Challenge

Project 1 Mechanical design of battery pack

DDarsi Shanthi Priya26 Nov 2021

ANR26650M1-B cell is used for Designing a battery pack:

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 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.

Dischage characteristics of Lithium ion phosphate cell:

Cell configuration:

Battery Pack capacity =18kWh

Assuming complete battery pack voltage = 300v

Nominal voltage = 3.3v

No. of cells on series = 300/3.3 = 91cells

Current = 18000/300 = 60Ah

Cell capacity = 2.5Ah

No. of cells in parallel = 60/2.5 = 24 cells

so total no. of cells in battery is given by 91*24 = 2184cells

The battery pack combination = 91S24P

In series connection the voltage increases and current remains same = 91*3.3 = 300.3 V

In parallel connection the current increases and the voltage remains the same = 24*2.5 = 60A

Mass of the battery pack = total no. of cells in a battery pack * mass of the single cell

= 2184*0.076

=166kg

Total volume of battery in terms of area = total no. of cells in battery pack * volume of a single cell

= 2184*0.0345

= 75.34m^3

Now considering single cell dimensions:

Diameter of a cell = 26mm = 0.026m

Length of the cell = 65mm=0.065m

mass = 76g=0.076kg

single cell voltage =3.3V

Capacity = 2.5Ah = 0.5C rate

Single cell volume = 3.14*0.026^2*0.065

=0.03451m^3

The volume of each cell is used to find the total volume of battery pack = cell capacity*cell voltage

=2.5*3.3 = 8.25Wh

Energy density of single cell is used to calculate the total energy density of battery pack:

Volumetric energy (Wh/m^3) = single cell energy /volume of each cell

= 8.25/0.03451

= 239.0 Wh/m^3

Gravimetric energy (Wh/kg)=single cell energy / mass of each cell

= 8.25/0.076

=108.55 Wh/kh

Cell peak current(A) = Max charge current (0.5C)*single cell capacity

= 20*2.5

= 50A

Battery pack peak Current(A) = cell peak current * no of cells connected in parallel

= 50*24 = 1200A

Battery pack peak power(W) = battery pack peak current *voltage if cells connected in series

= 1200*91=109kW

Battery holders and spacers:

To make the battery pack, you have to connect the 18650 cells together by means of Nickel strips or thick wire. Generally, Nickel strips are widely used for this. In general two types of nickel, strips are available in the market: nickel-plated steel strips and pure nickel strips.

Now by considering nickel strips and foil based component and battery with length = 70mm

width = 26+4 = 150mm

volume of the module = L*b*h

= 70*150*150 = 1575000mm^3

Volume of the battery pack = 1575000*75.34=118660500mm^3

weight of the battery pack = weight of the single cell *no of cells = 0.076*2184 = 166kg

As we know

Power = voltage*current

3.3*2.2 =8.25kW

To get 18kWh , we are assuming 91S24P combination = 2184 cells

Series connection = 91*3.3 = 300.3V

Parallel connection = 24*2.5 = 60A

Now voltage *current = 300*60 = 18kWh battery capacity

Design for battery pack of 91S24P combination along with battery holders and for connecting them nickel strip is used and when they are connected to cell spot welding should be done so that there wont be any damage.

Few designs of battery pack

Cell Holder