## 1 Introduction  
Robotics technology has become popular in many fields, ranging from military to industry applications, and particularly within the physical security field. Security robots are expected to be a cost-effective method for protecting people and their property by deterring crime. State-of-the-art security robots are much more capable than previous ones. However, it is noteworthy that they are still lacking as a total replacement for human guards, who are able to respond intelligently and spontaneously to anomalies. Therefore, in current scenarios, security robots help human guards in tasks that they can perform better. For example, security robots can collect and process far more data. They can also make more regular and extended facility rounds than human guards. These rich capabilities are enabled by recent advances in hardware devices and software applications. Software applications primarily require computer vision and artificial intelligence solutions. Hardware devices involve parallel embedded computing, wireless networks, and smart sensors. Additionally, energy-dense power sources are essential for these systems to operate for a long time. As shown in Figure 1 and Table 1, there are several security robot models that are currently available or will be available soon [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. However, their detailed specifications are not yet open to the public. A majority of manufacturers only make public the overall size, appearance, and rough capabilities of their security robots. Only SMP discloses the full specifications of its S5.2 model, including the operating time and distance, in which we are particularly interested. It is noteworthy that most current models are limited in terms of these capabilities. In response to future market demands, development must be geared toward versatile capabilities for expanding the availability of robots in the physical security field. Knight-scope are currently developing a new model called the K7 in addition to their existing line-up of the K1 (stationary), K3 (indoor), and K5 (outdoor, but only on roads) models. It has been reported that K7 will provide outdoor, multi-terrain, all-weather, relatively high-speed, and long-distance capabilities [7; 12]. Therefore, K7 could be well-suited to perimeter patrolling in large facilities such as airports, seaports, power plants, and refineries, which are out of reach for smaller models. It is clear that the more capabilities security robots provide, the higher energy they require. Although only a few models have provided specifications regarding their power sources, outdoor models generally demand more energy than indoor models. Among outdoor models, ROAMEO and S5.2 are equipped with 3.1 and 5.12 kWh lithium-ion batteries (LIBs), respectively. Such large batteries allow them to patrol continuously for 10.6 h and up to 16 h, respectively. However, they require a long time to recharge. ROAMEO requires 9.6 h and S5.2 requires 4 to 6 h depending on the available charging options. Security robots are expected to operate 24 h per day and 7 days per week, but battery-powered robots could not fulfill this requirement due to their not-short charging time. For refueling, they must leave off their work at least once a day or twice, meaning multiple, at least two, units have to work in shifts. Deploying a greater number of units leads to higher costs, which is contrary to public expectations that security robots are cost-effective