Study of cell balancing techniques of a battery pack.

Aim:

To study cell balancing techniques of a battery pack.

Objectives:

01.

To study, how weakest cell limits the usable capacity of the battery pack.

02.

To study, the cell balancing techiniques.

Introduction:

First we will look into the precise definitions of EV battery’s “cell”, “module” and “pack”.

Electric Vehicle Battery is Composed of “Cell → Module → Pack”

To operate an electric vehicle, an enormous amount of power thousand times stronger than that of a smart phone is required. That is why EVs need from dozens of battery cells up to as many as thousands. The composition of an EV battery might vary slightly depending on the types of electric vehicles, but generally EV batteries are composed of cells, modules and a pack.

In fact, to safely and efficiently manage the countless battery cells mounted in one EV, the cells are installed in forms of modules and packs. Simply put, cells, modules and packs are units of gathered batteries. A cluster of cells make up a module and a cluster of modules make up a pack. Ultimately, in an electric vehicle, one form of battery is installed: a pack.

Let’s take a look at the example of a typical pure electric vehicle. In a BMW i3, a total of 96 battery cells are installed. Twelve cells are combined into one module and eight modules are put together to go into the vehicle in the form of a pack.

Let's examine each term one by one. A cell which is the basis of a battery must possess high capacity per unit volume in order to show maximum performance in a restricted area inside a vehicle and the cell also needs to have much longer lifespan compared to batteries used in general mobile devices. Furthermore, cells must endure shocks transmitted during the drive and possess high reliability & stability to the extent of being able to withstand high and low temperatures.

When a number of cells are put into a frame to protect them better from external shocks such as heat or vibration, this is called a module. And when a number of modules come together with a BMS (Battery Management System) and a cooling device that control and manage battery’s temperature, voltage, etc., this is called a pack. This is how numerous cells are installed in an electric vehicle through the form of a pack.

Fig01: Assembling a battery pack.

Nominal capacity of the battery pack:

Battery capacity shows how much energy the battery can nominally deliver from fully charged, under a certain set of discharge conditions.

Usable capacity of the battery pack:

By programming the system to limit the Depth of Discharge, the effective usable capacity of the system is limited.

Usable Capacity (Ah) = Depth of Discharge (%) * Nominal Capacity (Ah)

In general this will increase the number of available life cycles of the battery – the lower the programmed depth of discharge, the longer the battery will last.

Weakest cell of the battery pack:

The battery configuration in series or parallel connection can have a different voltage on each cell. The weakest cell in the battery pack could have more stress than other cells so that it will become the fastest damage. This research will find the weakest cell in the battery pack. The identification of the weakest cell is performed on the charging and discharging process with direct measurement. The result shows that the weakest cell located in the middle of the battery pack. The weakest cell have 2.75 V and the health battery cell have 2.9 V.

What is Cell Imbalance?

Fig02: Imbalanced cells.


If lithium cells are overheated or overcharged, they are prone to accelerated cell degradation. They can catch fire or even explode as a thermal runaway condition can occur if a lithium ion cell voltage exceeds 4.2 V by even a few hundred millivolts.

Every pack that we design and manufacture at Epec has an overvoltage protection circuit (sometime even a backup) to go along with standard cell balancing that will prevent such an event from ever occurring. In a multi-cell battery pack, which is commonly used in laptop computers and medical equipment, placing cells in series opens up the possibility of cell imbalance, a slower but persistent degradation of the battery.

What is Cell Balancing?

Cell balancing is the process of equalizing the voltages and state of charge among the cells when they are at a full charge. No two cells are identical. There are always slight differences in the state of charge, self-discharge rate, capacity, impedance, and temperature characteristics. This is true even if the cells are the same model, same manufacturer, and same production lot. Manufacturers will sort cells by similar voltage to match as close as possible, but there are still slight variations in the individual cells impedance, capacity, and self-discharge rate that can eventually lead to a divergence in voltage over time.

Most typical battery chargers detect full charge by checking whether the voltage of the entire string of cells has reached the voltage regulation point. Individual cell voltages can vary as long as they don’t exceed the limits for overvoltage protection. However, weak cells (ones with lower capacity / higher internal impedance) tend to exhibit higher voltage than the rest of the series cells at full charge termination. These cells are then weakened further by continuous overcharge cycles. The higher voltage of the weaker cells at charge completion causes accelerated capacity degradation. If the maximal recommended charging voltage is exceeded even by as little as 10 percent, it will cause the degradation rate to increase by 30 percent.

On the discharge side, the weak cells tend to have lower voltage than the other cells, due to either higher internal resistance, or a faster rate of discharge that results from the lower capacity. This means that if any of the weak cells hits the cell under voltage protection limit while the pack voltage is still sufficient to power the system, the full capacity of the battery will never be used as the pack protector will prevent over discharge (which would damage the cell) by stopping the discharge of the whole pack when one cell voltage goes below the cell under voltage threshold (usually around 2.7 V).

