## 5 Development of an Agricultural Drone BMS
### Cell Voltage Deviation and SOC Measurement  
The SOC and cell voltage deviation of the battery management board (see Figure 16) were measured to determine whether the voltage deviation values between the SOC and cell measured in charge/discharge units when under the control of the BMS were appropriate. The experimental method was as follows. After two lithium polymer batteries were connected in series and the batteries were discharged to 40 V and recharged to 45 V, the Ah value was measured by both the charge/discharge device and the BMS system. Table 7 is calculated to know the SOC and cell voltage deviation measurement data values. In addition, Table 8 is a voltage value measured for each of the 12 cells of the agricultural drone battery. If the system is working properly, the measured value should be within \(\pm 8\%\) for the SOC. The cell voltage deviation was determined by measuring the voltage between the 12 cells of the battery while charging to 45 V and comparing this outcome to the voltage measured using the BMS. Ideally, the measured voltage should be within 50 mV or less of the target.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Division** & **Only Battery** & **BMS** & **Remarks** \\ \hline \multirow{2}{*}{0.2 C} & Discharge amount (Ah) & 21.77 & 22 \\  & Discharge amount (Ah) & 15,463 & 18,565 \\ \hline \multirow{2}{*}{6 C} & Discharge amount (Ah) & 22 & 22 \\  & Discharge amount (Ah) & 810 & 840 \\ \hline \multicolumn{2}{c}{Cell voltage deviation (mV)} & 120 & 50 \\ \hline \multicolumn{2}{c}{Discharge rate (\%)} & 94.6 & 95.6 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Comparing the discharge of the battery alone versus that with the battery management system (BMS).  
Figure 15: A 50 V charge time comparison per cycle.  
Figure 19 compares the battery discharge of agricultural drones with the developed BMS (Figure 19a) and without the BMS (Figure 19b). In the charged state, it was confirmed that the voltage differences between the cells with the BMS installed and with only the battery were not large. This occurred because the C-rate is not high in the charged state, resulting in little to no difference. In the discharged state, voltage differences were observed between the cells when only the battery was installed on the right side and the BMS was installed on the left side.  
Figure 19: Cell change comparison in the discharge interval of the drones (**(a)**: BMS, **(b)**: Battery only).  
Figure 18: Comparison of a 0.6 C discharge outcomes: (**a**) Battery only, and (**b**) with the BMS.  
In this case, the battery had six cells per pack, although the system also supports two packs with 12 cells each. When 7-12 cells were installed, a large difference was observed at the seventh cell. This occurred because the voltage deviation increased when the sixth and seventh cells were connected to the first cell, as shown in Figure 20.  
Based on these results, it was confirmed that when the BMS was used, the voltage deviation was reduced and the DOD was improved.  
In this study, we found that the efficiency was improved by 5% when the developed BMS was installed compared to when the battery-only method was used. In actual flight, the 5% efficiency improvement is improved even further due to the climate and temperature, showing an efficiency rate exceeding 10% while charging and discharging (Refer to Table 9).  
We anticipate that if the proposed battery pack and integral BMS are commercialized, it will be shown to be useful during the operation of agricultural drones due to the extended battery life and reduced charge time.