# 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  
Batteries are characterized by ten key parameters: voltage, capacity, maximum current draw for short time, maximum continuous discharge current, maximum charging current, temperature range for storage and operation requirements, chemistry, weight, and overall \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **Lead-Acid** & **AGM** & **Gel** & **NiMH** & **LiPo** & **LiFePO4** \\ \hline Specific energy [Wh/kg] & 35–40 & 35–40 & 35–40 & 60–120 & 100–265 & 90–160 \\ Specific power [W/kg] & 180 & 180 & 180 & 250–1000 & 245–430 & 2000–4500 \\ Charge current [C] & 0.2 C & 0.25 C & 0.25 C & 1 C & 1 C & 1 C \\ Discharge current [C] & 0.2 C & 0.25 C & 0.25 C & 5 C–15 C & 5 C & 30 C \\ Self-discharge per month [\%] & 10–15 & 3–4 & 3–4 & 0.08–2.9 & 0.3 & 0.3 \\ Max cell voltage [V] & 2.15 & 2.15 & 2.15 & 1.4 & 4.2 & 3.65 \\ Nominal cell voltage [V] & 2.1 & 2.1 & 2.1 & 1.2 & 3.7 & 3.7 \\ Min cell voltage [V] & 1.8 & 1.8 & 1.8 & 0.9 & 2.7 & 2 \\ Cycle durability [cycles] & 350 & 500 & 500 & 180–2000 & 500 & 1200–2000 \\ Max discharge capacity [\%] & 50 & 20 & 20 & 0 & 3 & 3 \\ Operating temperatures [\({}^{\circ}\)C] & \(-35\) to +50 & \(-40\) to +49 & \(-20\) to +45 & \(-20\) to +45 & \(-20\) to +60 & \(-30\) to +80 \\ Charge temp range [\({}^{\circ}\)C] & \(-20\) to +50 & \(-20\) to +50 & \(-20\) to +50 & 0 to +45 & 0 to +45 & +5 to +45 \\ Price [EUR/Wh] & 0.14 & 0.21 & 0.26 & 0.63 & 1.04 & 2.96 \\ \hline \hline \end{tabular} where: AGM—Absorbent Glass Mat battery, Gel—Gel battery, NiMH—Nickel-metal-hydride battery, LiPo— Lithium-ion-polymer battery, LiFePO4—Lithium iron phosphate battery.  
\end{table} 
Table 2: Battery parameter comparison based on the material composition. dimensions. Integrators are mostly interested in the battery pack characteristics, which are a result of internal battery cell connections and battery cell properties. Battery capacity is one of the key design parameters required by system integrators. It is commonly expressed in Ah (Amp-hours) or Wh (Watt-hours); for larger capacities kWh are used and for small capacities mAh is a popular unit. It should be noted that Ah describes battery capacity in terms of the battery's capability to provide current over time; for example, a 10 Ah battery can either supply 2 A of current for 5 h or 1 A of current over a 10 h period, in either way discharging completely a fully charged battery. On the other hand, capacity can be expressed in terms of work (power over time), which can be done using the unit of Wh. In such a case, a 10 Wh battery could provide 1 W of power to an appliance over 10 h, which corresponds to a complete discharge cycle. The relation between Ah and Wh is linked by the battery voltage. While comparing battery packs having the same voltage, both capacity ratings can be used for capacity comparison; nevertheless, comparing batteries with different nominal voltages should be based on Wh instead of Ah. From the definition of the unit Watt for electrical power, one can derive that 1 W of electrical power is equivalent to the current of 1 A flowing across 1 V of voltage potential in a conductor. This means that capacity expressed in Ah gives an estimate of how many hours a system consuming 1 A of current at the battery's nominal voltage could be powered by the specific battery pack, while the Wh rating gives more universal information on how long the battery pack can supply specific power for an appliance (irrespective of the actual voltage requirement). Integrating current and voltage into power makes more sense in robotic applications, where the work, and therefore power demand over time, is expressed in Watt-hours, while current and voltage are often secondary derivatives from the energy rating. Interfacing the battery to electronic control systems, sensors, and actuators requires matching a voltage range.