Modular Scalable Design for Active Balancing of Li Ion Batteries

The project aims at designing a market solution for active balancing of li ion cells in a battery pack that is scalable and modular in its design. The requirement for such a design is primarily due to inherent mismatches in series or parallel connected cells in a battery pack and also due to degrada

Project Title

Modular Scalable Design for Active Balancing of Li Ion Batteries

Project Area of Specialization

Electrical/Electronic Engineering

Project Summary

The project aims at designing a market solution for active balancing of li ion cells in a battery pack that is scalable and modular in its design. The requirement for such a design is primarily due to inherent mismatches in series or parallel connected cells in a battery pack and also due to degradation of the cell’s capacity over time. The modular design shall be embedded with every cell such that when it is connected to the string in either way (series or parallel), the State of Charge (SOC) for all the cells remain the same throughout the charging or discharging cycle and any spill current due to the mismatch flows to the remaining cells, thus giving an equal terminal voltage on every single cell. The design under consideration is compatible with both, Lead Acid batteries and Li-Ion Batteries but the use case is Li-Ion specific because the upcoming market is predominantly based on Li-Ion batteries due to higher energy density and longer life with considerably high depth of discharge limit. Currently available BMS (Battery Management Systems) are very specific and depend on the series and parallel connected cells in the battery (e.g. 3S4P or 3S 10A) but the design proposed will be at a cell level and aims to work with any number of series or parallel cells. With more and more electric vehicles penetrating in the market, long lasting battery chemistries and their efficient charging and discharging ways are being figured out. From increasing backup requirements for a system with UPS to Tesla filing a patent for the “Million Mile Battery”, batteries hold an important place in future of electricity storage and their lifetime is contingent upon the efficient use of the battery.

Project Objectives

1) Implementation and design of a DC to DC bi-directional Converter.

This objective is further divided into several parts. The first step of this objective is to design a bi-directional converter with maximum efficiency. The proposed system not only promises a comparatively long lasting electrical energy storage alternative, it also aims at efficient management of the Li-Ion cells. The proposed design ensures minimum wastage of input electrical energy into the battery pack. The design will be tested in various simulation programs to ensure if the proposed outcomes are being met.

2) Implementation and design of BMS (Battery Management System).

Circuitry of BMS will be implemented in a way which ensures maximum efficiency. BMS will sense voltage across each Li-Ion cells included in the battery pack. It then intelligently controls flow of current into the cells. The cell with lower voltage levels will be fed with comparatively greater current coming from the charging circuitry. Balanced charging and discharging of the LiIon cells in the battery pack is hence ensured. Simulation of our proposed design will be tested in various simulation software. The proposed design will then be designed on a PCB.

3) Implementation of an Actively Balanced Li-Ion Battery Pack by compiling the bidirectional converters and cells together.

All the elements discussed above will be cascaded with each other by the end of the project. The cascaded system will be simulated on various simulation software. The behaviour of the proposed circuitry will be tested. On successful testing of the design, the proposed system will be then implemented on PCB. The final objective of the project is to produce a full Actively Balanced Li-Ion battery pack.

Project Implementation Method

The implementation of the project involves cascade of three stages.

The first stage involves the bi-directional energy transferring capacity topology called cuk converter. The cuk converter has an inductor (L1 and L2) on the input of the two batteries to allow smooth flow of current with low voltage ripple. Further to add it involves a capacitor (C1) which is the main energy storage element and two self-controlled switches to control the diverted current from the batteries in both directions. The figure below shows how the dc-dc Cuk converter is implemented. Pulse width modulated (PWM) signal is used to control the On time and Off time of the two switches (Q1 and Q2) with switching time Ts. For the energy transfer from battery 1 (B1) to battery 2 (B2), Q1 is turned on while Q2 is off whereas for the energy flow from B2 to B1 switch Q2 is switched on while Q1 is switched off.

Thus the direction of power flow in the converter and hence the state of charge of the batteries can be can be controlled by controlling the states of the switches Q1 and Q2.

