Mechanical design of battery pack
Aim: For a battery capacity of 18KWh, prepare a detailed battery pack drawing using ANR26650M1-B cells.
Procedure:
The given capacity for the required battery is 18KWh. Assuming 18KWh is the capacity including the margin of 15-20%, the actual battery capacity required is around 15KWh. In order to design for safe conditions we can have a capacity between 15-18KWh.
The cell to be used is ANR26650M1-B. The specifications of the cell are:
For making a battery from cells for any given capacity, we need the cell voltage and capacity.
For the given cell :
voltage = 3.3V capacity = 2.5Ah
To get the energy capacity of the cell we mulpity voltage and ampere hour rating.
Voltage * Ampere = Watts;
Voltage * Ampere * Hours = Watthours;
We need 18000Watthours energy capacity.
Assuming the designed voltage for the system is 200Volts
The ampere hour capacity will be,
200 * Ampere hour = 18000;
Ampere hour = 90;
Designing for 200V and 90Ah,
One cell has 3.3V, hence to convert into 200 we need,
200/3.3 = 60.60 =~ 60 cells in series connection.
Since voltage increases in the series connection and current remains constatnt.
Similarly, for 90Ah,
one cell has 2.5Ah and to get 90Ah,
90/2.5 = 36 cells in parallel.
Since curent increases in parallel connection and voltage remains same.
Hence, for 200V and 90Ah from a (3.3V & 2.5Ah) cell, we need
60S36P connection. i.e. 60*36=2160cells.
60*3.3*90*2.5= 17820Wh =~ 17.8 KWh (within margin 15-18kwh)
Cell Dimensions:
Diameter = 26mm
Height = 65mm
Weight = 76gram
Cell holder dimensions:
Height = 10mm
length = 29mm
width = 29 mm
weight = 5gram
Assumptions:
1. The given 18KWh energy requirement is including a margin of 15-20%.
2. The vehicle in which this power is being used is an electric Autorickshaw.
3. The battery pack is air cooled.
Assembly:
Arrangement of the cells in 60S36P together will not be the best arrangement.
it will take 29*60 x 29*36 mm (1740x1044mm)of space. And all the cells in a single matrix will lead to very poor thermal management. The ventilators will not be able to cool the inner cells as the air will not reach till the inside cells.
Also according to the 18KWh capacity, the application might be in an Autorickshaw. Hence 1.74x1.04meter area is taken by the battery only.
A solution to the thermal and space challenge will be creating a module of 10S6P cells. This will create 33V 15Ah module. Arranging such 10S6P modules in 6S6P manner will create 60S36P array giving 198V 90Ah.
Also, only the proper connection in series and parallel is what matters and not the position of this modules.
step1: assemble cell holders in 10S6P manner
step2: assemble cells in all the cell holders
step3: add cell holders on the upper side as well
step4: add nickel strips for series and parallel connection
One module is complete.
Now such 36 modules are required for 6 series and 6 parallel connection. The position of the module does not matter but the connection should be made in 6S6P manner only.
The below image shows arrangement of 36 such modules which will be able to ventilate all the cells and their position can be altered as per the space requirement.
step4: arrange 36 modules as one battery pack.
The battery pack in the above image is fitted below the passanger in an electric rickshaw. The part of the battery where the modules are one above the other is at the rear side of the vehile. This position ensures good traction to the rear wheels. As the battery is in lowest position possible the CG of the vehicle is also well maintained.
Conclusion:
A 17.82KWh battery pack is generated by arranging ANR26650M1-B cells in 60S36P manner.
The total weight of this pack is, [(cell weight x2160)+ (holder weight x2160*2)=[(76*2160)+(5*2160*2)]=(164160+21600)=185760grams = 185.76 Kg.