Project 1 Mechanical design of battery pack

Battery pack capacity: 18 kWh

Cell: ANR26650MI-B

Prepare a detailed battery pack drawing along with its enclosure. State your assumptions.

INTRODUCTION

As electric vehicles are increasing in the market is gradually increasing, there is considerable growth in the production of lithium-ion batteries. As it is the electrical source for the EV.

A lithium-ion battery or Li-ion battery is a type of rechargeable battery in which lithium ions move from the negative electrodes through an electrolyte to the positive electrode during discharge, and back when charging. Li-ion batteries use an intercalated lithium compound as the material at the positive electrode and typically graphite at the negative electrode.

Lithium-ion batteries are made use of various different electrodes. The most common material use is lithium cobalt oxide and graphite which are most commonly found in portable electronic devices such as cellphones and laptops. Lithium manganese oxide and lithium iron phosphate are also used as cathode materials.

A battery is made up of an anode, cathode, separator, electrolyte, and two current collectors (positive and negative). The anode and cathode store the lithium. The electrolyte carries positively charged lithium ions from the anode to the cathode and vice versa through the separator. The movement of the lithium ions creates free electrons in the anode which creates a charge at the positive current collector. The electrical current then flows from the current collector through a device being powered (cell phone, computer, etc.) to the negative current collector. The separator blocks the flow of electrons inside the battery.

CHARGE/DISCHARGE

While the battery is discharging and providing an electric current, the anode releases lithium ions to the cathode, generating a flow of electrons from one side to the other. When plugging in the device, the opposite happens: Lithium ions are released by the cathode and received by the anode.

Nanaophosphate High Power Lithiumlon Cell ANR26650M1-B

A123's high-performance Nanophosphate® lithium iron phosphate (LiFePO4) battery technology delivers high power and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package. Our cells have a low capacity loss and impedance growth over time as well as high usable energy over a wide state of charge (SOC) range, allowing our systems to meet end-of-life power and energy requirements with minimal pack oversizing.

Applications of the battery.

There are various applications of Lithium-ion cells

1. COMMERCIAL SOLUTIONS

This is a large-scale application for the battery where high power is required.

• Datacenter UPS
• Telecom backup
• IT backup
• Autonomously guided vehicles
• Industrial robotic and material handling equipment

2. GOVERNMENT SOLUTIONS

• Military vehicles
• military power grids

3. GRID SOLUTIONS

• Frequency regulation
• Renewable integration
• REverse capacity
• Transmission and distribution

4. TRANSPORTATION SOLUTIONS

• Plug-in hybrid vehicles, hybrid cars,
• electric vehicles
• Commercial vehicles
• Off-highway vehicles
• passenger vehicles.

The performance of the cell is given in the datasheet that is referred to for this project.

These images show the power discharge, current discharge, and the life cycle of the cells used for the battery pack.

Here you can see the voltage capacity of the battery for different power ratings.

DESIGN CALCULATION

All the parameters of this calculation are taken from the datasheet

PROCEDURE FOR THE CALCULATIONS

The capacity of the battery pack is set to be 18kWh.

For making a high voltage battery pack many cells are arranged in a particular pattern and sequence to improve the voltage and the power rating.

Individual battery cells may be grouped in parallel and/or series as modules. Further, battery modules can be connected in parallel and/or series to create a battery pack. Depending on the battery parameters, there may be several levels of modularity.

The total battery pack voltage is determined by the number of cells in series. For example, the total (string) voltage of 6 cells connected in series will be the sum of their individual voltage.

In order to increase the current capability of the battery capacity, more strings have to be connected in parallel. For example, 3 strings connected in parallel will triple the capacity and current capability of the battery pack

Now for calculating our required battery pack based on the cell manufacture data provided.

The calculation for the battery pack is as follows.

1. Volume of each cell

Cylindrical volume Ve [m³]

Vcc =(pi.D^2bc/ 4)*Lbc

From the datasheet we have

Dbc 26mm = 0.026m

and

Lbe 65mm = 0.065m

.. VcC=(pi*0.025^0.2* 0.065 /4) = 3.5 x 10 ^-5m^3

The volume of each cell is 35mm^3

2. Battery Cell Energy

Ebc=Cbc Ube

we have,

Cbc 2.5Ah

Ube = 3.3V

Substituting the values in above equation we get

Ebe = 2.5 x 3.3 = 8.25Wh

3. Battery cell energy density

• Volumetric energy density

uv=Ebc/Vcc(pc)

Substituting the value

UG=Ebc/Vcc=8.25/3.5 x 10^-5=235714.3 Wh/ m3

• Gravimetric energy density

UG=Ebc/mbc

Substituting the above calculated values in the equation, we get,

UG=Ebc/mbc=8.25/0.076=108.55*Wh/kg

4. Total number of the battery pack

The total energy of the battery pack which is required is given as 18 kWh

Ebp = 18000Wh

5. Number of cells connected in series

It is calculated by dividing the nominal battery pack voltage Upp [V] by the voltage of each battery cell Upc [V]. The number of strings must be an integer. Therefore, the result of the calculation is rounded to the higher integer.

Ncs=Ubp/Ubc

Let us assume that the battery pack voltage which is required to be 330V and the nominal voltage of the battery as per the datasheet is given as 3.3V.

