Multi Cell Battery Pack

Aim:

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

Model:

MULTI CELL BATTERY PACK:

The performance of electric vehicles (EVs) and plug-in hybrid electric vehicle (PHEVs) strongly relies on their battery storage system, which consists of multiple battery cells connected in series and parallel. However, cell state variations are commonly present, which reduces the energy conversion
efficiency of the battery system. Furthermore, in a large battery system the risk of catastrophic faults of cells increases because a large numbers of cells are used. To solve these problems, this paper proposes a novel power electronics-enabled, self-X, multicell battery system design. The proposed battery system can self-heal from failures or abnormal operations of single or multiple cells and self-balance from cell state variations. These
features are achieved by a cell switching circuit and a high-performance battery management system (BMS). The proposed design is validated by simulation studies in MATLAB Simulink for a battery system containing five modules connected in series, where each module consists of 6×3 cylindrical lithium-ion cells. The proposed design is scalable to large battery systems for EV/PHEV applications. We will also study about the BATTERY-TECHNOLOGY in general.

Introduction:

Electric vehicles (EVs) and plug-in hybrid vehicles (PHEVs) use a battery system consisting of a large number of cells to meet the required voltage, current, energy, and
power density. However, several design deficiencies are present in current multicell battery systems, which are

1) Adopting a fixed configuration for cell connections, resulting in low reliability and low fault-tolerance capability from abnormal operating conditions, such as high temperature, overcharge, over-discharge, and over-current.

2) Lacking an effective method to utilize cell state variations, resulting in non-optimal energy conversion efficiency, and

3) Lacking a capability for flexible dynamic power management, resulting in non-optimal system performance. In the existing battery design, safety circuits are commonly used to protect cells from high temperature, overcharge, over-discharge, and over-current by monitoring the temperature, voltage, and current of each cell.

However, lacking an effectively reconfigurable topology, the safety circuits will cut off the whole battery system when any single cell is operated in an abnormal condition. Moreover, cell state variations are commonly present in multicell batteries. In this case, the fixed-configuration design can only utilize a part of the total battery capacity. To overcome this deficiency, cell balancing circuits are used. However, most existing balancing circuits for EVs/PHEVs use dissipative resistors, resulting in energy loss and generating considerable heat. To reduce energy loss, active balancing circuits were proposed by using transformers, capacitors, and dc/dc converters. The latest products of cell balancing integrated circuits (ICs) use electronic converters to transfer charge from cell to cell during operation. However, these solutions substantially increase the cost and volume of the battery system and might not be able to handle faulty cells. Recently, several reconfigurable multicell battery topologies have been proposed to handle cell failures or to optimize the performance of battery systems for portable electronic devices. However, these topologies are too complex and unrealistic for the battery systems in EVs/PHEVs. This paper extends the work in by proposing a power electronics-enabled, self-X, multicell battery design for EVs/PHEVs. The resulting battery system can dynamically configure itself during operation to achieve self-healing from failures of single or multiple cells and self-balancing from cell state variations. Compared to existing active balancing circuits, the number of balancing components, such as inductors, capacitors, and switches, in the proposed design is significantly reduced. This reduces the cost, complexity, and control effort of the total battery system.

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

Common multiple cell configurations for Li-Ion cells in battery packs consist of three or four cells in series, with one or more cells in parallel. This combination gives both the voltage and power necessary for Portable Computer, medical, test and industrial applications. While these configurations are common today, they may not be as efficient as they could be. The reason for this is that any capacity mismatch between cells in a series connection of cells results in a reduction of overall pack capacity.

There are two kinds of mismatch in the pack, State-of-Charge (SOC) and capacity/energy (C/E) mismatch. Each problem limits the pack capacity (mAh) to the capacity of the weakest cell. It is important to recognize that the cell mismatch results more from limitations in process control and inspection than from variations inherent in the Lithium Ion chemistry. As such, these types of cell to cell variation more likely occur in Li-Ion prismatic cells, due to more extreme mechanical stresses, and in Li-Ion Polymer, due to the newer processes involved.

The use of cell balancing can improve the performance of series connected Li-Ion Cells by addressing both State-of-Charge and Capacity/Energy issues. SOC mismatch can be remedied by balancing the cell during an initial conditioning period and subsequently only during the charge phase. C/E mismatch can be remedied by balancing during both charge and discharge periods. Even though the defect level for a given cell manufacturer may be very low, the extra confidence that a pack will not be returned, because of an early end of life, may be well worth the extra effort. It provides another level of quality assurance.

To provide some insight into the issues, this article discusses cell balancing, offers some guidelines for implementing cell balancing, and provides some actual results achieved through cell balancing techniques.

