Week 8 Multi cell Battery Pack

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 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.

While charging, most typical battery chargers detect full charge by checking whether the voltage of the entire string of cells has reached the maximum voltage or 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.

For eg, if the weak cell (cell no:2) has reached the upper cut off limit while charging, the overvoltage protection circuit is enabled and the charging is disconnected while the other cells in the string have not reached its upper limit. So the full utilisation of capacity of other cells are limited by the weak cell. If there is no overvoltage protection, the weak cell will be overcharged and as a result in long run, it fails

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 (cell no 2) 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.

Because of these, the BMS employs cell balancing as a control measure to avoid the overcharge or over-discharge of battery cells and this balancing must take place while the cells are charging or discharging. Cell balancing is done to ensure that the delivered energy to the cells during the charging process is maximized, and the energy released from the cells during the discharging process is also maximized. The EVs battery pack contain a series of multi-cell batteries in order to achieve higher operating voltages. The high number of cells in a battery pack, are subject to failure and worse still this will affect reliability since higher failure rate is likely to take place than in the case of a single battery cell. This implies that when the cells are many, the chances of failure are higher which means that if there is n battery cells in a pack, failure of the battery is equivalent to n times the failure rate of an individual cell. Due to the variances in battery cells, charge and discharge occur at different rates. This results in a non-uniform voltage across the battery cells in series in the pack. This phenomenon can make cells to overcharge or undercharge respectively yielding to decrease in battery pack lifetime, safety hazards, and loss of pack capacity over time. To safeguard batteries from these and many other unforeseen issues, cell balancing or cell equalization technique is employed by the BMS to ensure that there is uniform charge level among different cell by basically dissipating excess charge as heat, or transferring the excess charge to the less charged battery cells.

Cell Balancing

Cell balancing is a technique in which voltage levels of every individual cell connected in series to form a battery pack is maintained to be equal to achieve the maximum efficiency of the battery pack. When different cells are combined together to form a battery pack it is always made sure that they are of the same chemistry and voltage value. But once the pack is installed and subjected to charging and discharging the voltage values of the individual cells tends to vary. This variation in voltage levels causes cell unbalancing which will lead to one of the following problems

1. Thermal Runaway: as the weak cell is over charged, the temperature of the cell increases and ultimately it leads to thermal runaway.
2. Cell Degradation: When a lithium cell is overcharged even slightly above its recommended value the efficiency and life cycle of the cell gets reduced.
3. Incomplete charging of Pack: As the batteries in a pack get older few cells might be weaker than its neighboring cells. These week cells will be huge problem since they will charge and discharge faster than a normal healthy cell. While charging a battery pack with series cells the charging process should be stopped even if one cell reaches the maximum voltage. This way the if two cells in a battery pack get week they will charger faster and thus the remaining cells will not be charged to it maximum as shown below.

4.Incomplete use of Pack energy: Similarly in the same case when the battery pack is being discharged, the weaker cells will discharge faster than the healthy cell and they will reach the minimum voltage faster than other cells. The pack will be disconnected from load even if one cell reaches the minimum voltage. This leads to the unused capacity of the pack energy as shown below.

A fresh battery pack will always have balanced cells. But as the pack is put into use the cells get unbalanced due to the following reasons.

1. SOC Imbalance: Measuring the SOC of a cell is complicated; hence it is very complex to measure the SOC of individual cells in a battery. An ideal cell balancing technique should match the cells of same SOC instead of the same voltage (OCV) levels. But since it is practically not possible cells are matched only on voltage terms when making a pack, the variation in SOC might lead to change in OCV in due course.
2. Internal resistance variation: It is very hard to find cells of the same Internal resistance (IR) and as the battery age the IR of the cell also get changed and thus in a battery pack not all cells will have the same IR. The IR contributes to the internal impedance of the cell which determines the current flowing through a cell. Since the IR is varied the current through cell and its voltage also gets varied.
3. Temperature: The charging and discharging capacity of the cell also depends on the temperature around it. In a huge battery pack like in EVs or solar arrays the cells are distributed over a waste areas and there might be temperature difference among the pack itself causing one cell to charge or discharge faster than the remaining cells causing an imbalance.

