Project 1 Mechanical design of battery pack
ANS..
OBJECTIVE: Prepare a detailed battery pack drawing along with its enclosure.
THEORY:
The high voltage battery is one of the most important components of a battery electric vehicle (BEV). The battery parameters have a significant influence on other components and attributes of the vehicle, like:
- maximum traction motor torque
- maximum regeneration brake torque
- vehicle range
- vehicle total weight
- vehicle price
Pretty much all major aspects of a pure electric vehicle (EV) depend on the parameters of the high voltage battery.
For our electric vehicle battery design we are going to start from 4 core input parameters:
- chemistry
- voltage
- average energy consumption of the vehicle on a driving cycle
- vehicle range
A battery consists of one or more electrochemical cells (battery cells) which are converting chemical energy into electrical energy (during discharging) and electrical energy into chemical energy (during charging). The type of elements contained within a battery and the chemical reactions during discharging-charging events define the chemistry of a battery.
A battery cell consists of five major components: electrodes – anode and cathode, separators, terminals, electrolytes, and a case or enclosure. For automotive applications, there are different types of cells used Individual battery cells are grouped into a single mechanical and electrical unit called a battery module. The modules are electrically connected to form a battery pack.
The voltage level of the battery determines the maximum electrical power which can be delivered continuously. Power P [W] is the product between voltage U [V] and current I [A]:
P = U* I
The higher the current, the bigger the diameter of the high voltage wires and the higher the thermal losses. For this reason, the current should be limited to a maximum, and the nominal power obtained by having a higher voltage. For our application, we are going to consider a nominal voltage of 400 V.
the energy needed for propulsion, the high voltage battery must supply the energy for the vehicle’s auxiliary devices Eaux [Wh/km], like 12 V electrical system, heating, cooling, etc. Also, we have to consider the efficiency of the powertrain ηp [-] during the conversion from electrical energy to mechanical energy.
Battery pack architecture:
All high voltage battery packs are made up from battery cells arranged in strings and modules. A battery cell can be regarded as the smallest division of the voltage.
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 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.
An example is the high voltage battery pack of Tesla Model S, which has:
- 74 cells in a parallel-group
- 6 groups in series for a module
- 16 modules in series
- 7104 cells in total
Battery pack production line:
parameters of the cell:
cell dimensions:
ANR26650M1-B Specifications
Nominal Ratings
Voltage: 3.3 Volts
Capacity: 2.5 Ah
Energy: 8.25 Wh
Specific Power: 2600 W/kg
Impedance (1KHz AC Typical) 6 mΩ
Cycle Life at 1C/1C, 100% DOD: > 4000 cycles
Cell Type: 26650 Lithium Ion Power Cylindrical Cell
Discharging
Max Continuous Discharage: 50 A
Max Pulse Discharge Current >50% SOC (10s): 120 A
Minimum Voltage: 2 Volts
Temperature: -30ºC to 55ºC
Charging
Recommended Standard Charge: 2.5 Amps
Max Charge Rate: 10 Amps
Max Pulse Charge Current <50% SOC (10s): 25 A
Float Voltage: 3.45 Volts
Recommended charge V & Cut-off Current: 3.6 Volts, taper to 125mA
Temperature
(reduce charging current to 250mA when under 0ºC): 0ºC to 55ºC
Mechanical
Diameter: Ø25.96 +/- 0.5 mm (1.0")
Length: 65.15 +/- 0.5 mm (2.6")
Mass: 76 g
Certifications
Transportation: UN 3480 (UN38.3), CIQ
Safety: UL 1642, IEC 62133
The LithiumWerks ANR26650M1-B is the next generation of a pioneering 26650 cylindrical cell, now with greater power and energy density and lower impedance. This versatile lithium ion cell is suitable for a wide variety of applications and system designs. Proven performance in the toughest conditions, combining durability, reliability, and safety, the ANR26650 cell offers an excellent combination of price-performance.
