Project 1 Mechanical design of battery pack

AIM:

To create a mechanical design of a battery pack for 18kWh and state the assumptions and the process.

DESCRIPTION:

Lithium Werks 26650 cells are best for Power.Safety.Life applications. They deliver very high power due to their use of patented Nanophosphate battery technology. Based on lithium iron phosphate chemistry (LiFePO4 ), the cells are inherently safe over a wide range of temperatures and conditions. Whether the application requires outstanding cycle life or stable float reliability, the Lithium Werks 26650 cells are suitable for a wide variety of power, pulse, or stand-by applications.

Nano phosphate battery technology offers thermal-stable chemistry, faster charging, consistent output, low capacity loss over time, and superior total cost of ownership (TCO). It provides the foundation for safe systems while meeting the most demanding customer requirements. Multiple layers of protection are employed at the chemistry, cell and system level to achieve an energy storage solution with superior safety and abuse tolerance compared to metal oxide lithium-ion chemistries.

DIMENSIONS:

CELL SPECIFICATIONS:

DISCHARGE VOLTAGE PROFILE AND CYCLE LIFE W.R.T TEMPERATURE

This shows with increase in usage of the battery the cycle life decreases and capacity of the battery decreases and voltage gradually decreases with decrease in discharge.

ASSUMPTIONS:

Here we are assuming the overall voltage required to operate the motor and other accessories of electric vehicle as 400V.Therefore, we need to consider a battery pack with 18kWh with 400V input voltage.

CONSTRUCTION OF BATTERY PACK:

To construct a battery pack the battery modules should be created in such a way that over all voltage is matched with the required voltage.

The modules can be created in series or parallel connections with individual cells. Then every module can be connected in series or parallel to complete the battery pack construction.

In series : Voltage adds and Current is constant

In parallel : Current adds and Voltage is constant

Here for a single cell, voltage : 3.3V

Capacity: 2.56Ah

Energy: 8.35 Wh

STEP-1: For 18kWh battery pack we need 18000/8.25 = 2182 cells

STEP-2: We need to create a single module which provides 400V of voltage.

So, no of cells to obtain 400V = 400/3.3 = 122 cells

So, 122 cells need to be connected in series which gives 400V as output.

STEP-3:But energy of one module is not enough to run the vehicle as motor power rating is more.

So energy from one module = 122*8.25 = 1006.5Wh

STEP-4:So as voltage is already obtained by adding it in series now to increase the current to run the motor we need the number of parallel modules that need to be connected in parallel to increase the current output.

So for 18kWh battery pack = 18000/1006.5 = 17.88 modules approx. 18 are required to obtain the current required.

Thus, the battery pack model with 122S18P configuration is to be designed with 122*18=2196 cells of total.So, for this we need to find the enclosure volume required to make up the battery pack.

ENCLOSURE CONSTRUCTION:

To find the overall total volume required to make the battery pack we need to first know the volume taken by one module of battery pack (122 cells in series multiplied with 18 in parallel).

So, for single module,

Length = No of cells in row*(cell dia+Margin at both sides of cell) + clearance at side (2(cell dia+margin))

=122*(26+5)+2(26+5)

=122*31+62

=3844mm

Breadth of module = Cell dia + Margin of both side of cells + clearance

=26+5+62

=93mm

Height of the module= Height of cell + connecting strip/wires thickness

=65+35

=100mm

Thus the volume required for one module = 3.844*0.093*0.1 = 0.0357 m^3

So total volume for 18 parallel module connection = 18*0.0357 = 0.6434m^3

Thus from above total volume obtained we assume that approximately 1m^3 of volume can be used for the space with battery pack and space for ventilation.

MATERIAL AND PROCEDURES IN BATTERY PACK CONSTRUCTION:

It consists of 3 types namely cell level, module level and pack level.

The electrodes are made up from naturally mined materials and are combined with electrolyte to form various cell modules like cylindrical, prismatic and pouch type cells.

Then the case sealing is carried out and according to the requirements of voltage and current the modules are made by inter connecting cell to cell. These modules are combined to form a battery pack.

JOINING TECHNOLOGIES OF CELL:

To make the inter connections between cells/modules various welding techniques are used. Some of them are

-Ultrasonic metal welding

-Resistance welding

-Laser beam welding

-Wire bonding

-Mechanical joining

ULTRA SONIC METAL WELDING:

It is a solid state welding technique which forms a bond by applying moderate static pressure and an ultrasonic oscillation of about 20kHz between the two parts of the 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 oxides layers.

Some of the materials welded by this method are as follows

Aluminium wire welding on copper, copper and aluminium welding,, multilayer copper foil.

RESISTANCE SPOT WELDING:

This is one of the oldest methods of spot welding. In this technique, 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.

LASER BEAM WELDING:

In this technique the laser beam provides a concentrated heat source that fuses the materials together. LBW is commonly performed using key hole mode welding in which the extreme power density of 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 vapour pressure. The vapour pressure creates a hole which is known as key hole by depressing the molten metal.

Below image shows how the weld with laser looks like

ENCLOSURES:

The cells are connected in series first i.e +ve terminal to -ve terminal and -ve terminal to +ve terminal for 122S as below

Then they are connected in parallel so that they act as one large cell as +ve connected to +ve and -ve connected to -ve charge side as below

Then they are assembled in cell holders to avoid mis-assembly of cell from module as follows

Then after arrangement they are integrated into the module case and a single maximum positive and maximum negative terminal are made under for each module as follows

Then according to the requirements for cylindrical, prismatic and pouch battery pack they are manufactured and out of modules the battery pack is made with the spaces left for ventilation and for heat regulation. This differs from one manufacture to another.

Then the battery pack is assembled in the chasis below the floor as follows

CONCLUSION:

Thus the mechanical design for 18kWh battery pack for 122S18P has been designed successfully.

REFERENCES:

1. https://www.researchgate.net/publication/323084845_Mechanical_Design_and_Packaging_of_Battery_Packs_for_Electric_Vehicles
2. https://www.sciencedirect.com/science/article/abs/pii/S1364032116002483