Selection of voltage for DC systems

For example whether to select 24V DC system or 48V DC system or 110V DC system. What factors are to consider for this selection of like UPS or solar power system?

The same thing that made Edison's DC system a loser, Ohm's Law and voltage drop. Remember Ohm's Law tells us that for a given load the higher the applied voltage the lower the current through the wires. So for a given conductor size higher voltage means more power delivered for the same ampacity of the wire. As an engineer you should know that you have the choice to keep the voltage low and increase the wire size, or to raise the voltage and use smaller wires.

In addition, the voltage chosen can also be a function of the amount of energy storage capacity (kWh) that is required for applications like UPS or battery backup.

Battery manufacturers produce cells with fixed storage capacities given in units of amp-hours based on a cell voltage of approximately 1.8-2.2 volts/cell. If you require a certain energy storage capacity of E [kWh], then pick a standard battery storage capacity like C [amp-hour] and voltage V. The battery energy storage capacity will then be equal to E = CV and the number of cells will be roughly equal to V/2-volts-per-cell if one series string is designed. Other string combinations are also possible.

To refine your calculations further, the depth of discharge (minimum voltage) needs to be considered (based on battery chemistry) along with the number of charge/discharge cycles to determine the battery lifetime. Battery manufacturers can assist with these calculations if they are provided with the design requirements for the application.

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