Nbc= UbpU/be=330/3.3= 100

The total number of cells in series is 100

6. Energy Content of a string

The product between the number of battery cells connected in series and the energy of a battery cell

Ncs=Ubp/Ubc

Ebs=Ncs* Ebc=100 x 8.25=825Wh

7. Number of strings in the battery pack

It is calculated by dividing the battery pack's total energy Epp [Wh] by the energy content of a string Eps [Wh]. The number of strings must be an integer. Therefore, the result of the calculation is rounded to the higher integer.

N sb=Ebp/Ebs

Substitute the above value

Nsb=Ebp/Ebs=18000/825=21.81

All the 22 strings will be connected in parallel

8. Battery pack Capacity

It is calculated as the product between the number of strings Nsb [-] and the capacity of the battery cell Cbc [Ah].

Cbp=Nsb*Cbc

Cbp=Nsb*Ccb=22*2.5=55Ah

9. Total number of cells of the battery pack

It is calculated as the product between the

number of strings Nsb [-] and the number of cells in a string Ncs [-].

Ncb= Nsb*Ncs .

Ncb =Nsb*Ncs=22*100=2200

The total number of cells required for the design pack is 2200.

10. Mass of the battery pack

It is the product of the total number of cells Ncb [-] and the mass of each battery cell mbc [kg].

Mbp=Ncb*mbc

`m_(bp) = N_(cb)m_(bc) = 2200 * 0.076 167.2 Kg =

11. Volume of the battery pack

It is the product of the total number of cells Ncb [-] and the mass of each battery cell Vcc(pc) [m³]. This volume is only used to estimate the final volume of the battery pack since it does not take into account the auxiliary components/systems of the battery.

into account the auxiliary components/systems of the battery.

Vbp = Ncb. Vcc(pc)

Substituting the value

Vbp Neb. Voe = 2200 3.5 105 0.077m³

The total volume of the battery pack is 77000mm^3

12. String peak Current

Is the product between the peak C-rate of the battery cell C-rate pcp [h¹] and the battery cell capacity Cpc [Ah].

Ispc=C-ratebep*Cbc.

From the datasheet, the battery pack peak C rate is 24 and the nominal capacity is 2.5Ah.

= 24 X 2.5= 60A

13. Battery peck peak current

It is the product between the string peak current lspc [A] and the number of strings of the battery pack Nsb [-].

Ibpp = Ispc*Nsb

Substituting the above values

= 60 X 22 = 1320 A

14. Battery pack peak power

Is the product between battery pack peak current Ippp [A] and the battery pack voltage Ubp [V].

Pbpp = Ibpp⋅ Ubp

= 1320 x 330 = 435600 W = 435.6 kW

15. String continuous current

It is the product between the continuous C rate of the battery cell C-ratepcc [h=¹] and the battery cell capacity Cpc [Ah].

Iscc=C-ratebec*Cbc

From the datasheet, the battery pack continuous C-rate is 10 and the nominal capacity is 2.5Ah.

= 10 x 2.5 = 25A

16. Battery pack continuous current

Is the product between the string continuous current Iscc [A] and the number of strings of the battery pack Nsb [-].

Ibpc = Iscc*Nsb

= 25 x 22 = 550 A

17. Battery pack continuous power

Is the product between battery pack continuous current Ippc [A] and the battery pack voltage Upp [V].

Pbpc = Ibpc Ubp

= 550 x 330 = 181500W

The battery pack continuous power will be 181.5 kW

All the above parameters that are calculated are in the table below.

Battery dimensions

1. Length of casing

length = number of cells x (cell diameter + margin at both side of cells) + clearance at side

L = 100 x (26+4)+ 60 = 3060 mm

2. Breadth of the casing (B)

Breadth = number of cells x (cell diameter + margin at both side of cell) + clearance at side

B = 22 x (26 +4) + 60 = 720 mm

3. Height of casing (H)

Height = Height of cell + margin for wiring

H = 65+35 = 100 mm

Assembly of the battery pack

Component of the battery pack

1. 2200 cells

2. Nickel strip

3. PVC heat sink film

4. spot welder

5. Connected

6. Cell holder

7. Solder

Step 1

The first step is to start snapping the square cell holder together. One can use the cell terminal block but they will take more space than a hexagonal block.

this is the cell holder block

This image shows the insulate gasket

Step 2

Placing the cells in the holder matrix.

Place all the cells in the holder and put the second holder in the place.

Step 3

Now we will be connecting the cells with nickel strips using the spot welding process. The series connection will be made by connecting the positive terminal of the cells to the negative terminal. The parallel connection will be made by connecting the same terminal.

Connecting the complete matrix of the cell together.

Step 4

Connecting the heat sink to the battery pack

Now we will be soldering the battery wire

Now the internal structure of the battery pack is assembled and all the terminals are connected as per the connections.

Step 5

Connect the balancing wire pin to the battery management system.

Step 6

After the battery management system, the next and final step is to place the enclosure for the battery pack.

This is the final battery pack model created.

CONCLUSION

The design calculation for the battery pack was carried out and calculated and a table has been made for the same. The steps involved in the making of the battery pack have been listed out.