# Energy Sources of Mobile Robot Power Systems: A Systematic Review and Comparison of Efficiency
## Energy Sources of Mobile Robots
### Energy Storage and Battery Technologies
#### 3.1.2 Battery Ratings
Elements that contain internal voltage regulation circuitry usually have a wider range of accepted voltage, while devices without such circuitry will require a specific voltage to be provided either directly from the power sources or through an intermediate regulator. Battery voltage ranges throughout the discharge cycle from the above nominal voltage to a few percent below the nominal voltage. For example, a fully charged LiPo battery cell will show 4.2 V, with its nominal voltage being 3.7 V and the fully discharged battery voltage being 3.27 V. This shows a 0.93 V range between the fully charged and fully discharged battery cell level [55]. It must be noted that the fully discharged state should not be reached with any battery chemistry as it often leads to permanent battery damage. A safe minimum charge limit is around 15% of the total charge, which for a LiPo battery is 3.7 V. This implies that the actual battery capacity available for the robot is limited by the application's defined maximum discharge level. In the case of airborne machines or safety-critical devices, where power loss can cause damage to the equipment or injury to a human, the maximum discharge level is set to something above 50% of the total charge, ultimately limiting useful battery capacity to half. For remaining battery capacity measurement, it should be remembered that the voltage to capacity is a non-linear relationship. The number of cycles that battery can undergo before reaching the end of its lifetime varies depending on the temperature range in which battery is used, allowed discharge level and maximum charge level, and periods where the battery is not used at all. Each charge-discharge cycle of a battery contributes to its wear. The deeper the discharge, the more wear is produced. The maximum charge and discharge rate is limited by the internal resistance of a battery and the thermal conductivity from the battery to ambient conditions. This limit can be increased by providing heat sinks or forced cooling. This is especially important if a short charging time is required [54]. Similarly, as in the case of the discharge current, the charging current affects the total battery lifetime; fast charging with high currents shortens the battery life, whereas slow charging can extend the number of cycles that the battery can handle. As a drained battery can take significant current, the current during the charging cycle needs to be controlled by the charger. All batteries can be charged using a constant voltage, constant current approach, where the current is limited by automatic selection of the charging voltage during the initial part of the charging, and, once the battery reaches a predefined charge state, a constant voltage is applied that is equal to the maximum voltage the battery is supposed to reach [56]. In this last part of the process, when the current drops below a predefined threshold, or the time of charging reaches a preset limit, the charge cycle is assumed to be over. In fast charging, in addition to current limiting, battery temperature must be monitored, as battery performance over time degrades, leading to increased heat generation during use and charging. Each battery has a rating for the allowable temperature range when it can be used. Extreme temperatures affect the total capacity available for the appliance. Lower temperatures can extend battery lifetime, but at the expense of lowering apparent capacity, whereas elevated temperatures decrease battery lifetime for the benefit of increased apparent available capacity. Battery capacity can reduce below 50% when the temperature drops below \(-18\)\({}^{\circ}\)C, although the actual effect depends on the battery chemistry. Therefore, this effect should be considered while designing robots that should be used in freezing conditions. To provide optimal battery working conditions in special applications, systems for cooling as well as for heating are designed and integrated into the battery pack.