Project 1 Mechanical design of battery pack

Aim:

Design a battery pack with the capacity of 18Kwh by using the cell model ANR26650M1-B and prepare detailed battery pack report.

Cell Model:ANR26650M1-B:

The above picture shows cylinderical type cell model ANR26650M1-B.

The cell model ANR26650M1-B is a high-performance lithium cell.

Madeup of Nanophosphate lithium iron phosphate (LiFePO4) battery technology it delivers high power and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package.

These model cells have low capacity loss and impedance growth over time as well as high usable energy over a wide state of charge (SOC) range.

These ANR26650M1-B cell offers an excellent combination of price & performance.

Applications:

Portable high power devices

Stationary battery backup systems

Grid stabilization energy storage systems

Commercial truck and bus hybrid electric vehicles (HEVs)

The cell model ANR26650M1-B technical data is shown below.

The constant power discharge and constant current discharge at various temepratures charactristics is shown below.

Battery pack capacity 18 kWh design calculation:

For calculation purpose i have assumed certain parameters.

Now a days all the EVs battery pack voltage comes between the range of 400-800V DC. So here i have assume that the total voltage of the battery pack capacity is 500V DC.

From the technical data sheet of the cell model ANR26650M1-B are listed below.

Cell diameter (D)= 26 mm

Cell height (H)= 65 mm

Cell nominal voltage= 3.3V

Cell nominal capacity= 2.5Ah

Cell weight= 76g

Calculation procedure for 18Kwh battery pack:

1. Calculate the number of series cells required to obtain the battery pack voltage as 500V.

For give a 500V battery pack the cells are need to connect in series.

The number of cells connected in series will imporve the total battery pack voltage.

For example from the above fig, the total voltage of 6 cells connected in series will be the sum of their individual voltage. In this example assume the individual cell voltage is 2 V therefore the total voltage of the 6 series connected cells is 12V.

Similarly in our case we already know the total voltage of battery pack is 500V so we can calculate number cells requred to connect in series(Ns).

Total voltage of the battery pack= Number of Cells connected in series(Ns) * Individual cell nominal voltage

500= Ns*3.3V

Therefore number cells connected in series (Ns)= 152

2. Calculate the number of paralle cells required to obtain the desired battery pack current capacity.

First we need to calculate the Total current capacity of battery pack

We already know that total battery pack capacity= 18Kwh

W.K.T Total battery pack capacity= Total Voltage of battery pack(V) * Total current capacity of battery pack(Ah)

18000= 500 * Total current capacity of battery pack(Ah)

Total current capacity of battery pack(Ah)= 18000/500

Therefore Total current capacity of battery pack(Ah)= 36 Ah

Now we can able to calculate the number of paralle cells required to obtain the desired battery pack current capacity.

Total current capacity of battery pack(Ah)= Number of Cells connected in parallel(Np) * Individual cell nominal current capactiy

From the technical data of the cell we can get to know Individual cell nominal current capactiy= 2.5Ah

36= Np * 2.5

Np= 36/2.5 = 15 (approximately)

Therefore the number of paralle cells required to obtain the desired battery pack current capacity Np= 15.

3. Calculate total number of cells required to the battery pack:

Total number of cells required to the battery pack= Number of cells in series(Ns) * Number of cells in parallel(Np)

= 152 * 15

Therefore Total number of cells required to the battery pack= 2280

4. Calculate the volume of each cell

Here the cell is in cylindrical in shape

So, W.K.T Volume of cylinder = πr^2h

From the cell data sheet we can get to know Diameter of cell D= 0.026 m and height of cell h= 0.065 m

we already know radius (r)= D/2

r=0.026/2

r= 0.013 mm

Volume of each cell= π*0.013^2*0.065

Therefore the volume of each cell= 0.0345 cubin mm

5. Calculate volume of battery pack

Battery pack volume = Total number of cell in a battery pack * Volume of each cell

= 2280 * 0.0345

Therefore the total volume of battery pack= 78.66 cubic mm

7. Calculate mass of the battery pack

Total mass of battery pack= Total number of cell in a battery pack * Mass of a single cell

From the cell data sheet we can get to know mass of a single cell is 0.076Kg

Total mass of battery pack= 2280 * 0.076

Therefore the total mass of battery pack= 173.28 Kg

The above calculated values are listed below of table.

S.No	Calculated parameters	Obtained results
1	Number of series connected cells	152
2	Number of parallel connected cells	15
3	Total current capacity of battery pack(Ah)	36Ah
4	Total number of cells in the battery pack	2280
5	Volume of each cells	0.0345 mm^3
6	Volume of battery pack	78.66 mm^3
7	Mass of battery pack	173.28Kg

There the configuration of this battery pack is 152S15P

Battery dimensions:

1. Length of casin

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

L = 152 x (26 + 4) + 60 = 4620 mm

2. Breadth of the casing (B)

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

B = 15 x (26 +4) + 60 = 510 mm

3. Height of casing (H)

Height = Height of cell + margin for wiring

H = 65+35 = 100 mm

Battery pack assambling procedure:

Components used to build the battery pack is listed below.

1. 2280 cells
2. Nickel strip
3. Cell holder
4. spot welding
5. Solder
6. BMS
7. Heat sink

Cell Holder:

Here we have using 2280 cells to build the battery pack for that we need cell holder.

The cell holder how it look like is shown below.

Now we start to snapping the square cell holder together. We can use the cell terminal block also but they will take more space than a hexagonal block.

After fixed the cells in the cell holder it will look like above mentioned figure

Advantages of using these cell holders are

• You can make a custom pack of any size according to your requirement.It's like solving a puzzle.
• It provides space between the cells, which allow fresh air to pass and the battery gets cooled easily.
• It makes your battery pack solid and reliable.
• It provides safety anti-vibration to your battery pack

Nikel strip

• By using the nike strip we can configure the cells as series and parallel.
• One important thing always keep in mind is before connecting the cells in parallel, first, check the individual cell voltages. For paralleling the cells, the voltage of each cell should be near to each other, otherwise, a high amount of current will flow from the cell with a higher voltage to the cell with a lower voltage. This can damage the cells and even result in fire on rare occasions.
• Nickel strips are widely used for it lower resistance quality. Low resistance means, less heat generation during the charging and discharging, which leads to longer useful battery life.
• Nickel strips come with different dimensions and lengths. Choose the strips according to the current rating.
• 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.

• After fixed the nikel strips at cell terminals by using spot wielding process is look like above figure.

Spot welding:

Spot welding is preferred over soldering, why because the problem with soldering is that you apply a lot of heat to the cell and it doesn’t dissipate very quickly. This enhances the chemical reaction in the cell which damages the cell's performance. Ultimately you will lose some capacity and life the cells.

Battery Management System:

A battery management system (BMS) is an electronic system. Its main functionality in the battery pack is listed below.

• Monitors all of the parallel groups in the battery pack and disconnect it from the input power source when fully charged.
• Balance all the cells voltage equally.
• Doesn't allow the pack from over-discharged.

• The BMS attached battery pack is looks like the above figure

Solder:

• The solder is used to connect the battery pack terminal to the BMS.

Heat sink:

• Heat sink are used to maintain battery pack temperature within a limit, so it will improve its performance, safety.

CONCLUSION:

• The design calculation for the battery pack capacity of 18Kwh was carried out with itsnecessary assumption and calculated and obtained result from the calculation also tabulate.
• The process involved in the making of battery pack also listed out and seen brifly.