The principle of active balancing method for lithium battery packs
The active balancing method for lithium battery packs with charge shuttle includes a device that can discharge, store, and then transfer electricity from a specific battery unit to another battery unit. There are currently several charge shuttle schemes, among which the most concerning is the "flying capacitor".
Control the system to close the appropriate switch through battery unit B; Charge capacitor C. After the capacitor is charged, the switch will disconnect. Subsequently, the switch connecting battery unit B2 and capacitor C is closed, and capacitor C charges battery unit B2. The charging amount 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 unit with the highest charge in the battery pack will charge C, and then C will charge the battery unit with the lowest charge. In this way, the battery unit with the highest battery capacity will distribute the power to other low battery units. Implementing this method only requires a fixed sequence of switches to close or open the appropriate switches.
To improve the "flying capacitor" method, it is necessary to achieve intelligent selection of battery cells that require balance. In this way, the capacitor can charge from the battery unit with the highest charge and then transfer the power to the battery unit with the lowest charge. This method can significantly reduce the time it takes for the battery pack to reach equilibrium, especially when the highest and lowest battery cells are located at both ends of the battery pack. And this requires another control system to detect and select the target battery unit.
This scheme requires a high conversion rate (n+5) when capacitor C reaches its peak charging current. For an ideal system with significant battery imbalance (Bn=3.0V, B=4.0V) (capacitors and switches have no impedance), a 1000uF flying capacitor can balance these battery cells at a rate of at least 1Ahr per hour at an average conversion current of 1A and a conversion frequency of 1kHz. If considering the impedance of capacitors and switches, the time constant of system charging and discharging will greatly increase, reducing the actual equilibrium current by an order of magnitude or increasing.
Add the peak value of the switching current. The larger the capacitor used, the longer it takes to complete a usable charge, resulting in a decrease in clock rate and an increase in peak converted current. A large battery pack (100Ahr) requires a charge shuttle device containing a large capacitor with a high conversion current. This will result in a lot of energy being wasted in the form of resistance heat on capacitors and switches. A large portion of power balance is achieved through the simple waste of energy in the form of thermal energy in high battery cells.
Another method of charge shuttle is to share a "flying capacitor" between every two battery cells. The capacitor is continuously connected to two battery cells, allowing the battery to move from a high battery cell to a low battery cell. Each capacitor only requires simple control to activate the switch.
Several charge shuttling blocks can be cascaded to form a battery pack with higher voltage. As battery cells B2... Bn-1 share a flyover capacitor with their adjacent two battery cells, electricity can be transferred from one end of a series connected battery pack to another. If the high and low battery cells are located exactly at the ends of the entire battery pack, this method will spend a lot of time transferring electricity between them, as the remaining electricity will pass through each battery cell and consume a lot of time, resulting in a decrease in balancing efficiency. However, this method has packaging advantages: for each two battery cells, their control circuits, power supply, and capacitors can be integrated. Pack into a separate section. When it is necessary to add battery units, we only need to add this separately packaged part.
The electric shuttle technology is not very useful for hybrid vehicle batteries. The open circuit terminal voltage of lithium batteries is relatively stable when the remaining battery capacity is between 40% and 80%. The voltage of a battery unit at high residual charge is not much higher than at low residual charge, unless the residual charge is as high as 90% or as low as 20% or less.
The hybrid vehicle battery operates in a medium residual state, where the voltage difference between battery cells is minimized, thereby limiting the application of battery shuttle technology.
Electricity converter
Through a power conversion device, the battery is balanced by using an inductor or transformer to transfer electricity from one or part of the battery cells to another or another part of the battery cells. Currently, there are two modes of electricity conversion: switch transformer method and shared transformer method.
The method of switching transformers is to share the same switching topology as the method of flying capacitors. The current I is taken from the entire battery pack and converted at transformer T. The output of the transformer is corrected by diode D and transmitted to battery unit B., The specific transmission to which battery unit is determined by the setting of switch S. In this way, the entire device requires an electronic control device to select the target battery unit and set the switch S.
This method quickly balances low battery cells at the cost of consuming the energy of the entire battery pack. The drawbacks of this method include: complex structure, large number of components, and low efficiency caused by magnetic and switching losses.
The shared transformer has a separate magnetic core that has secondary distribution to each battery unit. The current I of the battery pack mainly varies within the transformer, and then induces current in each secondary component. The strongest induced current will be generated within the secondary component with the smallest reactance (due to the low terminal voltage of the battery unit Ba). In this way, the charging current of each battery unit is inversely proportional to its relative percentage of remaining battery capacity.
The only part of the shared transformer that functions together is the conversion transistor of the main transformer, which does not require additional closed-loop control. The shared transformer device can quickly balance a multi unit battery pack with minimal power loss. The disadvantage of this balancing method is the complex magnetic induction involved and the need for a large number of secondary circuit rectifiers. Moreover, the balancing circuit should be designed to meet the maximum number of battery units, as it is very difficult to increase the number of secondary circuits.
Using a set of primary coil coupling instead of coupling through a single magnetic core can achieve the same effect. The advantage of this method is that each battery cell can have its own magnetic core, which can increase the number of series battery cells without changing the main controller.
This method of sharing transformers is applicable to both electric vehicle batteries and hybrid vehicle batteries. If the current I is set very small (<100mA/Ahr), the balancing efficiency of this device during continuous operation will be higher than any active balancing method.