# Hybrid Power System for the Range Extension of Security Robots: Prototyping Phase
## 3 Conversion to Plug-In Hybrid Structure
### Simulation Results  
Under the most favorable conditions, we predicted that the plug-in HPS will run for 2.07 longer hours than the HPS. By discharging the larger battery (7S12P) over the widest SOC window (90%), the CD mode lasting for 2.07 h can be added before the transition to the CS mode. The CD mode was assumed to comprise surveillance during patrolling consuming 450 W. If it was replaced with only surveillance consuming 250 W, the CD mode can be extended to 3.73 h.  
Figure 3: Comparison of hybrid structures. (**a**) Plug-in HPS versus (**b**) HPS.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name [Units]** & **HPS** & **Plug-in HPS** & **Remarks** \\ \hline Nominal voltage [V] & 23.1 & & 3.3 \(\times\) 7S \\ Rated capacity [Ah] & 20 & 40 & 2.5 \(\times\) 12P \\ Initial state-of-charge [\%] & 50 & 60–100 & \\ Battery response time & 30 & & \\ Max. capacity [Ah] & 20 & 40 & \\ Cut-off voltage [V] & 17.5 & & 2.5 \(\times\) 7S \\ Fully charged voltage [V] & 29.4 & & 4.2 \(\times\) 7S \\ Nominal discharge current [A] & 20 & 40 & 1C \\ Internal resistance [ohm] & 0.01 & & \\ Capacity at nominal voltage [Ah] & 16.8 & 36.2 & 2.1 \(\times\) 8P \\ Exponential zone [V, Ah] & 26.18, & 26.18, & 3.74 \(\times\) 7S \\  & 5.84 & 8.76 & 0.73 \(\times\) 12P \\ \hline \hline \end{tabular}
\end{table}
Table 3: Battery model parameters.  
Nearly half of the total increase in operating hours are accounted for by the wider SOC. In the simulation, the plug-in HPS was allowed to discharge down to 10% starting from 100% SOC. This is in contrast to the HPS, which has almost no net discharge capacity. The gain from the wider SOC thus amounts to approximately 18 Ah (466.2 Wh). However, although a wider battery SOC range increases the operating time of the security robot, it is well known that high SOCs of over 90% are detrimental to the battery health. In addition, such high SOCs are not favored as the upper limit of SOC, considering the low quality of the components used to retrofit the HPS prototype. For instance, the charger was not customized to the battery. The charger voltage stops at approximately 28 V, which corresponds to a SOC of not higher than 81%. More importantly, only rudimentary monitoring capabilities are provided in the battery, in which the battery voltage can only be monitored across the entire pack and not across each series group of cells. Therefore, operation at high SOCs is prone to overvoltage faults, regardless of how well the SOC difference between the modules is balanced. The lower limit of the SOC is typically less than 20%, which is not low enough to affect the battery health. However, low SOCs are prone to operational instability. This is particularly serious in overload conditions in which a higher power demand is imposed on the plug-in HPS than can be delivered by the fuel cell alone. If such higher power demands occur frequently and/or last for a long time, the battery might need to bear the power demand without any time to recharge. The plug-in HPS might then fail to meet the voltage requirements as a result. In the simulation, a SOC of as low as 10% was favored as the lower SOC limit partly because of the relatively moderate power demand in the given drive cycle. However, the validity of this assumption in practice is questionable and needs to be confirmed through actual tests.  
The remaining half of the increase in operating hours can be accounted for by the larger battery. By upsizing the battery configuration from 7S8P to 7S12P, the battery size was increased from 20 Ah (518 Wh) to 40 Ah (1036 Wh). Accordingly, the gain from the wider SOC was doubled to nearly 36 Ah (932.4 Wh). We concluded that the total energy gain of 624.6 Wh from these two contributions is the maximum gain that can be expected by converting to the plug-in hybrid structure. However, as mentioned above, much of the gain from the wider SOC will be reduced in practice by various practical constraints.