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Principle of active balancing method for lithium battery pack

by:dcfpower     2021-03-18

The active balancing method of lithium battery pack packs uses an active reciprocating charging element, a voltage or current converter to make the power from one battery cell to another. These devices can be controlled by analog or digital control. The two main categories of active balancing methods for lithium battery pack packs are charge shutting and energy conversion.

Charge Shuttle

The charge shuttle's active balancing method for lithium battery pack packs includes a method that allows power to be discharged from a specific battery cell, stored, and then transferred to Another battery cell device. There are several charge shuttle schemes, the most interesting of which is the 'flying capacitor'.

The control system closes the appropriate switch and passes through the battery cell B; charges the capacitor C. When the capacitor is charged, the switch will open. Then the switch linking the battery cell B2 and the capacitor C is closed, and the capacitor C charges the battery cell B2. The charge capacity depends on the voltage difference between B1 and B2.

Then the capacitor is connected to B3, B1..., Bn, B... in the same way. The battery cell with the highest power in the battery pack will charge C, and then C will charge the battery cell with the lowest power. In this way, the battery cell with the highest power will distribute the power to other low-power battery cells. Only a fixed switching sequence is needed to achieve this way to close or open the appropriate switch.

To improve the 'flying capacitor' method, it is necessary to realize intelligent selection of battery cells that need to be balanced. In this way, the capacitor can be charged from the battery cell with the highest power and then transfer the power to the battery cell with the lowest power. This method can significantly reduce the time for the battery pack to reach equilibrium, especially when the battery cell with the highest power and the battery cell with the lowest power are located at both ends of the battery. This requires another control system to detect and select the target battery unit.

This solution requires a large conversion rate (n+5) when the capacitor C reaches the peak charging current. For an ideal system with large battery imbalance (Bnu003d3.0V, Bu003d4.0V (capacitors and switches have no impedance), the flying capacitor of 1000uF can be at least at 1A conversion current and 1kHz conversion frequency. These battery cells are balanced at a rate of 1Ahr per hour. If the impedance of the capacitor and the switch is considered, the time constant of the system charging and discharging is greatly increased, reducing the actual balancing current by an order of magnitude or increasing.

Add switch The peak current. The larger the capacitor used, the longer it takes to complete a usable charge, so the clock rate decreases and the peak current of the conversion increases. A large battery pack (100Ahr) requires a large battery pack (100Ahr). A large-capacitance charge shuttle device with a large conversion current. In this way, a lot of energy is wasted in the form of resistance heat on the capacitors and switches. A large part of the cell balance is simply wasted by the energy in the high-power battery cells becoming heat energy. Realized.

Another charge shuttle method is to share a 'flying capacitor' for every two battery cells. The capacitor is continuously connected to the two battery cells, so as to realize the power from the high-power battery cell to the low-power battery cell. The movement of power battery cells. Each capacitor only needs simple control to activate the switch.

Several charge shuttling blocks can be cascaded to form a battery pack with a higher voltage. Because of the battery cell B2 ….Bn-1 respectively shares a flying capacitor with two adjacent battery cells, and the power can be transferred from the end of a series-connected battery pack to another battery pack. If the high-power battery unit and the low-power battery unit happen to be located in the whole At both ends of the battery pack, this method will spend a lot of time on the power transfer between them, because the remaining power will pass through each battery cell, which consumes a lot of time and reduces the equalization efficiency. However, this method has packaging advantages. : For every two battery units, their control circuit, power supply and capacitor can be packaged into a single part. When the battery unit needs to be increased, we only need to increase this separately packaged part.

The power shuttle technology is not very useful for hybrid vehicle batteries. The open-circuit battery terminal voltage of lithium batteries is relatively stable when the remaining power is 40% to 80%. The voltage of a battery cell at high remaining power is no better It is much larger when the remaining power is low, unless the remaining power is as high as more than 90% or as low as 20% or less.

The battery of the hybrid electric vehicle is working in the state of medium remaining power, and the voltage between the battery cells at this time The difference is the smallest, which limits the application of power shuttle technology.

Power converter

Through the power conversion device, the battery balances the use of inductance or transformer to transfer power from one or a part of battery cells to Another or another part of the battery unit. At present, there are two power converter modes: switching transformer method and shared transformer method.< /p>

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