Project 4
For a vehicle of gross weight 300 kg, the motor ratings are 1200Wh, 65V. The range is 225km and the maximum speed is 60kmph. Calculate the capacity of the battery pack required and the number of series-parallel connections. Give a stepwise discussion on how a battery pack is built.
Note: Choose any relevant lithium ion datasheet
Given
Vehicle gross weight = 300kg
Motor Ratings - 1200 Wh and Voltage - 65V
Range= 225km
Max speed = 60kmph.
The lithium-ion battery used for the analysis is LIR18650
The motor current is to be calculated.
Power= Volatge * Current
Current= Power/ Volatge
Current= 1200/ 65 = 18.5 A
Consider the Motor current loss from 1-10%
Considering the motor current as 6%
The new motor current after the losses = 18.5* 1.06 = 19.61 A
Calculate the max power required at max speed
Power= Voltage * Current
At Max speed 60kmph
Power = 65V * 19.61A
Power = 1274.65 W
Calculate the Power factor
Power factor = Total range/ Max speed
Power factor = 225/60
Power factor = 3.75
Now calculate the total power required based on the power factor
Power = 1274.65 * 3.75
Power = 4779.9 W = 4.8 KW
Now calculate the power based on battery pack efficiency
Let us consider the batter pack efficiency in this model as 90%
Required battery pack capacity = 4.8kw/.9 = 5.33KW
Now calculate battery pack efficiency based on motor efficiency
Consider the motor efficiency to be 87%
The required battery pack capacity = 5.33KW/0.87 = 6.13 KWh
Now upon knowing the required battery pack capacity, we need to calculate the cell arrangement in the battery pack.
Series connection cells
number of series cells = motor voltage /( nominal voltage of the selected Li-ion battery model)
The number of cells in series = 65/ 3.7 = 18 cells
Parallel connection cells
The number of cells in parallel = Motor current / (Max discharge current of the battery model)
The number of cells in parallel = 19.61/ 2.6 = 8 cells
The total number of cells in the battery pack = 18 * 8 = 144 cells
Now designing the battery pack for better safety and reliability
Now I need a battery pack with 6.13KWh capacity to full the power demand of the motor and to run 225Km without hindrance.
2) As per the above calculation we have to use 18cells in series and 7 cells are in parallel, do the parallel connection first and then connect all parallel connected cells in series this eases the process to find a faulty cell while maintaining. Before the cells have to be checked whether it is balanced. The presence of an unbalanced cell might lead to unusual behavior or the battery cannot be used to its full potential.
3) I am using Cylindrical shaped cells for my modeling because they can be easily positioned and any destructive cell can be easily identified. Also, it provides better automation processes and consistency. The electrodes in a cylindrical cell are wound tightly and encased in a metal casing, This minimizes electrode material from breaking up from the mechanical vibrations, thermal cycling from charging and discharging, and mechanical expansion of the current conductors inside from thermal cycling
4) Now all the cells are placed in the cell holder and the Nickel strip is spot welded between all the cells. Nickel has low resistance. Even soldering can be used but it generates a lot of heat within the battery pack during conduction. Sometimes nickel-coated steel is also used which has high resistance and heats up wasting energy. This reduces the range and capacity of the battery.
5) On top of this construction BMS is configured which is a very crucial controller to monitor and control the cell's functionality.
6) Now the battery pack voltage is checked with the help of a voltmeter.
7) Once the required voltage is obtained then the battery pack is enclosed with an epoxy sheet for safety.