The second stage involves the implementation of switch (MOSFETS) driving circuitry and the generation of PWM whose Duty cycle (D) depends on the difference between the voltage levels of the batteries cuk is connected to. The principle behind the generation of PWM involves comparing a DC voltage with a saw-tooth wave using TL494. The cuk converter is designed for the switching frequency of 50kHz therefore the oscillator frequency of TL494 is adjusted to 50kHz using 2.2nF capacitor and 10k? resistor. The PWM output of TL494 is fed to MOSFET gate driver (IR2110) to drive the low-side and high-side MOSFETS. The circuit is configured with an external diode and a bootstrap capacitor of 1uF to hold the voltage (12V) with respect to source of the high-side MOSFET so that it can be switched ON. Following figure shows how isolated gate driver is used to apply voltage and and provide drive current to the gate of the power device.

The coupling capacitor of 1uF is used to remove the DC component from the signal so that an AC signal could be fed to the transformer and the arrangement of diodes regulates the output to 12V and 0V..

The third stage is the control circuit responsible for controlling the Duty cycle of the PWM signal and hence the amount of current being diverted from one higher charged battery to the lower charged battery. The duty cycle of the PWM signal fed to each MOSFET depends on the difference in voltages of the two batteries. Figure below shows how a summing amplifier is implemented. 

                                                                                  

V1 represents the voltage level of upper battery (B1) and V2 represents the voltage level of the lower battery (B2) connected in series with B1. Since a local ground exists between the two batteries the voltage V1 is positive with respect to the ground and V2 is negative with respect to the ground hence the sum of the two voltages will actually be the difference of V1 and V2. The difference is fed to TL494 to generate the PWM accordingly.   

Benefits of the Project

If we analyze UPS systems installed in Pakistan alone, there are around 2.8 Million UPS systems already installed. The requisite storage market is $1.2 billion every two years, if we take 2 years as average life of a battery. The proposed system, i.e. Li-ion based battery packs, ensures higher reliability with an increase in cost and performance efficiency. The huge market which exists for electrical power storage in Pakistan will provide the proposed battery system a huge business opportunity. Apart from UPS based storage systems, renewable based storages also require cost and performance efficiency. The proposed system will also benefit the huge Renewable based storage market. Telecommunication companies, which require backup storages, can also employ the proposed Li-ion battery system in order to increase the lifetime of their investment. 

Technical Details of Final Deliverable

The final deliverable is based on two major circuits:
1)    Control Circuit:
The control circuit establishes a local ground between two consecutive batteries and a voltage difference based PWM signal is generated at the gate of the appropriate MOSFET if any mismatch is detected between the batteries. This signal is responsible to ensure the power flow from one battery to another based on the mismatch. Since the duty cycle of the signal is dependent on the difference between the two batteries, more the difference, higher the duty cycle and consequently higher is the balancing current for the power circuit. The virtual grounds established, and the gate signals are at different levels, so an isolation is required between the generated signal and MOSFET’s gate. This is done by adding a gate isolation after gate driver circuit and is used to pull up the voltage at the gate with respect to the source. 
2)    Power Circuit:
The power circuit is based on Cuk Converter. The major advantage of using Cuk is because of its continuous input and output current, hence reducing stresses on the circuit. Moreover, the conversion ratio of input and output voltage is negative. This is beneficial because when a battery is connected in series to another cell, the positive terminal of lower cell is shorted to negative terminal of the upper cell. Such topology automatically favors an inverting gain of the converter and thus the charging and discharging current flow is consistent to the theoretical application. Since the power flow can be from either direction, the converter used is bidirectional and two MOSFETs are used that dictate the power flow in the desired direction. The component design of the power circuit comes from the basic power electronics knowledge and the symmetrical circuit can then be broken down from the center and divided into 2 parts, each goes with the individual battery as its indigenous power circuit and gets completed only when 2 batteries are connected to each other.

Block diagram below summarises the integration of two circuits to actively balance the battery pack.

Final Deliverable of the Project

Hardware System

Core Industry

Energy

Other Industries

Core Technology

Others

Other Technologies

Sustainable Development Goals

Good Health and Well-Being for People, Sustainable Cities and Communities, Responsible Consumption and Production

Required Resources

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