Project 1 Mechanical design of battery pack
PROJECT 1: Mechanical design of battery pack
Aim: design a Mechanical battery pack.
OBJECTIVE:
Battery pack capacity: 18 kWh
Cell: ANR26650M1-B
Prepare a detailed battery pack drawing along with its enclosure. State your assumptions.
INTRODUCTION:
As electric vehicles are increasing in the market gradually,therte 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 rechargable battery in which lithium ions move from the negative electrodes through an electrolyte to the positive electrodes during discharge,and back when charging.Li-Ion batteries used an intercalated lithium compound as 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 cell phones and laptops.Lithium-manganese oxide and Lithium-ion phosphate are also used as cathode material.
The basic internal structure
A battery i made up of an anode,cathode,seperator,electrolyte,and two current collectors(+ive and -ive).The anode and cathode stores the lithium.The electrolyte carries positively charged lithium ions from the anode to the cathode and vice versa through the seperators.The movement of the lithium-ions creates free electrons in the anode which creates a charge at the positive currrent collector.The electrical current then flows from the current collector through a device being powered (cell phones,computers,etc) to the negative current collector.The seperator 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.
A123’s high-performance Nanophosphate® lithium iron phosphate (LiFePO4) battery technology delivers highpower and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package. Our cells have 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:
- Datacenter UPS
- Telecom backup
- IT backup
- Autonomously guided vehicles (AGVs)
- Industrial robotics and material handling equipment
2.GOVERNMENT SOLUTIONS
- Military vehicles
- Military power grids
3.GRID SOLUTIONS:
- Frequency regulation
- Renewables integration
- Reserve capacity
- Transmission and distribution
4.TRANSPORTATION SOLUTIONS:
- Hybrid, plug-in hybrid and electric vehicleS.
- Commercial vehicleS
- Off-highway vehicles
- Passenger vehicles.
The performance of the cell is given in the datasheet that is referred 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 baterry for the different power ratings.
DESIGN CALCULATIONS:
All the taken parameters of this calculation are taken from the datasheet.
PROCEDURE FOR TEH 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 power rating.
Image: Battery cell
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.
Now for calculating our required battery pack based on teh cell manufacturer data provided.
The calculation for the battery pack is as follows:
1.volume of each cell
cylindrical cells, Vcc [m3]
`V_Cc = (piD_(bc)/4*L_(bc))`
From the datasheet we have
`D_(bc)= 26mm = 0.026m`
`L_(bc) = 65mm = 0.065m`
`therefore V_Cc = (pi*0.025^0.2 *0.065)/4 = 3.5xx10^-5 m^3`
The volume of each cell is 35mm^3.
2.Bttery cell energy
We have
`C_(bc) = 2.5Ah`
`U_(bc) = 3.3V`
Substituting teh values in the above equation we get,
`E_(bc) = 2.5xx3.3 = 8.25 Wh`
3.Battery Cell Energy density
- volumetric energy density
Substituting the value
`u_G= (E_(bc)/V_(cc) = 8.25/(3.5xx10^-5) = 235714.3 (Wh)/m^3`
- Gravimetric energy density
Substiituting the above calculated values in teh equation we get,
`u_G = E_(bc)/m_(bc) = 8.25/0.076=108.55wh/k_g`
4.Total number of battery pack
The total energy of the battery pack which is required is given as 18 kWh
`E_(bc) = 18000Wh`
5.Number of cells connected in series
It is calculated by dividing the nominal bapprty pack voltage `U_(bp)`[V] by the voltage of each battery cell `U b c` [V].The number of strings must be an integer.Therefore, the result of the calculation is rounded to the higher integer.
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
`N_(bc) = (U_(bp))/U_(bc) = (330)/3.3 = 100`
The total number of cells in series is 100.
6. Energy content of a string
The product between the nuber of the battery cells connected in series and the energy of a battery cell.
`E_(bc) = N_(cs)*E_(bc) = 100xx8.25 = 825Wh`
7.Number of strings of the battery pack
It is calculated by dividing the battery pack total energy Ebp [Wh] to the energy content of a string Ebs [Wh]. The number of strings must be an integer. Therefore, the result of the calculation is rounded to the higher integer.
Substitute the above value
`N_(sb)=E_(bp)/E_(bs) = 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].
`C_(bp)=N_(sb)*C_(cb) = 22xx2.5 = 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 [-].
`N_(cb) = N_(sb)*N_(Cs) = 22xx100 = 2200`
The total number of cells required for the design pack is 2200.
10.Mass of the battery pack
It is the product between the total number of cells Ncb [-] and the mass of each battery cell mbc [kg].
`m_(bp) = N_(cb)*m_(bc) = 2200xx0.076 = 162.2kg`
11. Volume of the battery pack
is the product between 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.
The total volume of the battery pack is 77000mm^3.
12. String peak current
It is the product between the peak C-rate of the battery cell C-ratebcp [h-1] and the battery cell capacity Cbc [Ah].
From teh datasheet the battery pack peak C-rate is 24 and the nominal capacity is 2.5 Ah.
24x2.5 = 60A
13. Battery pack peak current
It is the product between the string peak current Ispc [A] and the number of strings of the battery pack Nsb [-].
Substituting teh above values
60x22 = 1320 A.
14. Battery pack peak power
It is the product between battery pack peak current Ibpp [A] and the battery pack voltage Ubp [V].
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-ratebcc [h-1] and the battery cell capacity Cbc [Ah]
From the datasheet the battery pack contineous C-rate is 10 and teh nominal capacity is 2.5 Ah.
16. Battery pack continuous current
It is the product between the string continuous current Iscc [A] and the number of strings of the battery pack Nsb [-].
25 x 22 = 550 A.
17. Battery pack continuous power
It is the product between battery pack continuous current Ibpc [A] and the battery pack voltage Ubp [V].
= 550 x 330 = 181500 W
The battery pack contineous power will be 181.5 kW.
All the above parameters that are calculated are in teh table below:
| Manufacturer | A123-System |
| Number of cells in the string | 100 |
| String Energy | 825Wh |
| Number of Strings | 22 |
| Battery pack energy | 55Ah |
| Total no of cells | 2200 |
| Battery pack Mass | 167.2kg |
| Battery Pack Volume | 0.077 m^3 |
| Battery pack peak current | 1320A |
| Battery pack peak power | 435.6kW |
| Battery pack contineous current | 550A |
| Battery pack contineous power | 181.5kW |
Battery Dimensions:
1.Length of casing (L):
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 casing(B):
B = number of cells x (cell diameter + margin at both side of cells) +clearance at side.
B = 22 x (26 + 4) +60 = 720 mm
3. height of casing (H):
H = Height of cells + margin for wiring.
H = 65 + 35 = 100 mm
Assembly of the battery pack
Components of the battery pack
- 2200 cells
- Nickel strip
- PVC heat sink film
- Spot welder
- Connected
- Cell Holder
- Solder.
Step 1-
The first step is to start the 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 insulated gaskit.
Step 2-
Placing the cell in the holder matrix.place all teh cells in the holder and put the 2nd 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 case to the -ive 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 terminal structure of the battery pack is assembled and all the terminals are connected as per the connections.
Step 5-
Connecting the balancing wire pin to the battery management system.
Step 6-
After the battery management system,The next and the final step is to place the enclouser for the battery pack.
This is the final battery pack model created.
CONCLUSION:
The design calculation for the battery pack was created out and calculated and the table has been made for the same.The steps involved in the making battery pack have been listed out.