# Design of Direct Current Microgrid Converter with Cost-Effective Low-Voltage Battery Storage System
## 11 Conclusions  
According to the selected concept of the need for voltage processing, we have built a simulation model of the selected inverter in a suitable power scale based on the required parameters. Identical topology was chosen for both the charge and output converter. Hence, we decided to investigate only the output converter due to the larger number of steady-state parameters. For this inverter, three control algorithms were selected; simulation validation of the parameters, especially the efficiency, was performed, resulting then in peak efficiencies ranging from 95 to 91%. For the simulated efficiency results, we assumed  
Figure 21: Measured current waveform through inductor in two different cases (black for case 3, blue for case 2). the result from the simulated current waveforms in a single current-accumulating element in the inverter. These assumptions were validated, and we chose the mode of operation (b) as a reasonable choice based on the cost and complexity of implementation relative to the available output power (estimated efficiency up to 94%). For this mode, we further investigated the current waveform for all three possible operating modes of the inverter, concerning the ratios of the input and output voltages. We then implemented the MPPT algorithm in one of the inverter pairs to find the maximum power point from the available solar energy source. Given the maximum input power point under consideration, we chose a perturb and a observed control algorithm; these are commonly used in low-power applications mainly because of their simple implementation. For the whole system, we selected 10 Ah battery storage consisting of 18,650 recycled lithium batteries, which are mainly used in consumer electronics. The processor provides protection against undercharging and overcharging by controlling and disabling the inverters. Protection against asymmetrical voltage distribution on the cells is provided by an active balancing circuit based on a commercially available solution using an integrated circuit with implemented power switches. After composing a working prototype using the values fitted according to the calculations in the simulation, we proceeded to verify all the above units and device properties. After the initial composition of the battery, it was connected to a balancing inverter that kept the differential cell voltages within the tolerance specified by the manufacturer and operated with a set mean current value at the desired switching frequency. From this, we can conclude that the balancing circuit is working as expected. The output converter was subjected to an efficiency test when supplied by a laboratory power supply due to maintaining a constant input voltage at the converter terminals. The shape of the efficiency curve corresponded to the simulated waveform. The inverter exhibited slightly higher efficiency, probably due it having a higher protection interval time than the simulation. The maximum efficiency was 94.3% at the nominal battery supply voltage in the 3S circuit. The self-consumption of the whole device, including the indicating elements, is included in the efficiency calculations. The highest possible peak inverter efficiency was 95%. The inverters were then connected to the battery for further testing. After measuring the efficiency, we focused on verifying the assumptions of the current waveform, which, according to the measurements, is consistent with the described waveform. To validate the MPPT charger function, we have described an alternate methodology without needing a solar cell with constant illumination intensity. We have used a laboratory source substitute with artificially predefined internal resistance. Thus, the shape of the power curve was changed, but it contained just one maximum that the described algorithm can achieve. The measurement results show a maximum 4% deviation from the actual maximum power point, which, together with the average observed efficiency of 93%, gives an estimated efficiency of 89.2% for the charging inverter.