# ENDURUNS: An Integrated and Flexible Approach for Seabed Survey Through Autonomous Mobile Vehicles
## 3 System Functionalities
### Power Pack and Energy Management  
There are many AUV power-pack commercial systems for underwater energy storage, such as the magnesium-seawater battery system [62], the Li-ion battery pressurized module, or aluminium-hydrogen peroxide (A1/\(H_{2}O_{2}\)) semi-fuel-cell [63], depending on the power or design requirements. The ENDURUNS project develops a novel energy-source system, based on hydrogen fuel cells [64]. The AUV can autonomously plan its energy requirements based on its sensors-system collecting operational data. Moreover, while the project is ongoing, simulations of different operation scenarios are performed on the prototype vehicle, to assess hardware, software, and external conditions affecting battery-pack efficiency, similar to [65], with the goal to find the configurations for an optimal consumption rate [66].  
On the other hand, the power pack for the USV is simpler than the one needed for the AUV, where in the former case, energy is obtained by two photovoltaic panels, 160 W each [67]. Here an appropriate electrical installation is needed to manage energy production, transformation phase, and storage in the battery pack [68].  
The AUV battery pack consists of two 4 kWh Li-ion stacks, placed side by side. This array of batteries supplies the required power to the AUV internal components, for example, thrusters, onboard CPU, sensors (e.g., camera, MBES), acoustic modem, Wi-Fi antenna, buoyancy controller. The Alkaline Fuel Cell (AFC) onboard the AUV is responsible for converting the chemical energy of the stored hydrogen into electrical power for the battery pack. The AFC is placed on a separate pressure housing with respect to the required hydrogen and oxygen storage tanks. It has a maximum power output of 300 W with a maximum theoretical efficiency of 60%.  
The power pack employed on the USV is similar to the one used for the AUV, with the exception that photovoltaic panels are also incorporated. The photovoltaic panels recharges two 2 kWh stacks of Li-ion batteries continuously during the daytime. The USV is also equipped with AFC, but in this case output power is 500 W instead of 300 W, as the USV weight restrictions are less severe than the AUV ones.  
In the case of USV, the recharging of the Li-ion battery array is performed by the photovoltaic panels installed on its top side. If the discharging of the battery exceeds a pre-defined threshold (e.g., 20% of total charge capacity) the fuel cell switches on to recharge the battery. Once the recharging of the battery is completed, the fuel cell switches off again. In the case of the AUV a similar approach is employed, with the fuel cell switching on only when the total discharge of the Li-ion battery array drops below the pre-defined threshold (e.g., 20%). Once recharging is complete the fuel cell will switch off again until the next recharging cycle.  
In the event the battery was to run out completely of charge, the AUV is designed to have inherently a positive buoyancy permitting it to resurface without the help of the electric motor.