Week 8 Multi cell Battery Pack

How weakest cell limits the usable capacity of the battery pack? What is the solution?

ANS:

What is Battery Pack:

A battery pack is a set of any number of (preferably) identical batteries or individual battery cells  They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density. The term battery pack is often used in reference to Radio Controlled Hobby Toys and Battery Electric Vehicle.

Components of battery packs include the individual batteries or cells, and the interconnects which provide electrical conductivity between them. Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detect the end of charging. Interconnects are also found in batteries as they are the part that connects each cell, though batteries are most often only arranged in series strings.

Pack Design Options:

The design of the outer package or housing of the battery depends to a great extent on the components it has to accommodate and the physical protection it has to provide for them. These components are not just the cells, but also protection devices, electronic circuits, interconnections and connectors which must all be specified before the final battery case can be designed. For high power, high energy batteries robust packaging is required for safety reasons.

battery configuration:

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 damaged. 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 has 2.75 V and the health battery cell have 2.9 V.

The weakest cell may affect the battery pack by the following condition-

1. It will carry out a limit on the complete operation of the battery pack.

2. Due to the weakest cell reaching the limit earlier than others, the battery pack may be underutilized.

3. During the weakest cell conditions it is important to protect an individual cell overvoltage under voltage limits apply to all series-connected cells.

4. It imposes a limit on the overall operation battery pack.

Problem with Week cell in the Battery Pack:

1. Mainly there are two prime mismatches that affect the overall output of the battery pack and they are-

1. State of Charge(SoC) Mismatch

2. Capacity/ Energy(C/E) Mismatch

2. Each problem limits the pack capacity(mAh) to the capacity of the weakest cell and it is important to recognize.

3. These types of cell-to-cell variation more likely occurs in Li-ion prismatic cells, due to more extreme mechanical stresses.

4. 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.

5. 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 than the result from the lower capacity.

6. If the 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 4.2V by even a few hundred millivolts.

Why Proper Cell Balancing is Necessary in battery Packs:

When a lithium battery pack is designed using multiple cells in series, it is very important to design the electronic features to continually balance the cell voltages. This is not only for the performance of the battery pack, but also for optimal life cycles.

The use of cell balancing enables us to design a battery with larger capacity for an application because balancing allows the battery to achieve a higher state of charge (SOC). A lot of companies choose not to use cell balancing at the start of their design do reduce cost but without the investment in the cell balancing hardware and software, the design does not allow the SOC to approach 100 percent.

What is Cell Imbalance:
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).

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 into two types:

1. Passive
2. Active

Passive Cell Balancing
The passive cell balancing method is somewhat simple and straightforward. 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 the low-cost system applications. 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.

Fixed shunt resistor method:

The fixed shunt resistor cell balancing circuit is as shown in Figure. Where, V₁, V₂, V₃, V₄, …V_n is each cell voltage which is connected in series and R₁, R₂, R₃, R₄, …R_n 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 battery balancing circuit since these batteries are brought into overcharge conditions without any damage. This circuit is very simple, hence it requires fewer 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.

Switching shunt resistor method:

The switching shunt resistor cell balancing circuit is as shown in Figure. Where, V₁, V₂, V₃, V₄, …V_n is each cell voltage which is connected in series, Q₁, Q₂, Q₃, Q₄, …Q_n are semiconductor switches of each cell andR₁, R₂, R₃, R₄, …R_n is 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.
                                    

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 a higher cost.

Capacitor based active balancing method

This technique is known as the charge shuttling method in which charge from the higher energy cell is released, that charge must be stored in a capacitor, and then transfer into a lower energy cell. There are different topologies of capacitor‐based cell balancing schemes are available and shown in Figures. 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 Figure 4. Where, V₁, V₂, V₃, V₄, …V_n is voltage of each cell which is connected in series, Q₁, Q₂, Q₃, Q₄, …Q_n, Q_n + 1, Q_a, Q_b 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, Q_a(1), Q_b(2), and Q₂ are turned ON, and others are turned OFF, the capacitor C is connected in parallel with Cell V₁. When Q₃, Q_a(1), Q_b(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.

The several capacitors based cell equalizing circuit is shown in Figure 5. In this method, the charge is transferred between two neighboring cells. It is classified as a switched capacitor method and a double‐tiered switched capacitor method is shown in Figure 5A,B.