Problem Statement:

01.

How weakest cell limits the usable capacity of the battery pack?

02.

Explain the cell balancing techiniques with relevant figures.

Solution:

01.

How weakest cell limits the usable capacity of the battery pack?

No two cells are identical. There are always slight differences in the state of charge, self-discharge rate, capacity, impedance, and temperature characteristics. This is true even if the cells are the same model, same manufacturer, and same production lot. Manufacturers will sort cells by similar voltage to match as close as possible, but there are still slight variations in the individual cells impedance, capacity, and self-discharge rate that can eventually lead to a divergence in voltage over time.

Most typical battery chargers detect full charge by checking whether the voltage of the entire string of cells has reached the voltage regulation point. Individual cell voltages can vary as long as they don’t exceed the limits for overvoltage protection. However, weak cells (ones with lower capacity / higher internal impedance) tend to exhibit higher voltage than the rest of the series cells at full charge termination. These cells are then weakened further by continuous overcharge cycles. The higher voltage of the weaker cells at charge completion causes accelerated capacity degradation. If the maximal recommended charging voltage is exceeded even by as little as 10 percent, it will cause the degradation rate to increase by 30 percent.

On the discharge side, the weak cells tend to have lower voltage than the other cells, due to either higher internal resistance, or a faster rate of discharge that results from the lower capacity. This means that if any of the weak cells hits the cell under voltage protection limit while the pack voltage is still sufficient to power the system, the full capacity of the battery will never be used as the pack protector will prevent over discharge (which would damage the cell) by stopping the discharge of the whole pack when one cell voltage goes below the cell under voltage threshold (usually around 2.7 V).

This is how, the weakest cell limits the usable capacity of the battery pack.

02.

Explain the cell balancing techiniques with relevant figures.

Cell Balancing Techniques:
The fundamental solution of cell balancing equalizes the voltage and state of charge among the cells when they are at a fully charged state. Cell balancing is typically categorized in two types:

1. Passive
2. Active

Passive Cell Balancing:
The passive cell balancing method is somewhat simple and straight forward. Discharge the cells through a dissipative bypass route. This bypass can be either integrated or external to the integrated circuit (IC). Such an approach is favorable in low-cost system application. The fact that 100% of the excess energy from a higher energy cell is dissipated as heat makes the passive method less preferable to use during discharge because of the obvious impact on battery run time.

Active Cell Balancing:
Active cell balancing, which utilizes capacitive or inductive charge shuttling to transfer charge between battery cells, is significantly more efficient because energy is transferred to where it is needed instead of being bled off. Of course the trade-off for this improved efficiency is the need for additional components at higher cost.

Fig03: Active and Passive cell balancing of cells.

Passive balancing is further classified into:

a. Fixed shunt resistor cell balancing.

b. Switching shunt resistor cell balancing.

Active balancing is further classified into:

a. Single Capacitor cell balancing.

b. Single Inductor cell balancing.

c. Single Transformer cell balancing.

Now let's study all balancing circuits one by one.

A. Fixed shunt resistor cell balancing:

Fig04: Fixed shunt resistor cell balancing circuit.

The fixed shunt resistor cell balancing circuit is as shown in Figure04. Where, V1, V2, V3, V4, …Vn is each cell voltage which is connected in series and R1, R2, R3, R4, …Rn are fixed shunt resistors of each battery cell. This technique balances each cell voltage by connecting the fixed resistor in parallel with each series connected cell based on the required cell balance current. The balancing current is dissipated through the resistor which limits the voltage of each cell.

This method is suitable for nickel and lead-acid batteries balancing circuit, since these batteries are brought into overcharge conditions without any damage. This circuit is very simple, hence it requires less number of components and low cost. But the disadvantage of this technique is to provide the energy losses due to the energy are dissipated as heat in all the cells during balancing operation.

B. Switching shunt resistor cell balancing.

Fig05: Switching shunt resistor cell balancing circuit.

The switching shunt resistor cell balancing circuit is as shown in Figure05. Where, V1, V2, V3, V4, …Vn is each cell voltage which is connected in series, Q1, Q2, Q3, Q4, …Qn are semiconductor switches of each cell and R1, R2, R3, R4, …Rn are fixed shunt resistors of each battery cell.
This technique balances each cell voltage by connecting the resistor in parallel with each series connected cell through controlled on/off semiconductor switches or relays. In this method, the resistor value is properly chooses based on the required balancing current. This method requires a controller for controlling the circuit in two different two modes. In continuous mode, the switches on/off are controlled at the same time, in sensing mode, the voltage of each cell is detected by a voltage sensor, which detects the cell imbalance situation and decides which resistor should be shunted.
This method is also called as charge shutting method and is commonly used for Li-ion batteries balancing circuit. This circuit is more reliable than a fixed shunt resistor balancing circuit. Also, this technique is providing the energy losses due to the higher currents are flowing through the switches and resistors during balancing operation.

