Project 1 Mechanical design of battery pack
PROJECT 1 : MECHANICAL DESIGN OF BATTERY PACK
Objective:
To create a battery pack drawing of 18 KWH using series and parallel connection of battery modules made up of individual Li-ion cells.
Li-ion cell used here is of model ANR26650M1-B from the manufacturer of lithiumwerks.
To discuss about the enclosure used for battery pack.
Li-ion cell(ANR26650MI-B):
eThis LI-ion cell is manufactured by Lithiumwerks. They deliver very high power due to their use of patented Nanophosphate battery technology.
Based on the lithium iron phosphate chemistry ,the cells are inharently safe over wide range of temperature and conditions.
For outstanding cycle life stable float reliability this cells are suitable for a wide variety of power,pulse and stand by applications.
Nanophosphate battery technology offers thermal stable chemistry fast charging consistent output low capacity loss over time.
It provides extremely safe conditions for while meeting most demanding customers requirements.Multiple layers of protections are employed on chemistry cell and sysytem level to achieve an energy storage solution with superior safety.
CELL SPECIFICATION:
Discharge Voltage Profile and cycle life with respect to different operating temperature from data sheet:
ASSUMPTIONS:
We are assuming the overall voltage required to operate the motor and other accessories of electric vehicles as 400 V.
So we have construct the battery pack of 18KWh for a 400 V input voltage motor.
Costruction of battery pack:
TTo construct a battery pack initially we have to estimate the voltage requirement of the overall load of vehicle.
We had assumed as 400V as mentioned above.
Then the battery modules should be created in such a way that the overall voltage is matched with the load requirement voltage.
The modules can be created in series/parellel connections of individual cells. Then every modules should be connected in series/parallel to construct final battery pack.
In Series Connection- Voltage added and Current remains same.
In parallel Connection - Voltage remains same and Current remains gets added.
Here in our case single cell voltage is 3.3V and capacity of 2.56Ah .Energy provided from single cell is give 8.25Wh.
So, for 18KWh battery pack we need approximately(18000/8.25 = 2182 cells).
We have to create a single module to produce 400V for our load requirement.
So to obtain 400V output cells are connected in series where voltage become added and current remains same.
To obtain 400V output, no of cells required is approximately(400/3.3=122 cells).
Thus single module gives as 400 V from 122 cells but energy of one module is not enough to run the vehicle as motor power rating is higher. The energy generated from single module is (122*8.25Wh=1006.5Wh).
So to construct a battery pack with18KWh with not exceeding 400V the modules are connected in parallel .In this parallel connections the currents are getting added and voltage remains same as 400V.As current increases the power of battery pack increase to run the motor load of high power rating.
Now we have to find how many similar modules should be connected in parallel to achieve 18KWh battery pack.
This can be calculated as (18KWh/1006.5Wh = 17.88), Which gives as nearly 18 similar modules should be connected in parallel to achieve 18KWh battery pack.
Thus the battery model is said to be 122S18P configuration(122 series and 18 parallel connections) which gives us (122*18=2196 cells).From this we have created overall capacity of battery pack as (2196*8.25= 18117Wh) which is slightly higher than 18KWh.
The overall capacity(Ah) of battery pack is given as(2.5Ah*18=45Ah).
Which means it provided 45A of current continuously for one hour to full discharge.
Thus 18 modules should be arrenged in a way as battery pack to fir in an EV chasis.
Enclosure Construction:
To find overall enclosure volume we have to find the volume of single module volume consist of 122 cells in series connection and multiplied with 18 parallel connected module volume.
SINGLE MODULE,
Let as assume 122 cells(single module) are divided into 2 modules of 61+61 cells.
The length of the module = No of cell in row*(Cell dia + Margin at both side of cells)+ clearance at side (2(cell dia + margin)) = 61*(26+4)+60 = 1890mm.
The breadth of the module = (cell dia + Margin of both side of cells+cleaarance) = 26+4+60=90mm.
HEIGHT of the module = Height of cell + connecting strip thickness,wires say 35mm= 65+35=100mm.
Thus the volume required for single module is 1.8*0.09*0.1=0.0162m^3.
Total number of modules assumed are 18*2 = 36 modules since single module is divided into two modules.
Therefore total volume required for battery pack of 122S18P configuration construction is 0.0162*36= 0.583m^3.
Thus from the above result it is assumed approximately 1m^3 of volume should be used for EV battery pack with additional ventilation spaces for pack.
We can assume that two modules are combined into one cabinet. So 18 cabinets should be connected to form final battery pack.
Materials and Process in battery pack construction:
Battery pack construction consist of three major levels of processings 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:
TThe 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 is a solid state welding technique which forms a bond by applying moderate static pressure and an ultrasonic oscillation of about 20KHz between two parts of metal. The solid state weld is formed through the high frequency motion between parts causing continuous shearing and plastic deformation while also remiving 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,
1.Aluminium Wire weldinng on copper.
2. Copper and aluminium welding.
3.Multilayered copper foil welding.
4.Ultrasonic seam welding.
Resistance Spot Welding:
One of the oldest welding methods is resistance spot welding .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 inthe affected area the interface and eventually reaches the materials 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 is a weldinng technique in which a laser beam provides a concentrated heat source that fuses the materials together. LBW is commonly performed using keyhole 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 evaporate from the surface forces in the opposite direction of the evaporation causes a local vapor pressure. The vapour pressure creates a hole which is known as the keyhole by depressing the molten metal.
Here below shown the welding of busbars to cylindrical battry cans.
Enclosures:
Cells in Series Connection:
Which you connect cells together in series it does not change the amps or capacity, it only raises the voltage of the pack.Hooking them up in series mesns that you connect the positive end of one cell 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 through thwy are one large cell. Also when you first connect them toeach other ,they must be at same level of charge.
CELL HOLDERS:
These holders are used for holding the individual cells in a better poasture 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 a 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 polimer sheet after the interconnections are made. Mostly connections withcell holders should be recommended for better safety concerns.
Battery Cabinet Arrangements:
Each modules are 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 evvery manufacturers.Below shown the battery pack construction process of TESLA.BMW and NISSAN leaf
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