Project 1 Mechanical design of battery pack

1.

Lithium ion batteries are widely used in Electric vehicles.A battery cell consists of five major components.these are:electrodes (comprising of anode and cathode),separators, terminals,electrolyte and enclosure.

Safety and reliability are two major considerations in mechanical design of a battery pack.The design has to be based on the following technical criteria:

A.Mechanical vibration must be under control

B.High impact forces has to be limited

C.Better thermal runway for cooling

D.Ensuring serviceability and recyclability

For designing of 18kwh battery pack a base model of ANR26650M1-B Lithium ion cell is chosen.This is made of Lithium ion phosphate chemistry(LiFePO4) and is suitable for varying range of temperature conditions.Some superior features of the cell include

a)stable chemistry

b)fast charging

c)consistant output

d)very good cycle life

e)cost advantage

Table XIII shows the Technical Specifications of the Cell.

Calculations for the battery pack are given below.The calculations are based on the following assumptions:

I.Vehicle range is upto 250 Km

II.Nominal voltage for the series connection is 145.2 V.

For a single cell the following parameters will be applicable.

Diameter=25.98/26 mm i.e.0.026m

Length=65 mm i.e. 0.065m

Mass=76 g i.e. 0.076Kg

Cell Voltage=3.3V

Capacity=6 Ah

Maximum Continuous Charging Current=10A

Maximum Pulse Charging Current=20A

Volume of a cell=Pi X (D/2)square X L =3.14X0.013X0.013X0.065=0.0000345 m cube.The volume of a single cell is used to find out the total volume of the battery pack.

Single cell Energy(Ah)=Cell capacity(Ah)XCell Voltage(V) =2.6X3.3 i.e. 8.58 Ah

Single cell energy density=8.58/0.0000345=248695.6 Wh/ m cube.

Gravimetric Energy(Wh/kg)=8.58/0.076=112.89Wh/kg

Per Km average energy=18120.96/250=72.483 Wh/km

Battery pack total energy=Average energyXkm=18120.96Wh

Number of cells connected in series=Nominal voltage of series connection/Nominal voltage of a single cell=145.2/3.3=44

Hence 44 cells are required to be connected in series to supply the proper voltage.

Number of cells connected in series X single cell energy = 44 X 8.56 =377.52 Wh

Number of cells connected in parallel

=Battery pack total energy/Energy for battery pack

=18120.96/377.52=48

Battery pack total energy=Number of cells connected in parallel X Energy for battery pack=48X 377.52=18120.96Wh

Battery pack capacity=Number of cells connected in parallel X Single cell capacity(Ah)=48X2.6 =124.8Ah

The value of battery pack capacity is used for calculation of total power requirements for the vehicle.

Total number of cells for the battery pack=Number of cells connected in parallel X Number of cells connected in series

This works out to be 48X 44 i.e.2112

So to make the complete battery pack we need to use 2112 cells.

Mass of battery pack=2112X0.076=160.512 kg

This is the total weight of all cells connected in series and parallel.

Total volume of the battery pack=2112 X 0.0000345=0.07285 m cube.

Cell peak current=Maximum Pulse Rate Charge Current X Single Cell capacity=20 X 2.6= 52A

Battery Pack Peak Current=Cell Pack Peak CurrentX Number of cells connected in parallel=52X48=2498 A

It is used for the operation of the battery pack.

Battery pack peak power=battery pack peak CurrentXVoltage of series connection=2498X145.2=362419.2 W or 362.42 kW

Cell Continuous Current=Maximum Continuous Charging Current X Single Cell Capacity=10X 2.6=26 A

Battery pack continuous current=Cell continuous currentX Number of cells connected in parallel=26X48=1248A

Battery pack continuous power=Battery pack continuous current X Voltage of series connection =1248X145.21.e.18120.96 W

Assembly of the full battery pack with 44S/48P.The arrangement of pack is as follows.

Package:44S/48P

Capacity:8Ah

Voltage:145.2 V

Weight:160.512(Only cells)

Number of cells:2112

Power:18120.96 Wh

The cells are placed in a cell holder to minimise vibration.The cell holder is made of polypropylene or Nylon bodies rated for 80 degree Celcius to 100 degree Celsius.Pack cover is made of Aluminium to keep it light weight.The casing/cover thickness is 0.5 mm.Lower floor case is also made of Aluminium.Positive and negative clips are used to connect the battery end cap.

The whole battery pack is subdivided into 66 packs in which each block had 32 cells and this configuration is prepared to set up 44S/48Pcell configuration.

Cylindrical cells have improved the thermal management efficiency because of compact shape,but have low packaging efficiency.

Reference:Mechanical Design and packaging of battery packs for Electric Vehicles by Shashank Arora and Ajay Kapur.