Definition of Cell Balancing:

For portable systems requiring 6V or more of operating voltage, battery packs utilize battery cells connected in series. A series connection results in a pack voltage equal to the sum of the cell voltages. For portable computers (PCs), the battery pack typically has 3 or 4 cells in series with nominal voltages of 10.8V or 14.4V. In the majority of these applications, the system requires more energy than is provided by a single series string of battery cells. Since the largest cell typically available (i.e. 18650) has a capacity 2000mAh, a PC requiring 50-60 Whr. of energy (5000-6000 mAh) requires three cells connected in parallel to each of the series cells.

Cell balancing is defined as the application of differential currents to individual cells (or combinations of cells) in a series string. Normally, of course, cells in a series string receive identical currents. A battery pack requires additional components and circuitry to achieve cell balancing.

Cell balancing is only considered when multiple cells in a battery pack are connected in series and usually when there are three or more series cells. Battery pack cells are balanced when all the cells in the battery pack meet two conditions.

1. If all cells have the same capacity, then they are balanced when they have the same relative State of Charge (SOC.) SOC is usually expressed in terms percent of rated capacity. In this case, the Open Circuit Voltage (OCV) is a good measure of the SOC. If, in an out of balance pack, all cells can be differentially charged to full capacity (balanced) then they will subsequently cycle normally without any additional adjustments. This is mostly a one shot fix. The customer usually has instructions with a new pack to provide an overnight conditioning on the first cycle. Overnight conditioning typically consists of one complete discharge, followed by one complete charge cycle. Conditioning the pack overnight reduces the demands on the cell balance circuitry by minimizing the load and maximizing the charge time.
2. If the cells have different capacities, they are also considered balanced when the SOC is the same. But, since SOC is a relative measure, the absolute amount of capacity for each cell is different. To keep the cells with different capacities at the same SOC, cell balancing must provide differential amounts of current to cells in the series string during both charge and discharge on every cycle. Since charge and discharge cycles times can be shorter than the initial charge time, this process demands higher currents. Therefore, it is a much more demanding issue.

When the cells in the battery pack are not balanced, the battery pack has less available capacity. The capacity of the weakest cell in the series string determines the overall pack capacity. In an unbalanced battery pack, during charging, one or more cells will reach the maximum charge level before the rest of the cells in the series string. During discharge the cells that are not fully charged will be depleted before the other cells in the string, causing early under voltage shutdown of the pack.

Manufactured cell capacities are usually matched within 3%. If less than optimal Li-ion cells are introduced in to a series string pack or cells have been on the shelf for a long period prior to pack assembly, a 150mV difference at full charge is possible. This could result in a 13-18% reduction in battery pack capacity.

Capacity/Energy Maximization

The term Capacity/Energy Maximization refers to the setting of all series cells in a pack to the same SOC even though they have different capacities. Managing the SOC at all times, maximizes the energy output of the pack.

To maximize energy output, the cells must all be fully charged. That is, the SOC of all cells is 100%. If the cells have different capacities, some cells will charge and discharge more than others. Consider the case where, in a 3 cell series stack, C1>C2=C3. The only way to balance the pack is to apply a differential charging current (cell balancing) to the cell with higher capacity (C1). The same has to be done as the pack discharges. If not done, then the total pack capacity diminishes because the pack discharge shuts off when the cell with the least capacity reaches its cut-off voltage. More important, in this case, is that the weakest cell reaches cut-off while capacity remains in the other cells. Over time, this degrades the weakest cells faster than the others, thus accelerating capacity loss over many charge/discharge cycles.

By matching the voltage of each cell in the string on the way down, more current is drawn from the higher capacity cells. During discharge this process requires that there is some additional voltage loss while balancing, but in the end, when all cells arrive at zero SOC, there is an increase in the total energy extracted from the pack over time.

Cell Manufacturing State of the Art with respect to Balancing

Li-ion cylindrical cells today generally have very good quality control, with cell capacity matched to ±3 percent. The input capacity is essentially exact and being within a few milli-ampere seconds. So the absolute amount of charge (mAh) is initially exact and the SOC is within a few percent.

Li-Ion self-discharge and the need for SOC matching

Li-Ion Chemistry does not create cell imbalance and has no mechanism for reversible self-discharge. On the other hand, there is a process by which the cell chemistry stabilizes and creates what is known as irreversible loss. Most of this loss is experienced inside the factory. Since these cells are rejected in the factory, gross errors are not observed by the user. However, a small amount of irreversible loss can be experienced in high temperature discharge and over long periods of time at room temperature. The maximum amount tends to be less than 10 percent and all cells stored or used together will experience this effect at the same rate. So cell imbalance is not created by this mechanism. There is a small reversible loss associated with this irreversible loss but it is proportional. The amount of reversible loss created in the factory is recharged before capacity sorting. Therefore, loss is the field is very small and consistent from cell to cell. Again it is no cause for cell imbalance.

