# Design of Direct Current Microgrid Converter with Cost-Effective Low-Voltage Battery Storage System
## 9 Experimental Results
### Efficiency Prototype Measurements  
To test the efficiency of the whole inverter with the proposed control algorithm, we replaced the battery with a laboratory power supply because of the dependence of the battery voltage change on the current drawn and the battery state of charge, respectively, which would make it impossible to measure for constant input voltage values. These were chosen purposely, namely the minimum considered voltage, where the inverter should draw the largest possible current. Thus, the conduction losses and losses on the measuring current shunts should be the highest, the nominal voltage of the 3S lithium battery, i.e.,  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Calculated Ideal Values**} & \multicolumn{4}{c}{**Measured Values**} & \multicolumn{2}{c}{**Error**} \\ \hline \(\rm R_{i}\) **Nom.** & \(\rm R_{i}\) **Meas.** & & & & & & \\ \(\rm[\Omega]\) & \(\rm[\Omega]\) & \(\rm V_{mpp}\) **[V]** & \(\rm Ampp\) **[A]** & \(\rm P_{mpp}\) **[W]** & \(\rm V_{mpp}\) **[V]** & \(\rm A_{mpp}\) **[A]** & \(\rm P_{mpp}\) **[W]** & \(\rm 1\Delta P\) **[\%]** \\ \hline
13.6 & 13.718 & 12 & 0.874763 & 10.49716 & 11.89 & 0.85 & 10.1065 & 3.8654 \\
6.8 & 6.823 & 12 & 1.758757 & 21.10509 & 11.72 & 1.74 & 20.3928 & 3.4928 \\
3.4 & 3.429 & 12 & 3.499563 & 41.99475 & 11.92 & 3.41 & 40.6472 & 3.3152 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Measured values of the steady point of the prototype inverter and their percentage comparison with the theoretically calculated value.  
Figure 19: The position of the three measured points in their ideal characteristics.  
11.2 V, and the maximum input voltage equal to the maximum possible voltage of the 3S battery, i.e., 12.6 V. In the last case, the current will be the smallest possible, so conduction losses should apply the least.  
The measurement results for the control algorithm (b), i.e., the combination of the step-up and step-down converter, where the step-down converter operates with a fixed relative switching time, D1 = 80%, and the step-up converter is connected to the voltage regulator are shown in Figure 20.  
The measured dependence curve shape follows the simulated waveform Figure 12, but the real values are higher than the simulated ones. This may be due to an inaccurate transistor and coil model or possibly a model operating with the worst possible catalog parameters and the real transistors not reaching these worst extremes. Another possible cause is different safe intervals (deadtime), where in the model, the guard interval size is 100 ns. In the actual circuit, 150 ns was used.  
Better efficiency than simulated is not a problem, as the curve copying the simulated shape can be seen. Likewise, the assumption about the maximum and minimum currents and their influences on the conduction losses, hence on the overall efficiency, is confirmed. The inverter has the best efficiency when supplied with the maximum possible voltage. Conversely, the worst efficiency is obtained when supplied with the minimum voltage considered.  
The shape of the current at different operating modes remains to be investigated in the (b) type algorithm. The curve changes its slope in the second interval according to the ratio of input and output voltages (Figure 13). We verified this fact by attaching a contact current probe to the PCB path away from the coil when the inverter is loaded with the output power when supplied from the laboratory power supply. The resulting waveform is shown in Figure 21, by varying the input voltage between the maximum and minimum input range, the slope of the current waveform in the second interval changes accordingly, the blue waveform corresponds to case 2, i.e., the input voltage is greater than the output voltage. The black waveform is the stored reference for case 3 at the same output power. The input voltage is smaller than the output voltage.  
Figure 20: Efficiency of output converter.