Project 1 Mechanical design of battery pack
SOLUTION : MECHANICAL DESIGN OF A BATTERY PACK
We are given a Cell : ANR26650M1-B
We are asked to prepare a detailed battery pack drawing along with its enclosure . Also we take certain assumptions here .
The specifications of the cell is given below :
This cell is of Lithium Iron Phosphate chemistry .This technology willl provide high power and energy density with a good safety performance .
The Capacity of the given battery pack is 18 kWh . Inorder to make a high voltage battery pack many cells are arranged in a particular configuration to achieve the required voltage and current rating. To increase the voltage rating of a string in a battery pack , a number of individual cells will be conected in series. To increase the current rating , a number of strings will be connected in parallel.
Now we go onto the Design calculations :
1. Volume of each cell :
Cylindrical volume :
Vc = pi * (r^2) * h
r= radius = D/2
D = 26 mm = 0.026 m
h = 65 mm = 0.065 m
so uploading the values given in the datasheet into the equation we get :
Vcc = 3.14 * ( 0.013^2) * 0.065 = 3.5 X 10^(-5) m^3
2. Battery cell energy :
Ec = Ah Capacity * Voltage capacity
= 2.5 * 3.3 = 8.25 Wh
3 . Battery cell Energy density
a. Volumetric energy density
= Ec/Vc
= 8.25 / 3.5 X 10^(-5)
= 235714.3 Wh / m^3
b. Gravimetric Energy density
= Ec / Mc
= 8.25 / 0.076
= 108.5526 Wh / kg
4. Net energy of the battery pack
Etot = 18 kWh = 18000 Wh .
5. Number of cells connected in series
Total voltage required when divided by the voltage of an individual cell , we get the number of cells in series . So we should assume a voltage required by the load. Here we take as 330 V .
So we get the number of cells connected in series as :
Ns = 330 / 3.3
= 100
6. Energy of a string
Number of cells connected in series in a string when multipllied with the energy of an individual cell, gives the total energy of the string.
So , Ens = Es * Ns
= 8.25 * 100
= 825 Wh .
7. Number of strings in a battery pack
This value can be found by dividing the total energy of the battery pack by the energy of an individual string
So we get it as : Np = Etot / Ens
= 18000 / 825
= 21.81
So the number of strings in this battery pack is 22. We need an integer value for the number of strings .
8. Battery pack capacity
This capacity can be calculated by multiplying the number of strings connected in parallel with the Capacity of an individual cell
Cbp = Np * Cb
= 22 * 2.5
= 55 Ah
9 . Total number of cells in the Battery pack
There are 22 strings connected in parallel, with each string having 100 cells connected in series .
So we can get the total number of cells in the battery , Nb
= Ns * Np = 22 * 100
= 2200
10. Mass of the battery pack
Esch individual cell weighs 76 gm
So 2200 cells will weigh ,
Mb = Mc * Nb
= 2200 * 0.076
= 167.2 kg
11. Volume of the battery pack
Volume of the battery pack can be calculated by multiplying the number of cells in the pack with the volume of each battery cell
Vb = Vc * Nb
= 2200 * 3.5 X 10^(-5)
= 0.077 m^3
12. String peak current
Peak current of the string can be calculated by multiplying the string current with the maximum possible C-rating . In the datasheet it is given that the maximum C rating is 24.
So Ips = Is * 24
= 24 * 2.5
= 60 A
13. Battery pack peak current
Peak string current when multiplied with the total number of strings give us this value
= 22 * 60 = 1320 A
14. Battery pack peak power
Product of Peak voltage of the battery pack and Battery pack Peak current gives us the Peak Power of the battery
= 1320 * 330
= 435600 W
= 435.6 kW.
15. String Continuous Current
Product of the continuous current rating and the battery cell capacity , which we get as :
= 2.5 * 10 = 25 A
Where 10 is the continuous C rating of the battery pack .
16. Battery pack continuous current
Product of the number of strings in the pack and the String Continuous Current, we get
= 25 * 22
= 550 A.
17. Battery Pack Continuous Power
Product of the Voltage and Battery pack continuous current we get the Battery Pack Continuous Power
= 550 * 330
= 181500 W
= 181.5 kW .
Battery dimensions :
Length of battery casing :
Length = Number of cells X ( Cell Diameter + Margin on both sides of the cell ) + Clearance at side
= 100 * ( 26 + 4 ) + 60
= 3060 mm.
Width = Number of cells X ( Cell Diameter + Margin on both sides of the cell ) + Clearance at side
= 22 * ( 26 + 4 ) + 60
= 720 mm
Height = Height of cell + Margin for Wiring
= 65 + 35
= 100 mm
There are two types of cell confugration for a battery pack . They are :
1. Parallel
All the positive terminals are connected together and the negative terminals are connected together . Parallel connection keeps the voltage same but can be used to increase the capacity of the battery pack .
2. Series
Positive terminal of one cell is connected to the negativeterminal of the other cell in series configuration . Voltage capacity can be increased using this , keeping the current through the string same . 
Parts of a Battery Pack :
1. Cells
2. Battery Management System
The above image is that of a flowchart of the working of a Battery Management System . Voltage , current and Temperaturre are connected as input s to the Measurement block of BMS . SOC, SOH etc will be given as output .
3 . Connection System
With the help of Nickel Strip all cells are connected and kept together in a battery pack is a connection system . Nicke strips are usedin battery packing because of the efficiency in spot welding, soldering also it prevents early corrosion . Current transfer inside the battery will not be effected by Nickel Strips .
4. Housing
This helps the battery to be connected to other devices .
Also Thermal Management System is present inside the battery which helps in controlling the Temperature of the battery and monitoring it . Thermal runaway will be avoided with the help of TMS . The design of the Battery is shown in the figure below :
Battery Pack enclosure :
Lithium ION Battery pack , consists of an assembly of battery modules . This battery pack is subjected to lot of mechanical stress and vibrations under th harsh working conditions . The safety of the EV depends on the safety of the battery pack , where mechanical design of the battery plays a major role .
Conclusion :
So from the technical details of the cell given , we have designed the mechanical aspects of the battery .