C. Single Capacitor cell balancing.

Fig06: Single Capacitor cell balancing circuit.

This technique is known as charge shuttling method in which charge from the higher energy cell is released, that charge must be stored in capacitor, and then transfer into lower energy cell. There are different topologies of capacitor-based cell balancing schemes are available. This technique can be classified based on a single capacitor and several capacitors used equalization methods.

The single capacitor cell equalizing circuit is shown in Figure06. Where, V1, V2, V3, V4, …Vn is voltage of each cell which is connected in series, Q1, Q2, Q3, Q4, …Qn, Qn+1, Qa, Qb are semiconductor switches and C is a single capacitor. This balancing circuit requires n + 5 semiconductor switches and only one capacitor for equalizing the n number of cells charge levels. The basic principle of this circuit is equalizing the charge of each cell by charge or discharge of common balancer such as a capacitor which transfers the energy between any two cells in a battery string. For example, when Q1, Qa(1), Qb(2), and Q2 are turned ON, and others are turned OFF, the capacitor C is connected in parallel with Cell V1. When Q3, Qa(1), Qb(2) and Q4 is turned ON, and others are turned OFF, the capacitor C is connected in parallel with cell V3. Hence, the energy moved straightforwardly between any two cells at any position in the battery pack with this method.

This method is also known as direct cell to cell method and suitable for high power applications. It is very simple due to only one capacitor is used to balancing the overall battery pack. But, it has a slow balancing speed and requires a number of semiconductor switches and intelligent control techniques for controlling switches.

D. Single Inductor cell balancing.

Fig07: Single Inductor cell balancing circuit.

There are different topologies of inductor-based cell balancing schemes are available. This technique can be classified based on a single inductor and several inductors used equalization methods. The single inductor cell equalizing circuit is shown in Figure07. Where, V1, V2, V3, V4, …Vn is voltage of each cell which is connected in series, Q1, Q2, Q3, Q4, …Qn, Qn +1, Qa, Qb are semiconductor switches and L is a single inductor. The single inductor balancing method shares the same switching topology as the single capacitor balancing method. This balancing circuit requires two groups of n + 1 semiconductor switches (MOSFETs) and a diode is in series with each switch (MOSFET) to prevent the cell from the short circuits during equalizing
operation, and only one inductor for equalizing the n number of cells charge levels.

The basic principle of this circuit is equalize the charge of each cell by charge or discharge of common balancer such as an inductor that transfers the energy between any two cells in a battery string. Hence, the energy moved straightforwardly between any two cells at any position in the battery pack with this method.

This method is also known as a direct cell to cell method. It is very simple due to only one inductor is used to balancing the overall battery pack. It has a faster balancing speed than capacitor-based topology and higher efficiency. But it requires high implementation cost than capacitor balancing topology, a number of semiconductor switches, and intelligent control techniques for controlling switches.

E. Single Transformer cell balancing.

Fig08: Single Transformer cell balancing circuit.

There are different topologies of transformer-based cell balancing schemes are available. This technique can be classified based on a single transformer and several transformers used equalization methods. The single transformer cell equalizing circuit is shown in Figure08. Where, V1, V2, V3, V4, …Vn is voltage of each cell which is connected in series, Q1, Q2, Q3, Q4, …Qn, Qn+1, Qa, Qb are semiconductor switches, D is a diode, S1 is semiconductor switch for control in primary side and T is a transformer for a whole battery pack.

The single transformer balancing method shares the same switching topology as the single capacitor balancing method. This balancing circuit requires n + 5 semiconductor switches and only one transformer for equalizing the n number of cells charge levels. The basic principle of this balancing circuit is based on the pack to cell technique and cell to pack technique.

In pack to cell technique, the entire battery pack current is switched (S1) into the transformer T and the output of the transformer is rectified and carried into the lowest charge cell through corresponding semiconductor switches. Hence, control is required to choose a target cell and switching devices. In cell to pack technique, the target cell is a higher energy cell that is moved into the battery pack through a transformer to balance the voltage of the target cell with other cells in the battery pack. Therefore, this balancing circuit has a fast balancing speed than inductor based topology. But, it has a high system cost, low efficiency due to magnetic losses, and requires perfect electronic switch control.

Conclusion:

In this study we learnt the role of weakest cell in the battery pack and different cell balancing techniques used to balance the cells of the battery packs.