate calculation here I change kWh)

Now,

We will calculate per km average energy = 18120.96 / 250

= 72.483 Wh/km

Therefor,

Battery Pack total energy = Average energy × km

= 72.483 × 250

= 18,120.96 Wh or 18.120 kwh

Let’s assume Nominal voltage for series connection = 145.2 V, Basically, this depends upon the motor power, We can decide the value according to the motor.

• Number of cells connected in series

= Nominal voltage of series connection / Nominal voltage of a single cell

= 145.2 V / 3.3 V

= 44

44 Cells will be connected in a series connection to provide a proper voltage.

• Energy for battery pack (wh)

= No. of cells connected in series × Single-cell energy

= 44 × 8.58 wh

= 377.52 wh

• Number of cells connected in parallel

= Battery pack total energy (wh) / Energy for battery pack (wh)

= 18120.96 / 377.52

= 48

48 cells will be connected in parallel to provide a proper capacity power.

• Battery pack total energy (wh)

= No. of cells connected in parallel × Energy for battery pack (wh)

= 48 × 377.52

= 18120.96 wh or 18.120 kWh

• Battery pack capacity

= No. of cells connected in parallel × Single cell capacity (Ah)

= 48 × 2.6 Ah

= 124.8 Ah

The value of battery pack capacity is used for the total power required by the vehicle.

• Total number of cells for battery pack

= No. of cells connected in parallel × No. of cells connected in series

= 48 × 44

= 2112

2112 Cells will be used to make a complete battery pack.

• Mass of battery pack

= Total no. of cells for batter pack × Mass of single cell

= 2112 × 0.076 kg

= 160.512 kg

It is the total weight of cells connected in parallel or series.

• Total volume of the battery pack

= Total no. of cells for batter pack × Volume of single cell

= 2112 × 0.0345

= 72.89 m³

This is the volume of 2112 cells which are used in the battery pack system.

• Cell peak current (A)

= C (Max Pulse Charge Current) × Single cell capacity

= 20 × 2.6

= 52 A

• Battery pack peak current (A)

= Cell peak current × No. of cells connected in parallel

= 52 × 48

= 2496 A

It is used for the operation of the battery pack. By this way, a vehicle will decide how much power is required to move.

• Battery pack peak power (W)

= Battery pack peak current × Voltage of series connection

= 2496 A × 145.2

= 362419.2 or 362.42 W

• Cell continuous current (A)

= C_rate (Max Continuous Charge Current) × Single cell capacity

= 10 × 2.6

= 26 A

• Battery pack continuous current (A)

= Cell continuous current × No. of cells connected in parallel

= 26 × 48

= 1248 A

This is the power which is supply continue to the whole battery pack system.

• Battery pack continuous power (W)

= Battery pack continuous current × Voltage of series connection

= 1248 × 145.2

= 18120.96 or 18.120 Kwh

Above calculation is to find the total number of cells for a full battery pack for Electric vehicle, also the battery pack weight, current, capacity and voltage.

For design purpose and easy for installation here, I configure this large battery pack into a small pack and the small pack consists of 32 cells into 66 pack and every single pack will have 8S / 4P.

Single-cell dimension

• Diameter Ø26 +/- 0.5 mm
• Length 65 +/- 0.5 mm

A Cell is basically consisting of Anode, Cathode, Separator, Positive Cap, Negative Cap and Gasket.

The arrangement design of single-cell is shown below:

Arrangement and assembly of 8S/4P Cell pack with 32 cells are given shown below:

The detail technical specification of the single pack is as follow:

• Package: 8S/4P
• Capacity: 4 Ah
• Voltage: 26.4 V
• Weight:2.432 kg (Only Cells)
• of Cells: 32
• Power: 274.56 Wh

Assembly of the full battery pack with 44S / 48P. The arrangement of the pack is as follow:

Single Pack: 32 cells with 8S / 4P. 32 cells × 66 pack = 2112 cells

Then,

2112 cells will have 44S / 48P

Here I arrange the pack such that it will build a proper battery pack.

The detail technical specification of 2112 battery pack is as follow:

• Package: 44S / 48P
• Capacity: 8 Ah
• Voltage: 145.2 V
• Weight: 160.512 kg (Only Cells)
• of Cells: 2112
• Power: 18120.96 Wh or 18.12 kWh

Here I also configure battery pack into electric vehicle lower floor panel case, a simple layout of the battery pack in an electric vehicle is shown below:

The main function of a battery holder is to keep cells fixed in place safely and securely also it is used to keep cells in a proper arrangement so that there

will be a decrease in mechanical vibration, and have a better thermal runway in between the cell gaps. Most of the cell holder is made up of polypropylene

or nylon bodies rated for 80–100 °C.

A dimension and 3D is shown below:

• Positive and Negative Clip

A positive and negative clip is used to connect the battery end cap i.e (+) and (-). Design is shown below:

• Joining of Cylindrical Cell Battery Pack

Cylindrical cells have improved thermal management efficiency because of compact shape, but have low packaging efficiency because of their round cross-section. Here I use a positive and negative tab to connect multi cells.

Conclusion

As from the single-cell technical data here I have designed a battery pack for the electric vehicle which is having total energy of 18.121 kWh or 18 kWh. The total number of cells are 2112 which are in the configuration of 44S / 48P. Also, the nominal voltage from the 44 series connection of battery gives 145.2 V and the capacity power we get from the arrangement of 48 parallel cells is 124.8 Ah.

All design is used to achieve a 250 km range by the vehicle, which is an assumption to calculate the battery pack. Here if I will increase the number of cell in parallel then the power will increase but at that time the distance will not be achieved by the vehicle, that’s why here I use 44S / 48P cell configuration for the battery pack.

Also, the whole battery pack is subdivided into 66 pack in which each block has 32 cells and this configuration is prepared to set up 44S / 48P cell configuration.

Also, I used 2.6 Ah as single-cell capacity instead of 2.5 Ah, if we use 2.5 Ah then the battery pack configuration will be 44S / 50P.

Single cell configuration
Package	1S / 1p
Capacity	2.6	Ah
Voltage	3.3	V
Weight	76	Gram
Power	8.58	Wh
32 cells configuration
Package	8S / 4p
Capacity	10.4	Ah
Voltage	26.4	V
Weight	2.432	Kg
Power	274.56	Wh
2112 cells configuration
Package	44S / 48p
Capacity	124.8	Ah
Voltage	145.2	V
Weight	160.5	Kg
Power	18.12	Wh

The detail parameter of single, 32 sell and 2112 cells are shown in table below.

A battery discharge characteristic is also calculated based on single-cell configuration.

• Nominal Voltage = 3.3 V
• Capacity = 2.6 Ah

References:

1. Mechanical Design and Packaging of Battery Packs for Electric Vehicles, Shashank Arora & Ajay Kapoor, Springer International Publishing AG
2. Battery Management Systems for Large Lithium-Ion Battery Packs, Davide Andrea
3. Mooy, Robert & Aydemir, Muhammed & Seliger, Günther. (2017). Comparatively Assessing different Shapes of Lithium-ion Battery Cells. Procedia Manufacturing. 8. 104-111.10.1016/j.promfg.2017.02.013.

4.https://datasheetspdf.com/pdf/1424845/A123Systems/ANR26650M1-B/1

5. https://a123batteries.com/anr26650m1-b-lithiumwerks-nanophosphate-3-3v-2-5ah-lithium-iron-phosphate-battery/
6. http://www.a123systems.com/
7. https://www.powerstream.com/BPD.htm