Project 1

Li-ion Battery System Design in EV & ES 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.

Ans:

Aim of the Project:

To design a safe, reliable, and efficient battery pack, with a high energy density and power output for a car with the given below specifications:

Power: 150kW

Voltage: 120V

Cell capacity: 3.5Ah

Cell Voltage: 3.6V (NMC chemistry cells).

Also design & define topology of BMS to monitor and control the battery pack, ensuring safe and efficient operation.

A.To Determine the number of cells in series and parallel connections to meet the given battery pack requirement.

Given parameters:

Power: 150kW

Voltage: 120V

Cell capacity: 3.5Ah

Cell Voltage: 3.6V

A.To calculate the number of cells in series:

-Nominal voltage per cell is 3.6V.

- Rated voltage of the battery is 120V.

Number of cells connected in series = Battery rated voltage / Nominal voltage per cell = 120V / 3.6V =Approx. 33.33 cells=34 cells

Ns=34 cells

To calculate the number of cells in parallel:

The continuous currentto be provided by the battery, need to be known.

Power (P) = Voltage (V) * Current (I) , Current (I) = Power (P) / Voltage (V)

Current (I) = 150/ 120 = 1250 A

The nominal capacity per cell is 3.5 Ah.

Number of cells in parallel = Continuous current / Nominal capacity per cell = 1250 A / 3.5 Ah = 357.142 cells

Np= 358 cells connected in parallel.

The number of cells in series=Ns = 34 cells.

The number of cells in parallel=Np = 358 cells.

Hence, the battery pack configuration is 34 cells connected in series and 358 cells connected in parallel, with a total of 12172 cells in the battery pack.

B.BMS Topology for the given battery pack

Understanding BMS Architectures in Electric Vehicles

Battery Management Systems (BMS) are fundamental to maintaining the safety, performance, and longevity of electric vehicle (EV) battery packs. They oversee critical aspects such as charge levels (State of Charge, SOC), battery health (State of Health, SOH), and operational readiness (State of Function, SOF). In addition to monitoring, BMS units protect the battery from adverse scenarios like overcharging, deep discharge, and thermal overload. Selecting the right BMS architecture is essential to meet the specific needs of a battery configuration in an EV.

There are several architectural approaches to implementing a BMS, including centralized, distributed, modular, and embedded systems. Each design presents its own strengths and drawbacks, outlined below.

1.Centralized Architecture

A centralized BMS features a single control module that manages all the battery cells directly. Connections to individual cells are established through dedicated wiring or communication lines. While this structure is straightforward and budget-friendly, it introduces challenges such as intricate wiring, lower fault isolation, and decreased dependability in high-voltage applications.

2.Distributed Architecture

In a distributed BMS setup, each cell or a small group of cells is equipped with a local monitoring unit. These units relay data to a master controller that governs system behavior. This model enhances system robustness, provides improved fault handling, and reduces cable clutter. However, the added electronics and system integration make it more technically complex and increase the overall cost.

3.Modular Architecture

Modular BMS systems divide the battery pack into smaller, self-contained modules, each managed by its own BMS. These modules are interconnected to build the complete battery system. The modular approach supports scalability, facilitates maintenance, and enhances fault resilience. Nonetheless, it tends to be more intricate and costly, making it ideal for high-capacity or commercial-scale battery packs.

4.Embedded (Integrated) Architecture

Embedded BMS solutions integrate control and monitoring capabilities directly within each cell. With built-in micro-controllers, these cells independently track parameters such as voltage, temperature, and charge status. This design achieves high precision, minimizes wiring, and boosts system reliability. However, due to its higher implementation cost and incompatibility with legacy battery packs, it is typically reserved for next-generation systems.

(Figure 1: BMS Topology)(Refer attached document for figures and results)

Proposed BMS Architecture for Advanced Battery Packs

The Battery Management System (BMS) serves as the brain of a battery pack, governing its operation to ensure optimal performance and safety. It plays a vital role in overseeing critical parameters and coordinating with the electric vehicle’s control systems to manage energy flow and maintain battery integrity.

For this battery configuration, a suitable BMS framework would incorporate the following components and features:

Cell-Level Monitoring

Each individual cell is supervised by a dedicated monitoring unit that tracks essential data points such as voltage and temperature. These monitoring units function as the first line of defense, identifying irregularities like overvoltage, undervoltage, or thermal anomalies at the cell level.

Central Control and Coordination

All cell-level data is transmitted to a centralized supervisory module. This control unit collects, processes, and relays vital information to the vehicle's main controller. It is also responsible for regulating the battery pack’s energy flow during charge and discharge cycles, interfacing with power converters such as high-voltage chargers and DC-DC modules.

