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.
2. Build a 3S2P battery pack configuration with generic battery block using Matlab/Simulink.
Ans.
Objective:
The Main Objective of this project is to design the battery pack configuration for given requiremments and to select a suitable BMS topology for the same and also to build 3S2P battary pack configuration by using the MATLAB/SIMULINK tool.
A Battery Management System (BMS) is said to be the brain of a battery pack. The BMS is a set of electronics that monitors and manages all of the battery’s performance. Most importantly, it keeps the battery from operating outside of its safety margins.
The battery management system is critical to the battery’s safe operation, overall performance, and longevity. Moreover, it protects the battery and the people who are using it.
The primary function of the BMS is to protect the battery cells from damage caused by being overcharged or over-discharged. Additionally, the BMS calculates the remaining charge, monitors the battery’s temperature, monitors the battery’s health and safety by checking for loose connections and internal shorts. The BMS also balances the charge across the cells to keep each cell functioning at maximum capacity.
If it detects any unsafe conditions, the BMS shuts the battery down to protect the lithium-ion cells and the user.
Given Data:
Power = 150KW
battery pack Voltage = 120V
Cell Specifications:
Nominal Voltage = 3.6V
Capacity = 3500mAh = 3.5Ah
Cell Type = NMC
An NMC is a lithium-ion battery that uses a combination of nickel, manganese, and cobalt as the cathode.
For an NMC Cell,
Max Voltage rating Vmax= 4.2V and
Min Voltage Vmin = 2.7V
We know that Power = Voltage * Current
From the given data Power = 150*1000 W
voltage = 120V
⇒ Rated Current = 150,000/120 = 1250A
a. Design the battery pack configuration.
To Design a Battery Pack following two parameters need to be calculated:
1. Number of Cells in Series
2. Number of Cells in Parallel
1. Number of Cells in Series:
Number of cells in Series = Total battery pack voltage/ individual cell voltage
=120/3.6 = 33.33
Rounding of number of cells in series:-
| Parameter | Case-1 | Case-2 |
| Number of cells (Ns) | 33 | 34 |
| Nominal battery pack voltage = Ns* 3.6 V | 118.8 V | 122.4 V |
When compare with required rated battery pack voltage (120V), the battery voltage is greater in case 2 than case-1.
So, No.of cell in series = 34S
2. Number of Cells in parallel:
Number of cells in parallel = Required continuous current / cell nominal capacity
Np= 1250/3.5 =357.14
Rounding of number of cells in Parallel:-
| Number of cells in Parallel | total Capacity(Ah) | Requred Capacity | |
| Case - 1 | 357 | 357*3.5 = 1249.5 Ah | 1250 Ah |
| Case - 2 | 358 | 358*3.5 = 1253 Ah | 1250 Ah |
The Total battery pack capacity =number of cells in parallel * each cell capacity = 358*3.5=1253 Ah which is higher than the rated capacity required
So,we go with Np= 358 cells
So the battery pack configuration is 34S358P
The total number of cells required = 34*358= 12,172 cells
b.Selectiong Suitable BMS Topologies:
| Centralised | Master & Slave | Modular | Distributed | |
| No. of cells | 16-18 cells | 7000 cells | >7000 Cells | 16-18 Cells |
| No. of Slaves | Nil | Limited | Nil | 1 |
| Implementation | Easy | Medium | Hard | Easy |
| Cost | Low | Medium | High | Relatively high |
| Scalability | No | Yes | Yes | No |
| Size | Compact | Not Compact | Not Compact | Compact |
| Complexity | Low | Medium | High | Low |
| Aplication | 2 & 3 wheelers | 4 wheelers | Energy storage | Energy storage |
Topology selection criteria:
Since it is given that the system is 150 kW system, we dont have to use Modular topology. However, the number of cells in series is 34 which is more than 18 cells, it is advisable to use Master and Slave Topology.
As the BMS's AFE Supports the max 18 cells in series so we need to divide the 34 sereis cells to 17 cells series sub battery pack which leads to 2 series sub connections with 17 cells in series in each.
So the battery pack configuration is 17S358P+17S358P
Note: If we choose to make 17Series connection first and then paralleling 358 such strings, we need 358 AFEs to monitor all these strings, 1st we make paralleling 358 cells and then make series of such 17 packs, then 2 AFEs are enough, this is prefereble when all parallel connected cells are of similar type.
2. Build a 3S2P battery pack configuration with generic battery block using Matlab/Simulink.
3S2P Battery Pack Configuration using Simulink Model:
The objective of this article is to connect batteries in 3S2P configuration in Matlab/Simulink. That is 3 cells should be connected in series and 2 cells should be connected in parallel.
Total number of cells in battery pack = No. of cells in series * No. of cells in parallel
= 3 * 2 = 6 cells
Model Explanation:
The figure shows the 3S2P Battery pack simulink model. The voltage measurement block is connected across the battery pack to measure the battery pack voltage. The scope output used for the battery pack consists of SOC (State of Charge), Current and voltage. The controlled current signal is given as the load to the battery pack. The constant block is determined with battery capacity and it is given to the controlled current block.
Figure : 3S2P Battery pack simulink model
The parameter given inside the battery is given below,
. 
|
S.No |
Battery parameters |
Value |
|
1 |
Nominal voltage of a cell |
7.2V |
|
2 |
Rated capacity of a cell |
5.4Ah |
|
3 |
Maximum voltage of a cell |
8.3V |
Nominal voltage of the battery pack = no.of cell in series * nominal voltage of a cell
= 3 * 7.2v = 21.6V
Rated capacity of the battery pack = no.of cell in parallel* rated capacity of a cell
= 2 * 5.4Ah = 10.8Ah
The model is for an 21.6 V 10.8Ah battery.
The figure below shows the single battery output for SOC, voltage and current.
- The SOC (state of charge) is started to reduce from 100% to 87.14 in 1000 Sec corresponding to the load given.
- The voltage of a single cell is around 8.36V at the start,at the end of the 1000 sec, and is reduced to 7.738v at the end of the 1000 Sec
For the whole battery pack:
- The current signal is determined by the load
- Max. battery voltage= Max. cell voltage * no.of cells in series = 8.3 * 3 =24.9 V
- The voltage of the battery pack is around 25.14 V at the start,at the end of the 1000 sec, voltage of a single cell is reduced to23.21 V
