Project 1
1. Design a battery pack for a car roughly 150 Kw with 120 V. Use 3500 mAh 3.6V nominal NMC chemistry cell.
a. Design the battery pack configuration.
b. Draw the BMS topology for this battery pack.
Answer:
AIM:
To design a battery pack for a car roughly 150 kW with 120 V using 3500 mAh, 3.6V nominal NMC chemistry cell & to draw the BMS topology for the same.
a) DESIGNING OF A BATTERY PACK:
For designing a battery pack configuration we need to find out how many cells to be connected in series and parallel in order to meet the requirements.
The Battery pack specification is given as:
Battery Capacity = 150 kW
Battery Maximum/Rated Voltage = 120 V
Nominal Cell Voltage = 3.6 V
Nominal Cell Capacity = 3500 mAh = 3.5 Ah
Calculation for Series Cells:
We know that,
Number of Cells in Series = Battery Maximum Voltage / Nominal Cell Voltage
= 120 / 3.6
Number of Cells in Series = 33.33
Now in order to round off to a higher or lower value we will check the conditions.
Case I: Considering a lower value i.e. 33 Cells
Total Voltage = Nominal Cell Voltage * Number of Cells in Series
= 3.6 * 33
Total Voltage = 118.8 V
Case II: Considering a higher value i.e. 34 Cells
Total Voltage = Nominal Cell Voltage * Number of Cells in Series
= 3.6 * 34
Total Voltage = 122.4 V
So, as seen from both the cases as our required voltage is 120 V & in Case I it is satisfying the required voltage while in Case II it exceeds the required voltage.
Therefore we will go for Case I where the total voltage is 118.8 V which is less than our required voltage i.e. 120 V.
Hence we will require 33 Cells to be connected in series.
Calculation for Parallel Cells:
Battery Capacity is 150 kW
We will convert it into Ah
Ah = (kW * 1000) / V
= (150 * 1000) / 120
Therefore, Battery Capacity = 1250 Ah
Number of Cells in Parallel = Required Battery Capacity / Nominal Cell Capacity
= 1250 / 3.5
Number of Cells in Parallel = 357.14
Now in order to round off to a higher or lower value we will check the conditions.
Case I: Considering a lower value i.e. 357 Cells
Total Capacity = Nominal Cell Capacity * Number of Cells in Parallel
= 3.5 * 357
Total Capacity = 1249.5 Ah
Case II: Considering a higher value i.e. 358 Cells
Total Capacity = Nominal Cell Capacity * Number of Cells in Parallel
= 3.5 * 358
Total Capacity = 1253 Ah
So, as seen from both the cases as our required Capacity is 1250 Ah & in Case I it is satisfying the required Capacity while in Case II it exceeds the required capacity of the battery pack.
Therefore we will go for Case I where the total capacity is 1249.5 Ah which is less than our required capacity i.e. 1250 Ah.
Hence we will require 357 Cells to be connected in Parallel.
Therefore, we will require a battery pack configuration of 33S357P to meet our requirements with the given specifications.
b) BMS Topology:
The BMS topoloy selection is based on:
1) Number of cells in series 2) Application
So according to our requirement we will choose Master & Slave topology, as it consists of multiple AFE's because we have the requirement of 33 cells & so we can connect more than 18 cells in series. Also it has high scalability.
So we need two slaves, 18 cells connected in series in one slave & the remaining 15 cells in the second slave to have this requirement.
It consists of 2 Slaves each having AFE in which 18 cells are connected in series in the first slave & the remaining 15 cells are connected in series in the second slave. Here, the information of 18 cells in first slave is send to second slave & then the second slave information along with first slave information is send to the Microcontroller.
2. Build a 3S2P battery pack configuration with generic battery block using Matlab/Simulink.
Answer:
AIM:
To Build a 3S2P battery pack configuration with generic battery block using Matlab/Simulink.
Blocks used to model configuration:
1) Battery
2) Voltage Measuremnet
3) Controlled Current Source
4) Bus Selector
5) Scope
6) Display
7) Continuous Powergui block
To model 3S2P battery pack configuration we need to connect 3 Cells in Series & 2 Cells in Parallel.
Therefore, total number of cells required in a battery pack = Number of cells in series * Number of cells in parallel
= 3 * 2 = 6 cells
So total of 6 Cells will be required to configure this model.
Stepwise model configuration in Simulink:
Step I:
First we will connect the cells in series. In series connection, batteries are connected from terminal to terminal in a way that the positive (+) terminal of one battery is connected to the negative terminal (-) of the other battery.
Step II:
Now we will connect the cells in parallel. In parallel connection, batteries are connected from terminal to same terminal that is positive (+) terminal of one battery is connected with positive terminal (-) of the other battery & the negative (-) terminal of one battery is connected with the negative terminal (-) of the other battery.
Step III:
The Voltage measurement block is connected across the battery pack to measure the battery pack voltage.
The controlled current source is used as the load to the battery pack.
The bus selector block is used to extract the values of SOC, Current & Voltage.
The constant block is used to give the battery capacity value to the controlled current source.
The scope block is used to generate various results.
Simulink model for 3S2P battery pack configuration:
The parameters of the battery are set as default.
Default values are:
1) Nominal voltage of a cell = 7.2 V
2) Rated capacity of a cell = 5.4 Ah
3) Maximum voltage of a cell = 8.3 V
Nominal voltage of the battery pack = Number of cells in series * Nominal voltage of a cell
= 3 * 7.2
Nominal voltage of the battery pack = 21.6 V
Rated Capacity of the battery pack = Number of cells in parallel * Rated capacity of a cell
= 2 * 5.4
Rated Capacity of the battery pack = 10.8 Ah
So, the model is for an 21.6 V, 10.8 Ah battery.
RESULTS:
1) The SOC is reduced from 100 % to 50 %
2) The current is constant
3) The voltage is around 8.3 V
The below result shows the overall output voltage of the battery pack.
Overall output voltage of the battery pack = Maximum voltage of the battery * Number of cells in series
= 8.3 * 3 = 24.9 V
Overall output voltage of the battery pack = 24.9 V
