# Energy Sources of Mobile Robot Power Systems: A Systematic Review and Comparison of Efficiency
## Energy Sources of Mobile Robots
### Battery Management
#### 3.2.2 Voltage Level Shifting  
Adjusting the voltage level of the power supply to individual component requires level shifting. In small robots a single-cell battery can be sufficient to power all components, but the voltage requirements of the motors might require step-up voltage shifting to provide sufficient torque for the motors. Alternatively, a two-cell battery could provide sufficient power to the motors, and a step-down voltage converter could provide lower voltage for the controller. While both solutions will work, the second one will be slightly more efficient, as the motor current will most probably be much higher than the current required for the controller, and each level shifting introduces inefficiency in the power path, converting part of the energy to heat that needs to be dealt with. This indicates that the main power voltage should be adjusted to the needs of the most energy-hungry component in the robot, which, in practice, means the actuation system.  
It is rather difficult to create a robotic system that utilizes only a single voltage supply level since energy consumption of the motors will, in most designs, be significantly higher than that of the controller and sensors. However, even among sensors and controller components, two or three different voltages might be needed. Low-power devices and fast processors tend to operate at low voltages, of typically around 3.3 V, whereas simple microcontrollers and many integrated systems on module solutions prefer a 5 V input and internally lower and condition it for individual subcomponents. Sensors that are designed for battery-operated devices fall into the 1.8 to 3.3 V supply voltage category, while sensors that require more power, or are designed for simple interfacing with microcontrollers, prefer a 5 V level of voltage supply. Industrial grade sensors can have a wide range of supply voltages, either by design or using an integrated voltage-shifting module. They can typically work within 5-24 V.  
For low power (<500 mA current draw) components there is a wide selection of step-up, step-down, and integrated buck-boost converters offering high efficiency exceeding 90% in optimal conditions. In general, step-down converters reach slightly higher current capabilities and higher efficiency than boost converters. For larger powers, such converters can maintain high efficiency, but the total energy lost starts to be substantial and can significantly contribute to battery work time.  
Due to the tolerances of up to a few volts in electronic components, and low voltage drop on batteries composed of a single cell or two cells, it is possible to design a system that does not require voltage regulation at all outside individual components. If possible, the voltage delivered to the most power-hungry component should be in the range that would eliminate wasteful need for its regulation and shifting. However, in most practical applications, when multi-cell batteries are used, voltage regulation is necessary to keep components within their safe voltage levels as the span of the voltage between fully charged and the lowest allowable charge level is equal to several volts.