Cylindrical Cell Primary Applications:
- Portable high power devices
- Stationary battery backup systems
- Grid stabilization energy storage systems
- Commercial truck and bus hybrid electric vehicles (HEVs)
Lithium Ion Cell Benefits:
- Power: Over 2,600 W/kg and 5,800 W/L
- Safety: Excellent abuse tolerance and environmentally friendly
- Life: Excellent calendar life, 10X cycle life vs. conventional lithium ion
cell calculations:
Volume of cell: pi*r^2*h
From datasheet we have’r’ and ‘h’ values
r=0.013m and h=0.065m
volume of cell=3.14*0.013*0.013*0.065
= 0.000344929
The value of the volume of the cellis used to find out the total volume of the batterypack.
Cell energy= cell capacity(ah)*cell voltage(v)
=2.5*3.3
=8.25wh
The value of single cell is used to calculate the total energy of the battery pack
Single cell energy density
Volumetric energy(wh/m^3)= single cell energy/volume of each cell
=8.25/0.0000344929
=2391796572
Battery pack assumptions:
There are 100 cells in series and 50 cells in parallel, and we will use ANR26650M1B cells
Battery pack calculation:
Total no. of cells= series cells* parallel cells
= 100*50
=5000
So from series and parallel combination , we can calculate the voltage and current of battery pack
Voltage=no. of cells* nominal voltage
=100*3.3
=330v
Current= no. of cell in parallel* cell capacity
=50*2.5
=125ah
Now we have to calculate energy with parameters including coltage and current
Energy=voltage*current
=330*125
=41250(total energy)
For total volume:
Total volume = no. of cells* volume of single cell
=5000*0.0000344929
=0.1724645m^3(total volume required for battery pack)
For total weight of battery pack:
Weight= no. of cells*weight of single cell(in datasheet)
= 5000*0.76
=380kg
Energy density= volume*battery pack energy
=0.1724645*41250
=7114.160625 kwh/m3
Assembly of the full battery pack with 100s/ 50P. The arrangement of the pack is as follow:
Then,
5000 cells will have100S / 50P
The detail technical specification of 5000 battery pack is as follow:
- Package: 100S / 50P
- Capacity: 125 Ah
- Voltage: 330 V
- Weight: 380kg (Only Cells)
- No of Cells: 5000
- Power: 41250
A simple layout of the battery pack in an electric vehicle is shown below:
Design of cell holder (Bottom and Top)
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.
Design of pack cover
Basically, Pack cover is made up of Aluminium and the reason behind is to keep battery pack for lightweight. The casing/cover thickness is 0.5 mm and the design is as shown below:
A bottom cover is shown to cover below the bottom battery holder.
Design of Lower floor case for battery pack
Basically, the lower case is a part which is fitted in an electric vehicle for the assembly of the battery pack. A lower case is also made up of aluminum and it has top cover and bottom cover to battery pack from dust and environmental pollution.
The design of the lower case is shown
Positive and Negative Clip
A positive and negative clip is used to connect the battery end cap i.e (+) and (-). Design is shown below:
To make the battery pack, you have to connect the many cells together by means of Nickel strips or thick wire. Generally, Nickel strips are widely used for this. In general two types of nickel, strips are available in the market: nickel-plated steel strips and pure nickel strips. I will suggest buying a pure nickel. It is a little bit costlier than nickel-plated steel, but it has much lower resistance. 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.
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.
The suitability of joining methods to build battery packs using cylindrical cells is shown below:
RESULTS:
| NO | Calculated Parameters | Value Get |
| 1 |
Cell Volume |
0.0000344929m^2 |
| 2 | Cell Energy | 8.25Wh |
| 3 | Cell Density | 239179.6572kWh/m^3 |
| 4 | Pack assumptions | 100s 50p |
| 5 | Total no of cells | 5000 |
| 6 | Voltage | 330V |
| 7 | Capacity | 125Ah |
| 8 | Energy | 41250WAh |
| 9 | Total volume | 0.1724645m^3 |
| 10 | Total Density | 7114.160625kWh/m^3 |
| 11 | Weight | 380kg |