## 3 Batteries  
One of the key components of the laser beacon is the battery system. The power available in the NEMO tower was not enough to flash the laser, so a battery system was included in the laser beacon. This system can be operated normally, as only a few minutes of operation per week are required. Several models of batteries were analyzed, and finally, the Ni-MH (nickel-metal hydride) [15; 16] model was chosen, mainly due to the following reasons:  
* Although they have a lower charge density than Li-Ion [17; 18] and Li-Po batteries [19; 20], Ni-MH batteries do not need a protection circuit to maintain safe operation, reducing the complexity of the circuit.
* They tolerate maintaining a charge even when fully charged, without the need for discharging or half-charging before storage, as required by other technologies.
* They are more economical and ecological than Ni-Cd [21; 22] batteries since they do not contain highly polluting cadmium.
* The primary limitation stems from the need to charge using a current that is less than 10% of the load. In this specific scenario, this requirement is indeed an advantage, as it is mandatory to limit the load current due to the input power supplied to the laser beacon being limited to 12 V at 200 mA.  
The final rechargeable batteries chosen were the Supreme from Duracell, which are Ni-NM cells with a capacity of 2450 mAh and a voltage of 1.2 V. The main characteristics of the battery are shown in Table 1. To supply enough power to the laser, the battery has to include at least 10 elements or cells, each with a voltage of 1.2 volts. Each cell has a nominal unloaded state value of 1.2 volts; as it charges, this voltage value increases up to 120% of the nominal value. This increase can be seen in Figure 2.  
Therefore, the battery formed by ten cells will have a nominal voltage of 12 V, which will increase up to 14.4 V when the battery is fully charged. The current should not exceed 10% of its capacity. Maintaining the ambient temperature within the range of \(-20\)\({}^{\circ}\)C to 60 \({}^{\circ}\)C is necessary. Considering that the temperature at the seabed remains stable at 13 \({}^{\circ}\)C, this criterion should be met when deployed.  
The maximum charging current is then limited to 245 mA. The minimum charging current is limited by the requirements, which specify that the battery should be completely loaded within 72 h. To achieve this requirement, the minimum charge current has to be higher than 34 mAh. This category of rechargeable battery exhibits varying discharge times, influenced by the voltage required from the battery and the discharge current. For the NEMO laser beacon, employing a battery composed of 10 cells will result in a demanded current of under 1.5 A, ensuring operational longevity well beyond the required twenty-minute duration.  
\begin{table}
\begin{tabular}{l l l} \hline
**Property** & **Value** & **Units** \\ \hline Battery Type & Ni-MH 2450 mAh, AA & \\ Battery Capacity & 2450 & mAh \\ Battery Voltage & 1.2 & V \\ Manufacturing Technology & Nickel–metal hydride & \\ External Diameter & 14.5 & mm \\ External Height & 50.5 & mm \\ Weight & 28 & g \\ \hline \end{tabular}
\end{table}
Table 1: Specifications of the battery chosen for the NEMO laser beacon.