**4.1. Existing balancing topologies**

Many battery balancing methods have been presented in previous studies [53-66], which could be categorized as passive balancing methods and active balancing methods. For passive balancing methods [67-69], the excess energy would be removed from high-energy cell(s) through resistors. The energy is dissipated as heat through resistors, which not only wastes electricity energy, but also increases thermal control difficulty of the battery pack. Besides, only the high-energy battery cells work in the balancing process. A large amount of energy would be wasted if most of the battery cells have higher energy than some other cells.

For active balancing methods, active switching circuits are utilized to transfer energy between cells or between the cell(s) and the battery string. Compared to passive balancing methods, wasted energy will be very little. However, to fulfill the functions stated above, more switches and associated components are needed, which would lead to increased cost and reliability concerns. The cost and reliability should be carefully considered to make active balancing more competitive, including but not limited to, reducing the components, simplifying the structure of the topology, and reducing the control complexity.

A bi-directional buck-boost converter-based balancing topology [59-61] is shown in Figure 10a, which is referred to as Topology 1. In this method, a buck-boost converter is formed to transfer energy from a high energy cell to its adjacent cell. The problem is that cells can only be balanced by their adjacent cells. More than one step will be needed if the two cells that need to be balanced have a long distance (*C*1 and *Cn* for instance). In this case, longer time will be needed to transfer energy between these two cells to fullfil the balancing.

Figure 10b shows another widely used balancing topology [62, 63], which is referred to as Topology 2. In this topology, the primary winding of a bi-directional Flyback converter is connected to each cell while the secondary winding is connected to the battery string terminals. A cell to the battery string balancing method is formed in this method, t. cells could be balanced simultaneously since each cell owns a separate converter. The total balancing time could be dramatically reduced compared with Topology 1. However, for *n* cells battery string, 2*n* switches and 2*n* windings are needed. High voltage stress needs be applied to the switches in the secondary winding of the transformer. The turns-ratio of the transformer should be designed to be large enough to cover the large voltage difference between cells and battery string. These aspects would result in high costs for the system.

Another widely used balancing topology is depicted in Figure 10c [60, 64-66], which is referred to as Topology 3. Only one multi-winding transformer is utilized in this topology. *n* primary windings of the transformer are connected to *n* battery cells, respectively, and the secondary winding is connected to the battery string terminals. Only one single converter is utilized in this topology compared with Topology 2. The switches used in this topology are reduced to *n* + 1. Furthermore, high voltage stress will only be applied to one switch in this topology. The secondary windings are also reduced to one. Compared with Topology 2, the cost would be dramatically reduced. However, only one cell could be balanced at a time in this topology, and it is still a string to cell balancing method. Comparing to Topology 2, these aspects lead to a longer balancing time. Besides, the converter efficiency would be almost the same since the transformer turns-ratios are the same as those of Topology 2.

**Figure 10.** Active balancing topologies: (a) Topology 1, (b) Topology 2, (c) Topology 3.

## **4.2. The advanced cell to cell battery balancing topology**

among the cells in a battery string would lead to imbalance, which would cause many problems. Due to the "Bucket Effects", the actual available capacity of the battery string would dramatically decrease. Overcharge and/or overdischarge could also be potentially caused by the imbalance, which is dangerous for batteries. Battery balancing is of paramount importance

Many battery balancing methods have been presented in previous studies [53-66], which could be categorized as passive balancing methods and active balancing methods. For passive balancing methods [67-69], the excess energy would be removed from high-energy cell(s) through resistors. The energy is dissipated as heat through resistors, which not only wastes electricity energy, but also increases thermal control difficulty of the battery pack. Besides, only the high-energy battery cells work in the balancing process. A large amount of energy would be wasted if most of the battery cells have higher energy than some other cells.

For active balancing methods, active switching circuits are utilized to transfer energy between cells or between the cell(s) and the battery string. Compared to passive balancing methods, wasted energy will be very little. However, to fulfill the functions stated above, more switches and associated components are needed, which would lead to increased cost and reliability concerns. The cost and reliability should be carefully considered to make active balancing more competitive, including but not limited to, reducing the components, simplifying the structure

A bi-directional buck-boost converter-based balancing topology [59-61] is shown in Figure 10a, which is referred to as Topology 1. In this method, a buck-boost converter is formed to transfer energy from a high energy cell to its adjacent cell. The problem is that cells can only be balanced by their adjacent cells. More than one step will be needed if the two cells that need to be balanced have a long distance (*C*1 and *Cn* for instance). In this case, longer time will be

Figure 10b shows another widely used balancing topology [62, 63], which is referred to as Topology 2. In this topology, the primary winding of a bi-directional Flyback converter is connected to each cell while the secondary winding is connected to the battery string terminals. A cell to the battery string balancing method is formed in this method, t. cells could be balanced simultaneously since each cell owns a separate converter. The total balancing time could be dramatically reduced compared with Topology 1. However, for *n* cells battery string, 2*n* switches and 2*n* windings are needed. High voltage stress needs be applied to the switches in the secondary winding of the transformer. The turns-ratio of the transformer should be designed to be large enough to cover the large voltage difference between cells and battery

Another widely used balancing topology is depicted in Figure 10c [60, 64-66], which is referred to as Topology 3. Only one multi-winding transformer is utilized in this topology. *n* primary windings of the transformer are connected to *n* battery cells, respectively, and the secondary winding is connected to the battery string terminals. Only one single converter is utilized in

needed to transfer energy between these two cells to fullfil the balancing.

string. These aspects would result in high costs for the system.

to maintain the performance and enhance the cycle life.

of the topology, and reducing the control complexity.

**4.1. Existing balancing topologies**

100 New Applications of Electric Drives

Based on the analysis of the three popular balancing topologies discussed above (as shown in Figure 11), an advanced cell to cell battery balancing topology is proposed [70]. In this topoloy, the number of switches and the number of windings are reduced. Thus, the cost will be lower and reliability will be higher due to reduced component count. Simultaneously, a much simpler control and gate drive system is needed with fewer switches. Since the topology realizes the cell to cell balancing, balancing time and complexity of the control processes will be reduced a lot. Furthermore, since the transformer turns-ratios are low, almost equals to 1, a relatively higher balancing efficiency and small size transformer could be achieved in this topology.

In Figure 11, *L* <sup>1</sup>, *L* 2, to *L <sup>k</sup>* are coupled windings, and every two cells in the string share one of such windings. For example, *C*1 and *C*2 share the same winding *L* <sup>1</sup>. Current *I* is the main current applied to the battery string, while *I*1, *I*2,..., and *I*2*<sup>k</sup>* are the balancing current for each cell, respectively. To ensure every two cells sharing a winding, the total number of cells (*n*) in the battery string are designed to be even (*n* =2*k*, *k*=1, 2...).

**Figure 11.** The proposed advanced cell to cell battery balancing topology.

In this topology, the two adjacent cells, which share one winding (*C*1and *C*<sup>2</sup> for instance), would form a buck-boost converter similar to that of Topology 1. Meanwhile, due to the coupled windings, non-adjacent cells (*C*1and *C*4 for instance) and its windings form a Flyback converter. So balancing could be easy, no matter if the cells are adjacent or not. The cell to cell balancing method is realized and is considered to be time-saving. To compare this topology with the other above-stated topologies, the components needed for an *n* cell string are compared in Table 1.


**Table 1.** Dear authors, please add caption

As shown in Table 1, for the proposed topology, only one switch and one winding are needed for each cell. All the switches used in the proposed topology will not suffer high voltage stress. The cost would be dramatically reduced and the controlling complexity will also be lower.
