## 3 Topological Classification
### FCS + Battery + UC Hybridization  
For a vehicle to exhibit optimal acceleration, it requires both high power density and high energy density [63]. Taking advantage of the complementary benefits offered by batteries and UCs, the combination of FCS, battery, and UC hybridization results in a more efficient distribution of energy under various operating conditions [37; 64]. [PERSON] et al. [65] conducted a study on various designs of the FCHEV and found that FCS + battery packs + UC system excelled in reducing both mass and volume, enhancing efficiency, and increasing the battery packs' lifespan, all while maintaining the same power as the FCS. Similarly, the FCS + battery units + UC hybrid configuration was introduced by [PERSON] et al. [66], as portrayed in Figure 10(a), where the FCS offers the load's average power, the battery supplies supplementary power during high-power load, and the UC delivers transient power and stabilizes the bus voltage. Additionally, the current generated while braking is absorbed by the UC to protect the battery from high current impacts. Figure 10(b) [67] and Figure 10(c) [68] show the hybridization topologies which are beneficial in increasing the FCS' lifespan.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Advantage** & **Disadvantage** \\ \hline Direct parallel connection of both FC & Simple structure, well-organized & Component matching requirements, high \\ and battery & arrangement, cost-effective solution and & voltage variation in the DC bus, complex \\  & effortless implementation, and absence of & management of each component’s \\  & DC/DC converter drawbacks. & operational condition, and low efficiency. \\ \hline FC direct connection & Uncomplicated structure, effortless & High voltage variation in the DC bus, lower \\  & implementation, battery voltage value & total energy consumption, strict criteria for \\  & not essential, and improvement of the & dynamic responsiveness and FCS stability. \\ \hline Battery direct connection & A longer FCS lifespan, less voltage & Lower total energy consumption, urgent \\  & variation in the DC bus, less needed for & need for reliable and efficient battery \\  & transient response and stability, and & performance. \\  & improved FCS energy efficiency. & \\ \hline Indirect connection of both FC & Reduced component matching & \\  & requirements for the FC and battery, & \\  & stable DC bus voltage level, and simple & A bigger filter capacitor is necessary, higher \\  & management of each component’s & cost, lower efficiency, and more complicated. \\  & operational condition. & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Pros and cons of various connection strategies of FCS + battery hybridization setup.  
Figure 11: Cont.  
In Figure 11a, the connection of the FCS to a UDC helps to regulate the rapid changes in voltage and current in the DC bus, ultimately leading to a longer lifespan for the FCS. The FCS + battery packs + UC hybrid configuration is seen as the optimal solution for energy recovery during braking and for meeting the power needs during starting, hill-climbing, and acceleration [69]. However, this hybridization approach comes with challenges such as high production costs, difficulty in matching energy sources' parameters, and complex energy management systems. As a result, this combination is still technically underdeveloped and challenging to implement on a commercial scale. In conclusion, each topology has its unique features, and the best option can be determined by evaluating the characteristics of each topology and reviewing relevant models, as outlined in Table 2.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type** & **Advantage** & **Disadvantage** & **Model** \\ \hline FCS + Battery & Manufacturing at a lower cost, less demanding performance specifications for FCs, and improved starting capabilities. & Inadequate for absorbing and discharging power over an extended period. &