From the above reasons it is clear that we cannot prevent cell from getting imbalanced during the operation. So, the only solution is to use an external system that forces the cells to get balanced again after they get unbalanced. This system is called the Battery Balancing System. There many different types of hardware and software techniques used for battery cell balancing.

Cell Balancing Techniques

Cell balancing techniques can be broadly classified into the following the four categories which are listed below.

1. Passive Cell Balancing
2. Active Cell Balancing
3. Lossless Cell Balancing
4. Redox Shuttle

Passive Cell Balancing

Passive cell balancing method is the simplest method of all. It can be used in places where cost and size are major constraints. The following are the two types of passive cell balancing.

1. Charge Shunting: In this method a resistor connected across the cell is used to discharge the excess voltage and equalize it with other cells. These resistors are called as bypass resistors or bleeding resistors. Each cell connected in series in a pack will have its own bypass resistor connected through a switch as shown below.

The circuit above shows four cells each of which is connected to two bypass resistors through a switch like MOSFET. The Controllers measures the voltage of all the four cells and turns on the MOSFET for the cell whose voltage is higher than the other cells. When MOSFET is turned on that particular cell begins to discharge through the resistors.. The capacitor connected in parallel with the cell is used to filter voltage spikes during switching. This method is not very efficient because electrical energy is dissipated as heat in the resistors and the circuit also accounts of switching losses. Another drawback is that the entire discharge current flows through the MOSFET which is mostly build into the controller IC and hence the discharge current has to be limited to low values which increases the discharging time. One way to overcome the drawback is to use an external switch to increase the discharge current as shown below

The internal P-channel MOSFET will be triggered by the controller which causes the cell to discharge (I-bias) through the resistors R1 and R2. The value of R2 is selected in such a way that the voltage drop occurring across it due to the flow of discharge current (I-bias) is enough to trigger the second N-channel MOSFET. This voltage is called the gate source voltage (Vgs) and the current required to bias the MOSFET is called as biasing current (I-bias). Once the N-channel MOSFET is turned on the current now flows through the balancing resistor R-Bal. The value of this resistor can be low allowing more current to pass though it and thus discharging the battery faster. This current is called as drain current (I-drain). In this circuit the total discharge current is the sum of drain current and bias current. When the P-channel MOSFET is turned off by the controller the biasing current is zero and thus the voltage Vgs also gets zero. This turns off the N-channel MOSFET leaving the battery to get ideal again.

2. Charge Limiting

The charge Limiting method is the most inefficient method of all. Here only the safety and life time of the battery is considered while giving up on the efficiency. In this method the individual cell voltages are monitored continuously. During the charging process even if one cell reaches the full charge voltage the charging is stopped leaving the other cells half the way. Similarly, during discharging even if one cell reaches the minimum cut-off voltage the battery pack is disconnected from the load until the pack the charged again. Although this method is inefficient it reduces the cost and size requirements. Hence it is used in an application where batteries could be often charged.

Active Cell Balancing

In Passive cell balancing the excess charge was not made used of, hence it is deemed to be inefficient. Whereas in active balancing the excess charge form one cell is transferred to another cell of low charge to equalize them. This is achieved by utilizing charge storing elements like Capacitors and Inductors. There are many methods to perform Active cell balancing lets discuss the commonly used ones.

1. Charge Shuttles (Flying Capacitors)

This method utilizes capacitors to transfer charge from high voltage cell to low voltage cell. The capacitor is connected through SPDT switches initially the switch connects the capacitor to the high voltage cell and once the capacitor is charged the switch connects it to the low voltage cell where the charge from the capacitor flows into the cell. Since the charge is shuttling between the cells this method is called as charge shuttles.

