We are going to discuss the cell imbalance, reasons for cell imbalance, and techniques for cell balancing.
A group of cells will be connected together to have a higher voltage and capacity for the batteries used in many applications. An electric vehicle battery voltage is typically from 200 to 800 Volts.
Electric Rickshaws and Scooters have less voltage batteries.
A number of 4.2 Volt Li-ion cells are connected together to form a high voltage battery pack. If they are connected in series, it adds up the voltage. The parallel connection increases the capacity of the battery keeping the voltage the same.
The process of keeping the voltage of individual cells in a battery equal, to achieve the maximum out of the battery pack.
Reasons for Cell Imbalance
Why do the cells of a battery have different Voltage? The following are the main reasons why the voltages of cells in a battery pack are different.
- Differences during manufacturing
- Internal resistance variation
- Temperature distribution
- Different initial capacities
- Aging effect
- The difference in the rate of self-discharge
#1 Difference during the manufacturing
A minor change in the capacity of the battery, internal resistance, thermal characteristics during the manufacturing affects the balance of the cells.
#2 Internal Resistance Variation
The internal resistance of the cell depends on many factors such as temperature, cell chemistry, and cell voltage. The difference in internal resistance results in cell imbalance.
#3 Temperature distribution
Li-ion batteries are highly sensitive to the temperature. The charging and discharging of the battery emits heat energy. If the temperature distribution is uneven, it affects the internal resistance of the battery.
The temperature affects the rate of chemical reactions in a battery. If the reaction rate of the cells is different, it imbalances the battery.
#4 Different initial capacities
Slight variations in the initial capacity of the battery during manufacturing results in cell imbalance. Once one cell is completely charged, the BMS stops the charging of the battery.
#5 Aging effect
As the battery ages, the capacity fades. The rate of capacity fade might not be the same in all the cells. Cell imbalance is the result!
The irreversible reaction happens at the electrodes and it accumulates at the electrode-electrolyte interface.
This compound is called the Solid Electrolyte Interface (SEI). While manufacturing the battery, a thin SEI is usually formed and this prevents the exposure of the active lithium ions and the electrode material to the electrolyte.
However, as time goes by, the SEI begins to grow and interact with more electrode material and lithium ions to progressively degrade the battery.
#6 The difference in rate of self discharge
Self–discharge. Self–discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes or any external circuit. [Reference]
The self-discharge rate will not be equal in all the cells. So there are possibilities of cell imbalance in a Li-ion battery pack.
The above effects results in different capacities and SoCs for the cells.
How does the unbalanced cells affect the performance of the battery?
The variation of the capacity of each cell results in the following in a Li-ion battery.
#1 Thermal runaway
Li-ion batteries are highly sensitive to temperature. The difference in the voltages of cells causes overcharging of some cells. The cells will not be charging at the recommended voltage due to the imbalance. The possibility is more when the cells are connected in series. Finally, there are chances of a thermal runaway of Li-ion battery.
#2 Battery Degradation
Another problem caused by unbalanced cells is battery degradation. If one or two cells in the battery pack have less capacity (due to imbalance), the charging results in high voltage on those cells. Overcharging causes faster degradation of the battery.
It’s a cumulative effect.
The degradation causes overvoltage during charging and it further degrades the cell.
#3 Incomplete Charging of Battery Pack
If one or two cells in a battery pack lose their capacity, it will not allow the whole battery pack to completely charge. The battery management system disconnects the battery from the charging circuit once at least one of the cells gets fully charged.
#4 Incomplete discharge of the battery
The battery would not discharge completely if the capacity of a cell is lesser than that of all other cells. The lower capacity cells discharge faster and prevent the further operation of the battery pack. So we would not get the maximum out of the battery.
How to balance the cells?
Cell unbalance is not easy to prevent. The only method to get rid of the disadvantages of an unbalanced cell is external cell balancing.
The cell balancing techniques are categorized as follows.
- Passive cell balancing
- Active cell balancing
- Lossless cell balancing
- Redox Shuttle
#1 Passive Cell Balancing
The simple and less expensive method for cell balancing is passive cell balancing. Complex algorithms are not being used in passive cell balancing. The following are some of the different types of passive cell balancing. But these methods are less efficient.
#1.1 Charge shunting
Dummy load resistors are connected to all series cells to discharge the excess charge in the battery if necessary to balance the cell in this method. The resistors to dissipate the excess energy is known as bleeding resistors. The resistors are connected to each cell through static switches such as MOSFET.
The controller measures the voltage across each cell and turns on the switches to discharge those cells that have excess voltage. The resistor dissipates the energy as heat and makes the voltage of all the cells equal.
As we mentioned earlier, this is an inefficient method for cell balancing. The energy of the battery is being dissipated in this method. Another drawback of charge shunting is that the MOSFET carries all the discharge current and it would be limited to prevent it from failure. It increases the discharge time (cell balancing time)
External FET connection would also be provided sometimes in this method to reduce the discharge time making it more current to flow through it.
#1.2 Charge Limiting
Another technique to balance the cells in a Li-ion battery is the charge limiting. This is also an inefficient method for cell balancing.
The charging of the battery stops when at least one of the cells exceeds the voltage limit. All other cells need not be at the maximum SoC level when the charging stops. So we cannot use the battery at its maximum capacity.
The controller continuously monitors the voltage of each cell.
What happens during discharging?
The controller stops the battery discharging when one of the cells reaches the minimum SoC limit. All other cells might not be completely discharged by that time and it limits the optimal use of the battery.
Since this method is inexpensive, it’s commonly used for applications where there are options to charge the battery frequently.
#2 Active Cell Balancing
Active cell balancing is more efficient than passive cell balancing. In this method the excess charge in one cell transfers to another cell that has a low charge to equalize the voltage of the cells.
Inductors capacitors are the commonly used components for active cell balancing. Let’s have a look at the most commonly used active cell balancing techniques.
#2.1 Flying Capacitors
The method that also known as charge shuttles use capacitors to transfer charge from high voltage cell to low voltage cell.
Once the controller detects a mismatch of voltages in the cells, the Single Pole Double Throw (SPDT) switch connects and charges the capacitor from high voltage cell.
Then the switch connects the charged capacitor to the low voltage cell to charge it and equalize the cell voltages.
#2.2 Buck Boost Method
In this method, the charge from the cells having higher voltage transfer to the low voltage by the use of inductors and static switches. MOSFET switches connect and disconnect the inductor connected to the cells and transfers the energy to low voltage cells.
Flyback buck-boost converter cell balancing requires a transformer and it adds up the total expense of the system.
#3 Lossless Balancing
This is a method where the number of hardware components is less hence the loss for cell balancing. A matrix switch circuit helps out here to connect and disconnect individual cells during charging and discharging.
The cells with high voltages get disconnected from the circuit during charging and the cell voltages get equalized. Similarly, it disconnects the low voltage cells from the battery during discharging.
There are some other methods also to do cell balancing. For example the Redox Shuttle technique.
The cell imbalance is not that prominent in other types of battery technologies. So, techniques are being developed to alter the chemistry of Li-ion cells to minimize the cell imbalance.
We discussed the cell balancing of the Li-ion battery. Different techniques for cell balancing were explained here in this post. Balanced cells are essential to get the maximum out of the battery.
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- Cell imbalance happens in Li-ion battery
- Cell balancing helps out to get maximum out of the battery
- There are many techniques for cell balancing