Battery equalizers are a crucial component to ensure a safety operation in a battery bank. The balancing efficiency is an essential parameter in equalizers since the less power it consumes, the more energy transferred into the cell. In this aspect, passive methods present a poor performance when compared to active ones. Moreover, switched capacitor–inductor network equalizers and capacitor-based equalizers suppress the switching losses; hence, these equalizers offer good efficiency. The other active equalizers present switching and conduction losses; therefore, they present a lower efficiency [71].

Reference [31] discusses other factors that impact in the efficiency. The equalization variable used is crucial since the operating voltage leads to an inefficient process. This behavior is explained because the variable does not reflect the internal state of the cell. Thereupon, the equalization process will be over-activated. Moreover, the equalization strategy can also lead to repeated equalization, e.g., rationalize the equalization variable to a threshold, minimize the equalization time and maximize the battery capacity. A recommended strategy to avoid an inefficient process is to minimize energy consumption. However, it is difficult to obtain the proper data and increases the cost of the hardware needed [31,72].

Battery equalizers are a crucial part of the storage system of EVs. They take active measures to keep all cells within an allowed range of the equalization variable, even when they present a high dispersion in capacity and internal resistance [16,39,40]. In this way, the batteries are protected, which is the most expensive element in EVs. Further investigations in this area are needed to overcome the shortcomings of the reviewed topologies. Advancements need to be made to improve one or more of the critical parameters highlighted as the component count, power losses, equalization time, controller and implementation complexity, current and voltage stress in the switches, size and cost. The advantages and limitations of the topologies present in the literature were highlighted in this work. We think that this paper serves as a guideline for future research and investigations regarding the issues and challenges of this topic.

Table 1 summarizes the results obtained in the simulations of this work. The complexity of the low-level controller indicates the number of variables that are required to regulate. For example, in the Ćuk converter, it is necessary to control 3 variables, the current in both inductors and the voltage in the internal capacitor. The complexity in the high-level controller indicates if it only decides the stop condition (1) or if it also decides the cells for power transfer (2). Finally, for efficiency, a score of 0 was given to passive schemes, 1 to those that present switching and conduction losses and 2 to equalizers that only present conduction losses.

Metrics from Table 1 were used to compare the reviewed equlizaers with an idea equalizer. An ideal equalizer has few devices, low equalization time, low switch stress, low controller complexity and high efficiency. According to their approximation to the ideal equalizer, we assign them a number where 1 is the most desirable equalizer. For example, the shunt MOSFET equalizer is 1 in component count because it is the equalizer with the fewest components. All the places assigned in the previous step for a converter are added to take into account all the parameters. The last column of Table 1 shows the accumulated places for each converter. Finally they are ordered from lowest to highest, where the converter with the lowest score is the one with the most desirable characteristics. The best equalizer using this methodology is the switched capacitor. However, this procedure is quite simple and has many points that can be improved. For example, weighted coeficients can be used to highlight parameters of interest. In this way, using this table a designer can reach important conclusions to select an equalizer suitable for his application.

3. Conclusions

The main parameters to select in an EV are its range, charging time, and acceleration. All these elements are related to the battery of the vehicle. Moreover, this device represents the most expensive, heaviest and bulkiest component in the system. Therefore, it is imperative to protect this element against dangerous situations. The main challenge in a battery pack used in EVs is to keep all cells within an allowable range of SOC or voltage operation. The array of cells connected in series to form a battery pack are charged/discharged at a different rate. This behavior is due to the dispersion in internal capacity, resistance, self-discharge rate and uneven distribution of the temperature in the battery pack. If the battery pack is operated without a battery equalizer, it decreases the service life of the cells and can even explode. BECs prevent these situations by taking active measures to keep all the cells within the same SOC.

The equalizers are classified, taking into account the main component in its architecture and energy dissipation. Passive equalizers present a low efficiency because they burn the excess of energy through a resistor. For this reason, it is not widely used for high-power applications. In contrast, active equalizers are preferred since the cell/string of cells with the highest SOC transfer the excess of energy to other cells/string of cells with lower SOC. The capacitor-based equalizers are simple to control and inexpensive, but present surge currents that can damage the cells. In the inductor-based equalizers, the current is controlled and they have a short equalization time. However, they present magnetization losses, saturation problems, are expensive and are bulky. The equalizers based on converters control the current extracted from the cell and the current delivered. Nevertheless, they are complex to control and expensive. The equalizer selection depends on the application, the budget and the critical parameter to consider for the designer. In this work we used a methodology to compare the main topologies based in quantitative simulation data. According to the results, the best active equalizers are the switched capacitor and the switched capacitor–inductor network.

Promising research opportunities were highlighted to move forward in this topic of investigation. Combining the existent equalizers with reconfigurable batteries could improve the equalization process. However, there are more questions than answers in the reconfiguration of the battery online. Although Tesla, Microsoft and several top tier universities accepted the software-defined batteries as a promising technology for EVs we consider that this technology will not reach its full potential in the near future. Moreover, another research opportunity is to design the volume of the equalization circuit to operate the battery pack. We think that this research trend can play a major role in the near future, due to the flexibility in the design process. Since the number of components can be accommodated to the application, it can be designed to optimize several parameters such as the cost, equalization time, number of components, etc. Finally, another identified trend is the integration of the battery equalizer and the charger, to achieve a multi-purpose converter instead of two dedicated converters. This approach has proved to be cost-effective in other applications. In general, the batteries need in-depth research since it is well known that it is the main hurdle for the widespread use of EVs.