Project 1 Mechanical design of battery pack
Battery pack capacity: 18 kWh
Cell: ANR26650M1-B
Prepare a detailed battery pack drawing along with its enclosure. State your assumptions.
Aim: calculating of battery pack dimensions using given cells configuration & providing required assumptions.
Requirements of battery pack energy capacity is 18kWh.
Provided cell details as follows as per datasheet.
Assumptions:
To operate the motor and the accessories of electric vehicle the battery should have 400V. so, we need to construct the battery pack which can produce 18kWh energy & 400V voltage. To get those values we need to create modules by connecting the individual cells in series/parallel than these modules need to arrange in series/parallel.
In series connections current remains same & voltage added.
In parallel connections voltage remains same & current is added.
As per data sheet of given cell nominal voltage 3.3V & capacity 2.56Ah whose energy is 8.25Wh.
Total number of cells required for 18kWh = 18000/8.25 =2181.8182 ≈ 2182 (rounded)
We will get desired voltage adding the cells by series connection.
Number of series cells required to produce 400 V
Number of cells in series = voltage required / nominal voltage of one cell.
Each cell produces 3.3V voltage so, to get 400V no. of cells required = 400/3.3 =121.1212 ≈ 122.
One module created by connecting the 122 cells in Series. This module alone can produce 400V but the energy generated by this module = 122*8.25 = 1006.5kWh is not enough.
Without changing the voltage, we required energy of 18kWh battery pack. So, we need to connect the modules by parallel connection to get desired results. Because, by parallel connections voltage wont change and capacity added. So, the no. of parallel connections required between modules = total energy of battery pack / energy produced by each module
= 18000/1006.5 =17.884 ≈ 18P
So, finally we required 122 series & 18parallel connections to obtain desired rating battery pack.
In parallel connection capacity of battery increases.
Capacity = parallel cell row*individual cell capacity
Capacity = 18 * 2.5
Capacity = 45 Ah
That means this battery pack can deliver 45A current per hour. Which is considered as C rate.
If we considered powertrain power consumption 100wh/km than range will become 180km of the battery pack.
Range = Capacity / energy consumed per kilometre =18000/100 = 180km
Calculation of Battery pack enclosure:
According to our calculation of battery pack, we required 122S18P to generate 18kWh.
According to data sheet of ANR26650M1-B the dimensions of each cell as shown below:
To ease of battery pack, we will consider all series connections in one module which will be connected in 2 lines to compact the battery pack. 61+61 is in one module. This kind of 18 modules is connected in parallel.
The length of the module=No.of cell in row*(Cell dia + Margin at both side of cells) + clearance at side
Margin is considered as 4mm, clearance is considered as nearly 60mm (each side 30mm).
The length of the module = 61*(26+4) + 60 = 1890mm
The breadth of the module= (2*(cell dia +Margin of both side of cells) +clearance)
= 2*(26+4) + 60 = 120mm
Connecting strips and wires clearance considered as 40mm on top & bottom sides of module.
Height of the module= Height of cell + connecting strip thickness
= 65+40 =105mm
Each module dimensions are as follows 1.89m X 0.12m X 0.105m
For battery pack we required 18 modules.
So, total battery pack size 18*0.12m X 1.89m X 0.105m.
Approx. 2.2m length X 2m width X 105mm height. So, finally with additional thermal configuration the volume of space occupied by battery pack might be below 1m^3.
Materials and process in battery pack construction:
Battery pack constructions consist of three major levels of processing’s namely cell level, module level, pack level constructions.
The electrodes are combined with electrolyte to various cell models as cylindrical, prismatic, pouch type cells.
After case sealing cells are combined to form module level construction. Finally, these modules are combined to form pack level constructions.
Joining technologies of cells:
The interconnections between cells/modules are practised by various welding techniques. Some of them are,
- Ultrasonic metal welding.
- Resistance welding.
- Laser beam welding.
- Wire bonding.
- Mechanical Joining.
Ultrasonic metal welding
Ultrasonic metal welding (UMW), is a solid-state welding technique which forms a bond by applying moderate static pressure and an ultrasonic oscillation of about 20 kHz between two parts of metal. The solid-state weld is formed through the high frequency motion between the parts causing continuous shearing and plastic deformation, while also removing any oxide layers or contaminants. Most metals can be ultrasonically welded and the method is excellent for welding together thin foils, as well as thicker sheets with spot or seam welds and also wires.
The four types of materials welded in this process can be seen as,
a) Aluminium wire welding on copper.
b) Copper and aluminium welding.
c)Multi-layered copper foil welding.
d) Ultrasonic seam welding.
Resistance spot welding
One of the oldest welding methods is resistance spot welding (RSW). RSW is a technique where two or more metal sheets are welded together through the application of an electrical current and pressure between two electrodes. The electrical resistance of the material at the interface results in the generation of heat and melts the materials together without the use of filler material or gases.
The temperature increases in the affected area, the interface, and eventually reaches the material’s melting point. A weld nugget forms in the affected area after cooling. This is a small volume of the material which has melted and then fused together.
Laser beam welding:
Laser beam welding (LBW) is a welding technique in which a laser beam provides a concentrated heat source that fuses the materials together. LBW is commonly performed using keyhole mode welding in which the extreme power density of the laser beam causes the metal to melt and evaporate. As the metal atoms evaporate from the surface, forces in the opposite direction of the evaporation causes a local vapor pressure. The vapor pressure creates a hole which is known as the keyhole by depressing the molten metal.
Here below shown the welding of busbars to cylindrical battery cans.
Enclosures:
Cells in series connection
When you connect cells together in series, it doesn’t change the amps or the capacity, it only raises the voltage of the pack. Hooking them up in series means that you connect the positive end of one cell (or P-group) to the negative end of the other as shown below.
Cells in Parallel connections
If you take several cells and connect their same-type electrodes in parallel, they all will act as though they are one large cell. Also, when you first connect them to each other, they must be at the same level of charge.
Cell holders
These holders are used for holding the individual cells in a better posture without mis-assemble. The connections can be made accordingly to make series connections and parallel connections to form the required module dimensions.
After installing the cells in an individual module, the entire maximum positive terminal and maximum negative terminal are made under single pair of terminals as shown below.
Here shown the cells are arranged and welded without using cell holders as
They are just covered by polymer sheet after the interconnections are made. Mostly connections with cell holders should be recommended for better safety concerns.
Battery Cabinet arrangements
Each module is interconnected in series or parallel according to the requirement and packed as a single large cabinet pack as shown below.
Battery pack arrangement:
Finally, the overall battery pack after the arrangement of all cabinet connections with enclosure looks like this as shown below,
Vents holes are present with enclosure for cooling/heating the battery pack temperature according to its environment conditions.
Different cell types are used by different manufacturers of EV such as Cylindrical, prismatic, Pouch types. The pack constructions are different for every manufacturer.
Below shown the battery pack construction process of Tesla, BMW and Nissan Leaf.
Note: The data sheet of cell attched for more information about cell.