Project 1 Mechanical design of battery pack
AIM : Mechanical design of 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 is gradually increasing, there is considerable growth in the production of lithium-ion batteries. As it is the electrical source for the EV.
A battery pack is a set of many numbers of identical cells. They may be configure in a series, parallel or a mixture of both to deliver the desired voltage, capacity or power density.
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.
Figure : Lithium ion
The high-energy density and large number of discharge cycles provided by lithium-ion cells are the most important factors, which make them indispensable in mobile phones and electric vehicles alike. However, there are many other ways in which they are superior to traditional battery chemistries. As well as the high-energy density of the cells, they are able to discharge at a higher power and can then be re-charged much more quickly.In applications where charging power or time may be limited, such as solar PV systems or stop-start cars, it is not harmful to operate lithium-ion cells continuously at partial state of charge. This gives them greater operating flexibility than lead acid cells.
The introduction of lithium-ion cells was driven by the need for a lightweight rechargeable cell to power the rapidly growing market for portable electronic equipment in the 1990’s. Starting with cameras and mobile phones, the technology has become the power source of choice for everything from cordless power tools to large scale energy storage and automotive applications.
The interaction between lithium-ion cells and the environment is very mild. There are no gas emissions from charging cells and heat loss, due to charging inefficiency, is very low. This means that it is possible to use lithium-ion cells in closed cabinets, completely isolated from their surroundings. At end of life the cells should be recycled but they contain no controlled toxic materials such as cadmium, mercury and lead.
Nanaophosphate High Power Lithium Ion 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.
Figure : Nanophosphate lithium ion cell (ANR26650M1B)
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.
Battery Specification
Dimension
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 (Eachone having 2V).
Figure : Battery cells string
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.
Figure : Battery cell strings in parallel
Now for calculating our required battery pack based on the cell manufacture data provided.
The calculation for the battery pack is as follows.
- Volume of each cells
Cylindrical volume, Vc (m^3)
where : Dbc [m] – battery cell diameter
Lbc [m] – battery cell length
From the datasheet we have Dbc = 26 mm = 0.026 m
Lbc = 65 mm = 0.065 m
Vc = {Pi * ( 0.026)^2}/4 * 0.065
= 3.45 * 10^-5 ( m^3)
Volume of each cell is 34.5 (mm^3)
- Battery Cell Energy
where : Cbc[Ah] – battery cell capacity
Ubc [V] – battery cell voltage
From datasheet, Cbc = 2.5 Ah & Ubc = 3.3 V
Ebc = Cbc * Ubc = 2.5 *3.3 = 8.25 Wh
- Batttery cell energy density
Volumetric energy density
Substituting the value
uV = {8.25 /(3.45 * 10^-5)}
uV = 2.391304 *10^5 = 239130.4 Wh/m^3
uV = 239.13 kWh/m^3
Gravimetric energy density
Where mbc = Battery cell mass
Substituting the value,
uG = { 8.25/(0.076)}
uG = 108.55 Wh
- Total number of the battery pack
The total energy of the battery pack which is required is given as 18 kWh
Ebp = 18000 Wh
- Number of cells connected in series
It is calculated by dividing the nominal battery pack voltage Ubp [V] by the voltage of each battery cell Ubc [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 300V and the nominal voltage of the battery as per the datasheet is given as 3.3V.
Ncs = Ubp/Ubc = 300 / 3.3 V = 90.90 ~= 91
The total no. of cell in series is 91.
- Energy content in a string
The product between the number of battery cells connected in series and the energy of a battery cell
We have Ncs = 91 cells & Ebc = 8.25 Wh
Ebs = Ncs * Ebc = 91 * 8.25 ~= 750 Wh
- Number of strings in a battery pack
It is calculated by dividing the battery pack's total energy Ebp [Wh] by 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.
Ebp = 18000 Wh and Ebs = 750 Wh
Nsb = Ebp / Ebs = 18000 / 750 = 24
All the 24 strings will be connected in parallel.
- 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 = 24 * 2.5 Ah = 60 Ah
- Total number of cell in a battery pack
It is calculated as the product between the number of strings Nsb [-] and the number of cells in a string Ncs [-].
Ncb = 24 * 91 = 2184 cells
So total no. of cell required for battery pack is 2184.
- Mass of a batttery pack
It is the product of the total number of cells Ncb [-] and the mass of each battery cell mbc [kg].
- Volume of Battery pack
It is the product of the total number of cells Ncb [-] and the volume 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 or systems of the battery.
- String Pack 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].
As per the datasheet, C-rate of cell peak current is 2.4 and capacity is 2.5 Ah
Ispc = 2.4 * 2.5 = 6.0 A
- 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 [-].
Substitute these value in above eq.
Ibpp = 6.0 * 24 = 144 A
- Peak Power of Battery Pack
It is the product between battery pack peak current Ibpp [A] and the battery pack voltage Ubp [V].
Subtitute these value in the above e.
Pbpp = 144 * 300 = 43200 W = 43.2 kW
- 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].
As per the datasheet, Max. C-rate is 10 and battery cell capacity 2.5 Ah
Iscc = 10 * 2.5 = 25 A
- Battery Pack Continuous Pack Current
It is the product between the string continuous current Iscc [A] and the number of strings of the battery pack Nsb [-].
Ibpc = 25 * 24 = 600 A
- Battery Pack continuous Power
It is the product between battery pack continuous current Ibpc [A] and the battery pack voltage Ubp [V].
Pbpc = 600 * 300 = 180000 W = 180kW
The continuous power of batttery pack will be 180 kW.
All the above parameters that are calculated are in the table below.
| Manufacture | A123 - Systems |
| Consider voltage (Assume) | 300 V |
| Number of cells in the string | 91 |
| String energy | 750 Wh |
| Number of string | 24 |
| Battery pack energy | 60 Ah |
| Total number of cells | 2184 |
| Battery pack mass | 166 kg |
| Battery pack volume | 0.075 m^3 |
| Battery pack peak current | 144 A |
| Battery pack peek power | 43.2 kW |
| Battery pack continuous current | 600 A |
| Battery pack continuous power | 180 kW |
Battery dimensions
1. Length of casing (L)
Length = number of cells x (cell diameter + margin at both side of cells ) + clearance at side
L = 91 x (26 + 4) + 60 = 2790 mm
2. Breadth of the casing (B)
Breadth = number of string x (cell diameter + margin at both side of cell) + clearance at side
B = 24 x (26 +4) + 60 = 780 mm
3. Height of casing (H)-
Height = Height of cell + margin for wiring
H = 65+35 = 100 mm
ASSEMBLY OF THE BATTERY PACK
Component needed for battery pack
- 2184 cells
- Nickel strip
- PVC heat sink film
- Spot welder
- Making connection
- Cell holder
- 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
This image shows the cell assembly
Step 2
Placing the cells in the holder matrix. Place all the cells in the holder and put the second holder in the place.
This image shows the cell placement with holder
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 (positive terminal with positive and negative terminal with negative).
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 step by step and a table has been made for the same data.
REFERENCES :
- https://a123batteries.com/anr26650m1-b-lithiumwerks-nanophosphate-3-3v-2-5ah-lithium-iron-phosphate-battery/
- https://x-engineer.org/ev-design-battery-calculation/
- https://en.wikipedia.org/wiki/Lithium-ion_battery#Uses
- https://www.scribd.com/document/582801201/BatteryProductionBrochures-LithiumIonCellProduction-Edition3-online-eng