At present, the performance of battery equalizer is uneven in balance speed, balance efficiency and scalability, and often neglects one thing and loses another. Although the peak of the balance efficiency reached 94.5%, the value was obtained under certain experimental conditions. When the experimental conditions changed, the value would decrease. The balance speed slows down with the increase of the number of series batteries because it is seriously affected by the number of series batteries. In addition, the hardware and parameters of the equalizer are heavily dependent on the voltage level of the battery system. When the number of series batteries changes, the hardware and parameters of the equalizer need to be re selected and calculated, which limits the scope of application of the equalizer. In view of the problems existing in the current equalizer, a modular high-performance battery equalizer while taking into account the high balance efficiency and fast balance speed is proposed in this work. When the number of series batteries increases, only the number of equalizer modules needs to be increased, the hardware and parameters of the equalizer remain unchanged, and the equalizer still has high balance efficiency and fast balance speed.
The following measures are taken to improve the balance efficiency: First, the shortest energy path is realized, that is, an energy path contains only one switching device, and the energy path is the shortest. Second, a new balance object with high voltage value is constructed, so that the proportion of the turn-on voltage drop of the switching device is reduced. To sum up, a layered balance strategy for different balance objects. The first layer balance takes single battery as the balance object, and uses the shortest energy path. The second layer balance takes the battery unit (new balance object) as the balance object, and uses the shortest energy path. In addition, the first layer balance adopts complementary PWM control with dead zone, which further improves the balance efficiency.
A multi-objective parallel balancing strategy is proposed on the basis of the layered balance in order to effectively improve the balance speed. The number of parallel balancing targets directly determines the balancing speed. The number of parallel balance targets reaches the maximum, that is, the number of balance targets is equal to the number of series balance objects. The number of parallel balance targets in the first layer balance, reaches the number of single batteries in series. The number of parallel balance targets in the second layer reaches the number of battery units in series. The balance speed is fast and not affected by the number of series batteries because the number of parallel balance targets reaches the maximum.
The equalizer adopts modular design, which determines the scalability of the equalizer and the stability of its performance parameters and hardware parameters after expansion. The performance parameters include balance efficiency and balance speed. The hardware parameters include all the hardware and parameters of the equalizer, among which the most important is the rated voltage parameters of the switching devices. Because of the same structure and independent equalization modules in each layer, the shortest energy path, and the maximum number of parallel balance targets, the equalizer has high balance efficiency and fast balance speed, and these superior performances are not affected by the number of series batteries. The selection of the rated voltage of the switching devices in the equalizer is independent of the voltage level of the entire battery system. Therefore, when the scale of the battery system increases, only the number of balance modules needs to be increased. Once the rated voltage parameters of the switching devices are determined, they will remain unchanged. The equalizer is easy to expand and has a wider application range because of the modular design.
The experimental platform with eight series batteries is built for balance experiments, which can verify the advantages of the equalizer in the three aspects of balance efficiency, balance speed and modularity. In the balance experiment, the number of balance targets reaches the maximum, the balance efficiency reaches 95.1% and 97%, the rated voltages of the switching devices are 20V and 40V, and the rated voltage parameters are fixed and are independent of the battery system voltage level. The balance experiment proves that the balance efficiency is high and the numerical value is stable, the balance speed is fast and is not affected by the number of series connected batteries. The modular design makes the equalizer easy to expand. When the scale of the battery system becomes larger, only the number of balance modules needs to be increased, and the hardware and parameters of the equalizer remain unchanged.
In the equalization experiment, the number of equalization targets is maximized, the equalization efficiency is 95% and 97%, the rated voltage of the switching device is 20V and 40V, and the rated voltage parameters are fixed. The equalization experiment proves that the equalization efficiency is high and the numerical value is stable, the equalization speed is fast and is not affected by the number of series connected single batteries. The modular design makes the equalizer easy to expand. When expanding the use, only the number of equalizer modules needs to be increased, and the hardware and parameters of the equalizer remain unchanged.