To provide the hundreds of volts required for modern high performance batteries for electric vehicles, it is often necessary to connect several individual battery cells in series. Each battery cell in the battery pack has different battery cell capacity, self-discharge rate, temperature characteristics, and battery impedance, and the difference will increase as the battery ages. When the battery unit is charging, this difference can lead to a situation where some battery units are not yet full of enough power, but others are already full of charge. Unless additional measures are taken, the charging process must be terminated because if a battery unit is overcharged, it can be damaged or even completely destroyed.
A similar situation will also occur during discharge. Contrary to the previous situation, the situation is that a battery unit has been fully discharged, while other battery units still have enough energy to continue to power the car (theoretical). However, it is impossible for the car to continue to walk at this time because it would cause the weaker battery cells to be over-discharged, resulting in damage to the battery cells. In order to avoid the occurrence of these two kinds of situations, active balancing between individual battery cells is necessary.
Passive balancing method converts available energy into heat loss
The currently widely used method is passive balancing technology, which uses a resistor to discharge an already fully charged battery unit again so that other battery units can continue to charge. The disadvantages of this method are obvious:
* For balance purposes, the battery can only be discharged
* The discharge current of the shunt resistor causes power loss
* Valuable energy is converted into heat and cannot power the car
* Reduce the distance traveled by the car
The passive balancing method can only convert the energy stored in the battery unit into heat loss, while the active balancing can transfer the charge in one battery unit to another. There are several ways to implement charge transfer, such as using switched capacitors or inductors. When using the capacitive method, the capacitor is connected in parallel with the battery unit with a higher voltage. Once the battery unit is fully charged, it is in parallel with the lower voltage battery unit and can continue to charge it. This process is repeated until all cells reach the same voltage.
The method of using capacitors is highly cost-effective, but has the disadvantage that the average balancing current is limited to less than 50 mA. This limitation does not exist with the inductive method, and in this case, it is easy to achieve a balanced current of 1 A or more.
Fast and almost lossless charge transfer with active balancing
Active balancing is achieved by shunting inductors and batteries that require charge. This result causes the current in the coil to continue to increase.
Once the coil has been decoupled from the battery cells that are discharged through the transistor, the energy stored in the inductor can charge the adjacent battery through a diode. Thus the charge can be moved back and forth between two separate cells, achieving extremely high efficiency and virtually no loss. This method has some decisive advantages:
* Balance current may reach 1A or above
* Balance is essentially lossless
* Extremely fast balance
* Increased efficiency and battery capacity
* Increased driving distance of the car
Implementing active balancing using inductors is not a low-cost approach compared to the other methods mentioned, as relatively expensive inductor components are used. However, this is not entirely a problem. Modern high-performance batteries currently cost close to $10,000. Using the inductive balancing method, even with an additional 10% capacity, represents a value of $1,000 - this amount can be used to purchase a large number of inductive devices.
For safety reasons, lithium-ion batteries must monitor individual battery cells because they can burn when overloaded and, in extreme cases, even explode. As with overvoltage, undervoltage and temperature monitoring, additional functions such as accurate charging conditions are also required. Elements that enable all of these functions and different balancing methods have been provided in the semiconductor market. Advanced battery cell balancing solutions (such as the Atmel ATA6870 battery management circuit) each battery unit has separate electronic monitoring to provide measurements such as state of charge, active/passive balancing or overvoltage, undervoltage and temperature Monitoring and other functions
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