In the switched capacitor method, one switched capacitor is connected between every two neighboring cells through corresponding semiconductor switches is shown in Figure 5A for swapping the charge between higher charge cell to lower charge cell.^47-49 Where, V₁, V₂, V₃, V₄, …V_n is voltage of each cell which is connected in series, Q₁, Q₂, Q₃, Q₄, …Q_n are complementary semiconductor switches and C₁, C₂, C₃, C₄, …C_n−1 are capacitors of adjacent cells. This balancing circuit requires 2n semiconductor switches and n − 1 capacitor for equalizing the n number of cells charge levels. For example, when Q₁₍₁₎ and Q₂₍₁₎ is turned ON, and others are turned OFF, the capacitor C₁ is connected in parallel with cell V₁. When Q₁₍₂₎ and Q₂₍₂₎ is turned ON, and others are turned OFF, the capacitor C₁ is connected in parallel with cell V₂. Hence, the energy moved between these two neighboring cells by switching of these two positions.⁵⁰

This method is also known as a contiguous cell to cell method because of the transfer of energy from one cell to the neighboring cell through an individual balancer.⁵¹ This circuit does not require any intelligent control technique for semiconductor switches. Hence, it is easy to control and implement, low power loss and low switching voltage stress. But, it takes a long time for balancing the cell particularly the higher voltage and lower voltage cells are located at opposite ends of the stack because the charge has to travel through every cell.⁵²

To overcome this difficulty, the next level of several capacitors based cell equalizing circuit is implemented such as a double‐tiered switched capacitor method is shown in Figure 5B. Where, V₁, V₂, V₃, V₄, …V_n is voltage of each cell which is connected in series, Q₁, Q₂, Q₃, Q₄, …Q_n are complementary semiconductor switches, C₁, C₂, C₃, C₄, …C_n−1 are capacitors of adjacent cells and C_n, C_n+1, C_n+2, C_n+3 are capacitors of nonadjacent cells. The circuit operation and control are as same as that of the switched‐capacitor method. But, it has an additional second row of capacitors are in parallel with the conventional first row of capacitors in switched capacitor method.⁵³ Using the second level capacitors the charge transfer between nonadjacent cells can be led in one switching cycle. For example, the capacitor C_n exchange the charge between cell V₁ and cell V₃ as shown in Figure 5B. Hence, the charge is moved to distant cells in a small number of switching cycles. As an outcome, the balancing time using a double‐tiered switching capacitor can be reduced more than half compared with the switched capacitor method. Therefore, more rows (tiers) imply more balancing path between cells that exchange the charge with low impedance over a specific distance through the battery pack.

 Inductor based active balancing schemes:

Hardware Implementation of cell balancing:

One of the simple techniques to balance the cells would be by means of a current bypass [2]. The bypass transistors are placed in parallel with the cells and are turned-on when a voltage difference is detected using a comparator. Voltage-based control algorithms are used for the detection of voltage differences. However, the energy of the bypassed charge is wasted in the process. 

The other approach is based on charge redistribution. This overcomes the disadvantage of charge wastage in the current bypass technique by offering an efficient cell-balancing approach that allows the high cells to drain to the bottom. The best way to accomplish this is by not having any cells connected in series in the battery pack. This enables the step-up converter to ensure that sufficient voltage is attained by the device. However, the drawback of this technique is increased design complexity, size, and poor efficiency of the power supply.

There are other solutions that enable energy transfer from high cells to low cells using specific circuits rather than using the bypass resistor for the same. One such approach is to redistribute the energy between the cells by connecting the capacitor to a high cell and a low cell and is typically referred to as the charge shuttles method. This method also allows faster equilibration in cells placed far apart in the pack by providing the capability of remote cell connection as shown in Figure 2 [3]. The drawback of this technique is high losses incurred during the charging phase of the capacitors. The known efficiency of this process is only about 50% and efficiency is higher only during the end of discharge as the transfer rate is proportional to voltage differences.

A cell-balancing method called inductive converters overcomes the disadvantage of small voltage differences between cells. In this method, the battery pack energy is transferred to a single cell by channeling the battery pack current through a transformer as shown in Figure 3 [4]. The transformer is connected to the cell that requires an additional charge. The downside of this approach is the use of an additional transformer which leads to an increase in cost and size along with reduced overall efficiency. 

Now one takes a simple Simulink model for Exmple:

RESULT:

the red graph on SOC above is showing cell 10 fault at 0.5 and the other 19 cells are at the required battery state of charge.

The scope above is showing the simulation result of 20 cells, the yellow graph is of a fault cell undergoing high temperature causing to overheat, compared to the rest which is at nominal battery temperature