Active Equalization Research Articles

An improved equalization technique for fast charging of Electric vehicles

This paper describes a lithium-ion battery (LIB) hybrid equalization (HE) structure. There are two primary strategies for voltage equalization (EQ): passive and active. Active equalization (AE) moves energy between the LIBs, whereas passive equalization (PE) wastes energy. PE strategy has the benefit of being easy to put into action, but at the same time, it requires more time to equalize and results in a loss of energy. Although it is difficult to implement, AE has a shorter EQ time and less energy loss. Combining these two methods simultaneously controls a pack of LIBs and equalizes voltage. This article offers recommendations on a cell level (CL) and between stacks. Some LIBs were outfitted with varying state of charge (SOC) levels to simulate a cell imbalance. Different topologies were tested, including combined AE and PE at the pack level and PE and AE topologies at the CL. Response times and SOC levels are used to compare the results. Compared to strictly AE or PE strategies, the HE showed encouraging results regarding EQ time and autonomy. This combination offers a solution that enables one to achieve minimal HE time while maintaining appropriate SOC.

Capacity Measurements for Second Life EV Batteries

After they reached the end of their useful EV life, lithium-ion batteries are still satisfactory for second life (SL) energy storage applications. However, the spread in their SL cell capacities may be much wider than in the EV, and this raises a question as to what type of cell voltage equalizer (EQU) should be used. Most users plan to retain the same passive EQU (PEQ) from the EV, but this means the battery capacity will be the same as the worst cell in the battery, just as it was in the EV. Unfortunately, the SL cell capacity spread may be much wider than it was in the EV, and if so, most of the cells will be under-utilized. This can be corrected by using an active EQU (AEQ) or a hybrid, such as the bilevel EQU (BEQ), to provide a capacity close to the cell average; but first, measured data is needed on the actual size of the cell capacity spread. To simplify and reduce the cost of these measurements, a new method is proposed that provides the capacities of the worst cell and the cell average.

Bilevel vs. Passive Equalizers for Second Life EV Batteries

Once lithium-ion batteries degrade to below about 80% of their original capacity, they are no longer considered satisfactory for electric vehicles (EVs), but they are still adequate for second-life energy storage applications. However, once this level is reached, capacity fade increases at a much faster rate, and the spread between the cell capacities becomes much wider. If the passive equalizer (PEQ) from the EV is still used, battery capacity remains equal to that of the worst cell in the stack, just like it was in the EV. Unfortunately, the worst cell eventually becomes much weaker than the cell average, and the other cells are not fully utilized. If operated while the battery is in use, an active equalizer (AEQ) can increase the battery capacity to a much higher value close to the cell average, but AEQs are much more expensive and are not considered cost effective. However, it can be shown that the bilevel equalizer (BEQ), a PEQ/AEQ hybrid, also can provide a capacity very close to the cell average and at a much lower cost than an AEQ.

A Bilevel Equalizer for Large Lithium Ion Batteries

Due to variations among the cells, large lithium ion batteries (LIB) such as those in battery energy storage stations (BESS) and electric vehicles (EVs) must have an equalizer (EQU) circuit to balance the cell voltages. In spite of their significant losses and other limitations, passive equalizers (PEQ) are used in most applications because they are relatively simple and low cost. Active equalizers (AEQ) reduce these PEQ problems, but are not as widely used due to their much higher cost and complexity. A new hybrid circuit called the Bilevel EQU (BEQ) combines the PEQ and AEQ to provide much higher performance than a pure PEQ but at a much lower cost than a pure AEQ.