Multi cell Battery Pack
AIM: Multi-cell Battery Pack
OBJECTIVE: -*
1. How weakest cell limits the usable capacity of the battery pack?
2. Explain the cell balancing techniques with relevant figures.
OBJECTIVE 1 - How weakest cell limits the usable capacity of the battery pack?
The weakest cell limits the usable capacity of the battery pack, which is called an imbalance of the battery pack. To avoid that we need to balance the battery pack. Common multiple cell configurations for Li-Ion cells in EV/HEV battery packs consist of 90 to 100 cells are arranged in the series and 4 to 10 cells are in parallel. 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 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.
No two cells are identical. There is always a slight difference in the state of charge self-discharge rate, capacity, impedance, and temperature characteristic, even for cells with a similar model from the same manufacture and even for the same batch of production. However, variations in an individual cell’s impedance, capacity, and self-discharge rate can still lead to a divergence in its voltage over time. Since most 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 do not exceed the limits for overvoltage (OV) protection. These cells are weakened further by continuous overcharge cycles. The higher voltage of we
On the other hand, in discharge, the weak cells tend to have lower voltage than the other cells, due to either higher internal resistance or the faster rate of discharge that results from their smaller capacity. This means that if any of the weak cells hit the cell Undervoltage-protection limit while the pack voltage is still sufficient to power the system, the full capacity of the battery will not be used.
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, maybe we'll be worth the extra effort. It provides another level of quality assurance.
OBJECTIVE 2 - Explain the cell balancing techniques with relevant figures.
Defination - 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.
Types of Battery Cell Balancing:
Cell balancing techniques can be broadly classified into the following the four categories which are listed below.
- Passive Cell Balancing
- Active Cell Balancing
- Lossless Cell Balancing
- Redox Shuttle
1. 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.
Charge Shunting
In this method a dummy load like a resistor 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 sample 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. Since we know the value of resistors we can predict how much charge is being dissipated by the cell. 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. 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.
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. The below figure should help you understand better 
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
3. 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.
4. Redox Shuttle:
The final method is not for hardware designers but for chemical engineers. In lead acid battery we do not have the problem of cell balancing because when a lead acid battery is overcharged it causes gassing which prevents it from getting over charged. The idea behind Redox shuttle is to try 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.
CONCLUSION
Here we have studied the cell balancing of the battery pack. Seen various methods for the balancing of the cell for a better battery pack performance.