These capacitors are called the flying capacitors since the fly between the low voltage and high voltage cells carrying chargers. The drawback with this method is that charge can be transferred only between adjacent cells. Also it takes more time since the capacitor has to be charged and then discharged to transfer the charges. It is also very less efficient since there will be loss in energy during the charging and discharging of the capacitor and the switching losses also have to be accounted. The below image shows how the flying capacitor will be connected in a battery pack

2. Inductive converter (Buck Boost method)

Another method of Active cell balancing is by using inductors and switching circuits. In this method the switching circuit consists of a buck boost converter. The charge from the high voltage cell is pumped in the inductor and then discharged into the low voltage cell by using the buck boost converter. The below figure represents an Inductive converter with only two cells and single buck boost converter.

In the above circuit charge can be transferred from cell 1 to cell 2 by switching the MOSFETS sw1 and sw2 in the following manner. First the switch SW1 is closed this will make the charge from cell 1 to flow into the inductor with current I-charge. Once the inductor is fully charged the switch SW1 is opened and the switch sw2 is closed. Now, the inductor which is fully charged will reverse its polarity and begin to discharge. This time the charge forms the inductor flows into the cell2 with current I-discharge. Once the inductor is fully discharged the switch sw2 is opened and the switch sw1 is closed to repeat the process.

During the time t₀ the switch sw1 is closed (turned on) which leads to the current I charge to increase and the voltage across inductor (V_L) to increase. Then once the inductor is fully charged at time t₁ the switch sw1 is opened (turned off) which makes the inductor to discharge the charge that it accumulated in previous step. When a inductor discharges it changes its polarity hence the voltage V_L is shown in negative. When discharging the discharge current (I discharge) decrease from its maximum value. All this current enters the cell 2 to charge it up. A small interval is allowed from time t₂ to t₃ and then at t₃ the whole cycle repeats again. This method also suffers from a major disadvantage that charge could be transferred only from higher cell to lower cell. Also the loss in switching and diode voltage drop should be considered. But it is faster and efficient than the capacitor method.

3. Inductive converter (Fly back based)

The buck boost converter method could only transfer charges form the higher cell to the lower cell. This problem can be avoided by using a Fly back converter and a transformer. In a flyback type converter the primary side of the winding is connected to the battery pack and the secondary side is connected to each individual cell of the battery pack as shown below

The battery operates with DC and the transformer will have no effect until the voltage is switched. So to begin the charging process the switch on the primary coil side Sp is switched. This converts DC to pulsed DC and the transformer primary side is activated. Now on the secondary side each cell has its own switch and the secondary coil. By switching the MOSFET of the low voltage cell we can make that particular coil to act as a secondary for the transformer. This way the charge form the primary coil is transferred to the secondary coil. This causes the overall battery pack voltage to discharge into the weak cell. The biggest advantage of this method is that any weak cell in the pack can be easily charged from the pack voltage and not particular cell is discharges. But since in involves a transformer, it occupies a large space and the complexity of the circuit is high.

Lossless balancing

Lossless balancing is a recently developed method that reduces losses by reducing the hardware components and providing more software control. This also makes the system simpler and more easier to design. This method uses a matrix switching circuit which provides the capability to add or remove a cell from a pack during charging and discharging. A simple matrix switching circuit for eight cells is shown below.

During charging process the cell which is of high voltage will be removed from the pack using the switch arrangements. In the above figure the cell 5 is removed from the pack by using the switches. Consider the red line circles to be open switches and the blue line circle to be closed switches. Thus the rest time of the weaker cells are increased during the charging process so as to balance them during charging. But the charging voltage has to be adjusted accordingly. The same technique can be followed during discharging also.

Redox Shuttle

The final method is not for hardware designers but for chemical engineers. In lead acid battery there is no problem of cell balancing because when a lead acid battery is overcharged it causes gassing which prevents it from getting over charged. The Redox shuttle try for achieving the same effect on lithium cells by altering the chemistry of the electrolyte of the lithium cell. This modified electrolyte should prevent the cell from getting overcharged.