# Hybrid Power System for the Range Extension of Security Robots: Prototyping Phase
## 1 Introduction  
As the demand for mobile robots continues to grow, they are expected to perform long-duration missions [1; 2; 3; 4; 5; 6; 7; 8]. Physical security is one of the missions that are also difficult, dangerous, and tedious. Commercial security robots have been introduced as supplements to help human guards [9; 10; 11; 12; 13; 14]. For long-duration missions, power systems are of particular importance in a mobile robot. The energy demand will depend on a task given to the mobile robot, which usually lasts for a few days or even a week rather than just a few hours. The power demand will vary significantly during a task, often to levels that are instantaneously higher than that power systems can constantly supply. Power systems for use in mobile robots are constrained in their weight, volume, noise, and emissions. Conventional power sources are key limiting factors for mobile robots under many important missions. Internal combustion engines have high energy densities but produce toxic exhaust, loud noise, and strong thermal signatures, making them inadequate for confidential operations. Despite recent advances in their performance, rechargeable batteries still have relatively low energy densities and thus need to be stopped and recharged every few hours, making them ineffective for continuous operations. Therefore, there is a significant demand for an alternative power source that can supply as much energy as necessary for security missions that are long-lasting, clean, and quiet. Fuel cells are electrochemical devices that convert chemical energy into electrical energy. In terms of energy conversion, fuel cells are outwardly similar to batteries or internal combustion engines that are used to supply electricity. However, unlike internal combustion engines, fuel cells can generate electricity directly from electrochemical reactions without multiple energy conversions that involve heat and mechanical motions. In contrast to batteries, fuel cells can generate as much electricity as needed as long as they are supplied with fuels. This is the key advantage of fuel cells over batteries, to be specific, sealed batteries, enabling them to work for longer duration even with much shorter refueling times. While fuel cells show great promise in theory, they have a few critical issues that must be addressed for practical use. These include difficulties in fuel storage [15; 16; 17], lack of durability [18; 19; 20], and low power density [21]. Of these, the difficult fuel storage and low power density are inherent problems. Hydrogen is costly to store in bulk primarily because of its high energy content but low density. Fuel cells are limited in how rapidly they can increase their fuel supply rate, especially oxygen to the cathode, as the power demand grows. From a systematic point of view, the low durability is a problem in use, which can be mitigated by running fuel cells in a more controlled manner. Among the various factors known to influence the durability of fuel cells, voltage oscillations and high voltages during operation are of particular interest. The voltage oscillations are relevant to a situation where mobile robots operating in unstructured environments are subjected to large variations in load demand. Without proper control, the voltage oscillations can increase the risk of fuel cell degradation, resulting in a short life span and premature failure. In this regard, the poor durability can be overcome by power system hybridization [1]. By hybridizing with batteries, fuel cells can be controlled to run within a narrower operating area, which extends their lifetime. By regulating the operating area, higher efficiency can be obtained. Besides, the low power density can also be remedied to some extent. This paper is intended as a sequel to our previous work. In our original paper [22], we described our best practices related to hybrid power system (HPS) development with a focus on the specification development phase.