Another source of pack cell imbalance occurs when cells remain on the shelf for long periods of time prior to assembly. This is compounded if cells from different manufacturing lots are mixed within a pack. In this case, cells with varying reversible losses will have had time to accumulate significant differences after capacity sorting operations, but before pack assembly. Even after assembly, imbalances can develop over time within the pack, if cells within a pack vary significantly” especially if the pack remains “on the shelf” for a significant period.

Capacity Degradation

Capacity degradation is a loss of cell capacity due to manufacturing or process variations. The likelihood of production cells containing these anomalies is quite low and cells made at the same time generally perform the same. As a result, the vast majority of cells will not tend to become unbalanced in a pack situation. However, a small number of random imperfections can occur in cells. When this is coupled with a wide variation in pack operating temperature, the result is that some cells can degrade faster than others. As an example, consider a pack that covers a considerable area of a PC. The pack is large enough that some areas of the pack are in close proximity to the power supplies or CPU. These localized areas are subjected to more heat than cells in other parts of the pack. This problem may be remedied with cell balancing, but the balancing must be done during both charge and discharge in a process called capacity/energy maximization. This process is difficult to implement in practice and is not intuitively obvious. For these reasons, further discussion of C/E maximization are left to a subsequent study.

Keeping Cells Balanced

Monitoring cell voltages and temperature allows the tracking of cell voltage variations. When a cell voltage variation is larger than 10 mV (or some other value) circuitry enables a balancing routine that matches the voltages of the cells in battery pack.

The balancing routine operates by “shunting” current around the cell that needs balancing (the highest voltage cell). Typically a series combination of a power transistor and current limiting resistor connect parallel to each series cell in the pack to control the balancing. During a charge operation, turning on the power transistor diverts part of the charge current around the cell so it will charge at a slower rate than the other cells in the pack. During discharge, turning on the power transistor increases the effective load on the cell so it discharges faster than the rest of the cells in the pack. Thus it is possible to balance the cells during charge mode, or discharge mode, or both.

What is the solution?

There are numerous issues with the current techniques to keep cells balanced.

Balancing cells during a discharge phase:

1. Balancing during the discharge phase dissipates power that is not utilized. However, when done during a conditioning cycle this power loss does not impact the operating time of the system. This is more a problem if cell balancing is attempted during discharge while the system is operating.
2. It takes too long to balance a cell during discharge. Because the discharge rate is dependent on the value of the load resistance, balancing during system operation is less effective.
3. To balance during discharge requires an external power transistor with low on resistance if balancing needs to be done in a reasonable time. This is not a significant issue because low on resistance transistors are common and called MOSFETs or FETs.
4. Fast balancing during discharge requires low Ohm resistor in series with the power transistor to reduce power dissipation of the power transistor. Without this current limiting resistor, the transistor would effectively short out the battery. With an FET on resistance of 100mOhm (not uncommon) and a cell voltage of 4V, the transistor would dissipate 160Watts” quickly turning the transistor into a “blown” fuse.
5. Using a low ohm resistor requires a large wattage device. This takes board area and increases cost. In one of the examples, the resistor needs to dissipate .42W. In order to minimize the heat generated and the stress on the resistor, a 2W resistor should be used. Ideally, cell balancing would require less current, requiring lower wattage resistors. But this power dissipation can also be handled using multiple resistors located at places in the pack where heat dissipation is greatest.

Balancing cells during charge phase:

1. Measuring the cell voltage during charge is not accurate and can lead to premature cell balancing. This requires that the charging be stopped periodically, so cell voltages can be measured.
2. Voltage spikes on the output of the charger can result from voltage switching and inductive resonance of the charger. These unexpected conditions can affect cell balancing through inaccuracies in measurements and variations in cell balancing current.
3. Balancing during charge also requires an external power transistor with low on resistance to divert current from the cell to be balanced. This leads to the same limitations as balancing during discharge, however balancing during charge is usually to correct for soft shorts, so required balancing current is less.
4. Due to the low resistance of the unbalanced cell, it is not possible to divert all the charge current around the unbalanced cell. Therefore, some amount of current will flow through the unbalanced cell but at a lower rate than through the other balanced cells in the battery pack. This requires cell balancing to start at a lower voltage to allow enough time to balance the cells within the standard Li-ion cell safety limits.

Recommendations for Cell Balancing:

The recommended way to implement cell balancing is to perform balancing is to provide a conditioning cycle on initial pack charge that balances during discharge and charge. Subsequently, balancing is only performed during charging.