Energy Balancing

To maintain uniform charge levels across all cells, a balancing mechanism must be employed. This system may use either passive or active balancing methods. Passive systems dissipate excess energy from higher-charged cells, while active systems redistribute energy to undercharged cells, ensuring consistent performance and extending battery life.

Current Sensing

Accurate current measurement is essential for managing energy input and output. By integrating current sensors into the system, the BMS can track power usage in real time, contributing to precise state-of-charge estimation and safeguarding the battery from excessive current flow.

Thermal Oversight

Thermal sensors are embedded throughout the battery pack to monitor heat levels. This thermal data allows the BMS to activate cooling strategies or reduce load to prevent overheating and maintain safe operation under varying environmental conditions.

Built-In Protections

The BMS incorporates multiple layers of protection to guard against hazardous scenarios. Key features include safeguards against excessive current, voltage irregularities, and thermal stress—ensuring the battery operates within secure thresholds at all times.

Communication Interface

To enable seamless interaction with the vehicle’s electronic control units (ECUs), the BMS must support robust communication protocols. This interface provides real-time insights into battery condition, performance metrics, and fault diagnostics, forming a vital feedback loop for the overall vehicle system.

Recommended Topology: Modular Decentralized Configuration

Given the complexity and scale of this battery system, a modular decentralized BMS topology presents a highly appropriate solution. Sometimes referred to as a "star" or "master-slave" architecture, this design strikes a balance between distributed intelligence and centralized control.

In this setup, the battery pack is segmented into smaller units, each governed by its own local BMS module. These modules independently monitor parameters such as voltage, current, and temperature for their assigned cells. A central coordinator aggregates data from all modules and executes higher-level control and decision-making.

Benefits of Modular Decentralization

Enhanced Reliability: Localizing fault detection and control ensures that issues in one module don’t compromise the entire system.

Scalability: The system can be easily expanded by adding more modules, supporting future growth and design flexibility.

Serviceability: Faulty modules can be individually replaced or upgraded, reducing maintenance time and cost.

Improved Fault Isolation: Failures are contained within individual modules, minimizing systemic risk.

A modular decentralized BMS topology is the best solution for managing the battery pack in an electric car.

2.Build a 3S2P battery pack configuration with generic battery block using Matlab/Simulink.

Ans.

3S2P Battery Pack Configuration in MATLAB/Simulink

This article demonstrates how to model a battery pack with a 3S2P configuration in MATLAB/Simulink. In this arrangement, three cells are connected in series (3S), and two such series groups are connected in parallel (2P), resulting in a total of six cells in the battery pack.

Total Number of Cells

= Number of Series Cells × Number of Parallel Cells

= 3 × 2 = 6 Cells

(Figure 2: 3S2P Battery Pack Simulink Model)

Simulink Model Overview

The diagram below (Figure: 3S2P Battery Pack Simulink Model) illustrates the structure of the 3S2P battery pack within the Simulink environment. The system includes:

A voltage measurement block across the entire battery pack to monitor overall pack voltage.

A controlled current source acting as the load.

A constant block representing the pack's capacity, feeding into the controlled current block.

A scope displaying real-time output values including State of Charge (SOC), current, and voltage.

Battery Parameters Used

Nominal voltage per cell 3.6 V

Rated capacity per cell 3.5 Ah

Maximum voltage per cell 4.2 V

(Figure 3: Battery block settings)(Refer attached document for figures and results)

From these parameters:

Nominal Voltage of Battery Pack

= Series Cells × Nominal Cell Voltage

= 3 × 3.6 V =10.8 V

Rated Capacity of Battery Pack

= Parallel Cells × Cell Capacity

= 2 × 3.5 Ah = 7 Ah

The model effectively simulates a 10.8 V, 7 Ah battery system.

State of Charge (SOC) declines from 100% to 99.5%.

Cell Voltage starts at 4.19 V and drops to 4.10 V by the end of the simulation.

Full Battery Pack Behavior:

Initial Pack Voltage: ~12.6 V

Final Pack Voltage: ~12.2 V

Maximum Possible Pack Voltage

= Max Voltage per Cell × Series Cells = 4.2 × 3 = 12.6 V

Load current is driven by the applied current signal.

This simulation captures the discharge behavior and dynamics of a battery pack under load conditions, validating the effectiveness of the 3S2P configuration for moderate energy applications.

(Figure 4: Individual cell O/P)(Refer attached document for figures and results)