Usually, hybrid power systems with fuel cells and batteries can be arranged in topologies with active control systems (typical configuration with DC/DC converters) or passive (with direct coupling among components system)
Active hybrid power systems ( with DC/DC converters)
The active configuration allows a decoupling of sizing and operating conditions in batteries and fuel cell, thanks to the DC/DC converters, allowing also a more precise control of the power system. The main disadvantages of indirect hybrids (active) are the more complex system topology, reduced efficiency due to losses at the voltage, system cost, and higher weight and volume.
The most usual configuration includes a DC/DC converter after the fuel cell system, but other configurations are also possible.
Passive hybrid power systems (without DC/DC converters)
Passive configurations with direct connection to DC bus offer the advantages of lower losses, reduced cost and simple architecture. However, active power control is not possible, and a careful design and integration of fuel cells and batteries is required to ensure a similar voltage range operation and proper charging conditions of the batteries from fuel cell if this option is considered.
The choice of the most suitable configuration will depend on the requirements of the final application, in terms of power and energy, and its possible constraints, in terms of weight and volume, as well as the characteristics of the fuel cell system and batteries.