Recently, Associate Professor Wang Haoyu’s research group in the Center for Intelligent Power and Energy Systems (CiPES) of SIST has made important progress in the field of electric vehicle (EV) battery management systems. The group has proposed a lithium battery equalization system based on an LCC resonant circuit. The equalization system can realize open-loop constant current equalization from battery string to single battery (string to cell), which provides a new solution to the problem in EVs of power battery voltage mismatch. Relevant research results are published online in IEEE Transactions on Power Electronics, under the title of "An LCC based string to cell battery equalizer with simplified constant current control".
With the rapid penetration of EVs in transportation systems, the onboard traction battery has attracted increased research attention. Due to the application demand of high voltage and high power, the onboard energy storage system usually needs a large number of lithium battery cells in series and parallel. Because of the differences in battery cells, the battery system encounters problems such as mismatch of cell voltages, insufficient charge and discharge, and so on. At present, Tesla and other EV manufacturers primarily use passive equalization technology to dissipate excess power through dissipative resistance. This method causes attenuation of the EV range and leads to heat management problems.
Compared with passive equalizations, active equalizations usually require complex control circuits, and the attenuation of current in the equalization process will affect the equalization speed. To reduce the control complexity of the circuit and improve the battery equalization speed, the researchers systematically studied the output characteristics of LCC resonant converters and compensated for the non-ideal characteristics of lithium batteries in the process of charging and discharging. This work adopts the equalization architecture from string to cell, using the good constant current output characteristics of LCC resonant topology at the resonant frequency point, and the efficient equalization of open-loop constant current control is realized. It can be seen that the proposed equalization technology can effectively ensure the consistency of battery pack operation. Meanwhile, the soft switching technology used in this work can allow the equalizer to achieve 88.1% energy conversion efficiency.
SIST third year Master student Wei Zhengqi is the first author, and Prof. Wang Haoyu is the corresponding author.
Link to this paper: https://ieeexplore.ieee.org/document/9508163
Schematic of the proposed LCC-based S2C equalizer and flowchart of the proposed S2C equalization algorithm
Charging and discharging equalization results for four cells with different distributions