A Lithium Ion Battery Management System With Bilevel Equalization

Climate change has become one of the most important global challenges that both developing and developed nations face in the 21st Century. In the transportation sector, electric vehicles (xEV) have emerged as a viable solution to fight climate change. However, the short longevity of the battery system, and their limited range which depends on the battery performance, remains a drawback . In the electric power sector, renewable energy (solar and wind) have emerged as a strong alternative, but these sources are intermittent and cause fluctuations on the electrical power grid. To solve these issues, renewable energy systems are sometimes coupled with a battery energy storage system (BESS). Most of these large BESS consist of lithium ion batteries because they are becoming more cost effective than other types. However, there is an issue with both of these applications: as the battery ages, the performance of the battery pack is limited because large variations develop between the large number of series connected cells. This requires the use of an electronic equalizer (EQU) to balance the cell voltages. Currently there are two types of equalizers: passive and active. The passive EQU monitors the weakest cell (lowest voltage), and removes charge from the other cells by dissipating their energy as heat through shunt resistors until all cells voltages equal the weakest cell. This leads to energy loss (heating), reduced capacity, and poor performance. In spite of this, the vast majority of users prefer to use passive EQUs because they are cheap. The second type is called an active EQU, and it transfers charge from one cell to another to balance the cell voltages. This results in reduced energy loss, increased capacity, and higher performance for the battery pack. Although active EQUs have high performance, they remain expensive, costing about 10x the cost of passives. As a result, very few users use active equalizers. This research proposes a solution that has high performance close to an active EQU but at a low cost like the passive EQU. This solution is the Bilevel Equalizer (BEQ), which is a unique hybrid because it consists of both an active (AEQ) and a passive (PEQ) EQU. Current battery management systems (BMS) use either a passive EQU or an active EQU, but they do not use both. The term bilevel is used because the BEQ balances the cell voltages at two different levels. A large Li-ion battery consists of a large number of cells connected in series, e.g., 96 cells. The BEQ organizes the battery into sections of a few cells each, e.g., 4-12. For example, a 96 cell battery might be organized into 16 sections of 6 cells each. The voltages of the 6 cells in each section are balanced by a PEQ, for that section, and the 16 section voltages are balanced by an AEQ. As a result, a weak cell in a section only drags down the other cells within that section, instead of all cells in the battery pack. The AEQ then transfers charge to the weakest section from the other sections. As a result, the battery capacity is increased to a level close to that for a pure AEQ. The BEQ has a lower cost than the AEQ because of the lower number of active EQU units, e.g., 15 for the BEQ (number of sections -1) vs. 96 for an AEQ in the previous 96 cell example.