## 3 Test Bench Layout
### Load Unit  
The load unit is composed of an electronic load (EL) and a power supply. The nominal properties of the elements of the load unit are reported in Table 4. The EL must reproduce the power demand deriving from the external load. The adopted EL is the Aim-TTi LD400P manufactured by Aim-TTi. As was done for the power supply for the FC emulation, to obtain precise and fast response control, the EL is controlled analogically through the Arduino Mega. The control signal must be a 0-5 V analogue signal; therefore, also in this case, the Arduino generates a 0-5 V PWM, which is then converted to an analogue signal using an RC filter (cut-off frequency of 48 Hz). However, the load profile must be provided to the Arduino by a personal computer (FC) through a USB connection. Thus, the Arduino receives the EL analogue signal values from the PC through the USB port then sends these signals to the EL in order to emulate the power demand. However, in order to properly emulate the fuel cell powertrain, the power requested by the auxiliaries of the FC system must be taken into account since they have a non-negligible impact on the powertrain efficiency. These auxiliaries generally include the air compressor, the cooling pump, the hydrogen recirculating pump, etc., and according to studies available in the literature [14; 44; 45], they usually absorb about 10-20% of the gross power generated by the stack. However, the power consumption of these components is generally dependent on the operative conditions of the fuel cell system. For the FC system considered in [45], \begin{table}
\begin{tabular}{l c} \hline \hline  & **Aim-TTi LD400P** \\ \hline Nominal continuous power & 400 W \\ Max short-term power & 600 W (max 60 s) \\ Voltage range & 0–80 V \\ Current range & 0–80 A \\ Operative modes & Constant current; Constant power; Constant voltage; Constant resistance; Constant conductance \\ \hline \hline  & **EA PS 2042-10 B** \\ \hline Nominal continuous power & 160 W \\ Voltage range & 0–42 V \\ Current range & 0–10 A \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Load unit element main properties. \begin{table}
\begin{tabular}{l l} \hline \hline Nominal power & 800 W \\ Input Voltage range & 20–70 V \\ Output Voltage range & 2.5–58 V \\ Max Input current & 20 A \\ Max Output current & 30 A \\ \hline \hline \end{tabular}
\end{table} 
Table 3: DC-DC converter main properties. the power absorbed by the auxiliaries was approximately equal to around 13% at low loads and increased to around 19% at high loads. Thus, the power absorption was a function of the power delivered by the stack. According to this approach, an estimation of the power absorbed by the auxiliaries of the FC system was performed during the tests and was added to the external load profile in order to consider its impact on the overall efficiency of the emulated powertrain. The estimated power absorption was determined based on the power delivered by the stack so that, according to [45], it was approximately equal to 13% at low loads, or 10-20% of the stack nominal power, and increased linearly to 19% at high loads, or 90-100% of the stack nominal power. As a consequence, the power requested by the load unit was determined by the sum of the external load, which is provided to the Arduino by the PC, and the estimated absorption of the auxiliaries of the FC system, which is evaluated at each time step by the microcontroller based on the actual power delivered by the emulated FC. Therefore, the overall power requested by the EL was expressed by Equation (1). \[P_{tot}=P_{EM}+P_{aux_{FC}} \tag{1}\]  
where \(P_{tot}\) means the total power requested by the EL, \(P_{EM}\) means the power requested by the electric motor of the emulated powertrain, and \(P_{aux_{FC}}\) means the power absorption of the FC system auxiliaries. As for the power supply of the load unit, the adopted PS is the EA PS 2042-10 B manufactured by Elektro-Automatik (EA, Elektro-Automatik GmbH & Co. KG, Viersen, Germany).