When balancing during charge, the controller in the battery pack controls the current applied from the charger, likely through management of the charge control FET in the pack. It is recommended that the charger be able to deliver a pulse of current for a relatively short duration. The battery pack voltage and cell voltages are measured during a rest time between the current pulses. If a mismatch is developed between the cells, then the balancing power transistor is turned on and the unbalanced cell will be charged with less current. During the next rest period the cells will be measured again and if balancing is reached the power transistor will be turned off.

The duration of the current pulse can vary. If the battery is fully discharged, the current charging period can last longer and the measurement period between current charging can be less frequent. As the battery voltage increases and the battery gets closer to the full charge limit, the current pulse duration decreases and the frequency of battery voltage measurement increases. If any cell in the battery pack reaches an over voltage level during charge time before the cells are balanced, then a period of discharge must take place by extending the rest period to allow the particular battery pack cells to stay in the safe zone. In this case several cycles of charge and discharge cycles should take place until all the cells are equalized.

Improving the control on the charger such that the current that the charger delivers can be increased or decreased is very important. If the temperature during charge exceeds a pre-assigned level, the battery pack can request less current from the charger to help reduce the temperature while still charging the battery but at a lower rate.

Measuring the voltage of each cell provides monitoring of the cell balancing operation. It is important that the load be constant during the monitoring period, so the current consumed by the battery pack does not change while the voltage for each cell is monitored.

Balancing the cells during discharge in a conditioning operation is a simpler process. The controller in the battery pack removes the load, by controlling the pack discharge FET. The controller then turns on the transistor across the cell with the highest voltage. When this voltage matches the others, the transistor is turned off. Trying to balance the cells during discharge while the system is operating is more difficult, because of varying loads. Changing loads affects the rate of balance and the accuracy of cell voltage and pack current measurements.

As described above, there are a number of factors that go into the cell balancing algorithm. One implementation of this algorithm was developed by PoweReady, Inc. in San Diego. In their implementation, PoweReady uses a microcontroller and the Xicor X3100 to monitor the individual cell voltages and controls the pack charge/discharge FETs and the cell balancing FETs. Since the microcontroller is flash-based, changes to the cell balancing algorithm are possible even after pack assembly. These changes can incorporate new balancing parameters or compensate for variations in the cell chemistry.

Pack Balancing Electronics

In order to implement cell balancing, the battery pack needs a number of components. It must be able to:

• Accurately monitor the voltage on each cell independently. This requires differential amplifiers that can operate with a supply voltage of up to 20V, have an input differential range of between 2 and 4.5V and have an accuracy of better than 10mV.
• Determine the highest voltage cell and the voltage differential between all the cells. This requires a number of hardware comparators or a computer controlled comparison mechanism (A/d converter plus software algorithm.)
• Make decisions on the cell voltage differences and initiate cell balancing. This is either a microcontroller, or a hardware state machine.
• Control cell balancing FETs. These FETs are generally individual discrete components. The control signal needs to be able to provide gate voltages that range between 3V and 17V.

These components must be integrated as much as possible to avoid a significant increase in the cost of the battery pack electronics. The Xicor X3100 Safety/Monitor IC shows an example of one way of integrating these components. This device provides a level shifter and a monitor on each cell voltage. The monitors are differential op amps. An analog multiplexer allows a microcontroller (with built-in A/D converter) to read each of the cell voltages. Through software on the IC, the pack can determine the balancing error and implement corrective actions. The X3100 also provides the FET drivers to eliminate the need to add level shifting circuits to drive high voltage from the 5V powered microcontroller.

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 favourable 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.

Summary
Cell balancing is not only important for improving the performance and life cycles of the battery, it adds an element of safety to the battery. One of the emerging technologies for enhancing battery safety and extending battery life is advanced cell balancing. Since new cell balancing technologies track the amount of balancing needed by individual cells, the usable life of battery packs is increased, and overall battery safety is enhanced.

Conclusion:

Cell balancing can increase the useable capacity of the Li-Ion systems when there is charge loss or capacity loss within a series string. Cell balancing then improves the life of the battery pack.

Adding cell balancing is now possible with new devices that integrate elements of the cell balancing control to reduce the cost. Devices like the X3100, from Xicor, integrate the voltage monitor circuits and FET control elements and allow an external microcontroller to evaluate the need for cell balancing. The microcontroller in the pack then uses level shifters to control the FETs according to the specific algorithm. Using an algorithm programmed into a flash-based microcontroller provides the ability to change the cell balancing operation even after pack assembly. This approach gives the battery pack designer the ability to use the latest cell technology and provides the flexibility to create a battery pack to meet the precise needs of the system.