## 5 Comparison with Conventional Balancing Methods  
Table 3 shows the comparison of the proposed topology and existing topologies in terms of a number of required components within the case of n cells connected in series. A buck-boost  
Figure 16: The percentage of the voltage drop of three cases (0–2.5 h).  
Figure 15: The balancing time vs the voltage difference between two modules.  
converter-based balancing topology [12; 14; 36; 37; 12] or switched capacitor converter-based balancing topology [11; 15; 22; 26; 27; 30] can be constructed with a reasonable number of components. However, the energy transfer of these topologies is limited between two adjacent cells (i.e., cell to cell balancing) when a large number of cells in SCBS increases (i.e., approximately 80 or more cells) resulting in relatively slow balancing speed and low balancing efficiency. Moreover, the buck-boost converter-based balancers need intelligent control and a large number of the switches resulting in relative complex control scheme and high cost. In the MWT-based balancing topologies [10; 17; 18], the multi-winding transformer needs to be considered as the main drawback due to not only the stringent requirement for parameter matching for the turns ratio but also the requirement to change the number of secondary windings. These lead to a mismatch between the leakage inductance that caused a charge imbalance drawback by itself. The circuit used the voltage multiplier to balance the cells [34]. The voltage multiplier circuit used the diodes and the capacitors. Therefore, there is no leakage mismatch issue. However, this circuit depends on the impedance of the capacitors. If \(R_{\text{eqi}}\) and \(V_{\text{D}}\) are not unified, they will cause a residual voltage imbalance. Additionally, the larger capacity of the capacitor results in a longer balancing time. The balanced voltages of the cells indicate a reduction much lower than the average voltage of the cells.  
In this study, the proposed balancing topology is constructed with the existing transformer by increasing the number of the transformers. The proposed circuit consists of the MWTFC-based intra-module balances and SCC-based outer-module balancer in transferring the energy from a high to a low level of both the cells in each module and the modules simultaneously. In other words, the intra- and outer-module balancers deal with \(N\) cells in each module and \(M\) modules, respectively. However, in the conventional methods, one MWTFC-based balancer or one SCC-based balancer should equalize \(N\times M\) cells. Hence, the proposed circuit deals with a small number of the cells, so that a balancing time for one MWTFC-based balancer can be a significant reduction. When compared to conventional balancing topologies using the MWTFC or switched capacitor converter, the proposed balancing topology offers some advantages as follows.  
* Basically, the proposed balancing circuit has some advantages originally found in the MWTFC-based balancer and SCC-based balancer such as the repudiation of the voltage sensors for the feedback control loop, simple control scheme.
* The MWTFC-based balancer is applied to a small number of cells. Therefore, the problem of mismatched leakage inductance can be minimized.
* The voltage stress of switches is low by applying the SCC-based balancer to the outer-module balancer.
* The number of cells in series can be easily extended.