Mechanical design of battery pack

AIM : Making and analsing A123 cell having battery pack capacity of 18 KWh

DESCRIPTION:

Cell : ANR26650M1-B 

Considering the Specifications of the given A123 cell, different combination of battery pack are possible.

But before moving forward we need to re-evaluate the required parameters.

Required Capacity - 18 KWh battery pack.

But we know, due to various different losses, such as Leakage, shelf life, Temperture effects, ageing Etc needs to be considered. 

Taking Efficiency = 90%

Required battery pack capacity = 18.000/0.9 = 20.000 KWh 

A standard car running entirely on the battery pack requries voltage range from 300V-800V. 

Therefore let us assume that our battery pack has 330V Voltage. 

hence, the Required Capacity comes out to be 20.000/330 = 60 Ah

We have A123 cell of 3.3V/2.5Ah rating

How to decide number of cells in series and in parallel ?

For 330 V pack

Number of cells in series = 330/3.3 = 100 cells connected in series. 

Also, for 60 Ah capacity

Number of cells in parallel = 60/2.5 = 24 modules to be connected in parallel. 

Total Number of cells required = 100*24 = 2400 Cells

Each Module

let Every Module have 4 Series connected cells Arrangement with 24 cells in parallel. (13.2V/60Ah) 

  When connected in 4S24P set gives ->  

The DOF (Degree of freedom) of cell is restricted with help of upper and lower housing and cover respectively.  Appropriate Tolerances are added to have a proper fit and allow batteries to swell incase of heating. 

This entire Assemly is enclosed withing a another enclosure. The positive and negative terminal are accessible from outside to have a further electricial connection within the modules. 

EXPLODED VIEW

When such 25 modules are connected further in series, the required EV voltage is optained. 

  Complete Battery Pack (Top View) Having 18000 KWh (330V/60Ah) Top View (Without Module Enclosure) 

The Dimensions of the Each Module are = 196mm*740mm. 

The Dimensions of the entire Battery pack are = (196mm*5)*(720mm*5) + Gap between each module side ways =  1.1m*3.75m 

This can be accomodated in an Electrical car.

SCOPE :

• The Dimensions of the battery pack could further be reduced by having a better arrangment of cells in the module.
• Cells of higher cell voltage can be used to further reduce the weight of the EV.

FURTHER ANALYSIS :

Now if we Consider a Car who's weight is 900Kg having a range of 100km with top speed of 100kmph. Would this be possible with our battery pack ?

let us check.

100kmph = 27.7m/sec 

Kinetic energy = 1/2*m*v^2 = (900*27.7^2)/2 = 347050 J

If we want the car to accerlate from 0 to 100kmph in 15 sec,

then Power required = 328050/15 = 23147 W

Having a 330V battery pack, the current required to meet this power requirement would be (Power/Volatage)

23147/330  = 70A. This Current would be used for 15sec, which can easily be provided from a 330V/60Ah battery pack. But it will use only 1-5 % of the battery every time the drive performs such accerelation. 

For 100km range at 100kmph, Battery must supply current for 100km/100kmph = 1 hour

based on capacity, 60Ah/1hr= 60A, it will draw during that period at 330V; I.e 330*60 = 19800W to drive at 100kmph

Looking at this it can be said that, 100% of the battery can be used to cover the required range. (Theoretically, when driver drives at constant speed without much acceleration and considering minimum losses.)