An issue that has been discussed among the photovoltaic professionals is that of the battery voltage in the residential storage systems.
Is there a better configuration than others?
Basically there are three type of systems:
» those in low Voltage, about 48V;
» those said in “high Voltage”, 400V approximately;
» high voltage modular systems (from 250 to more than 500V). The latter are realized by composing several battery packs, like in Lego® bricks, until the wanted capacity is reached.
In addition to assessing the type of battery, we need to consider the complete system, including the electronic power that performs the coupling with the network or with the build system: you can think that the greater economy and efficiency is obtained by connecting the battery to the DC side of a standard photovoltaic inverter and using the DC/DC converter input stage.
With this configuration, the most suitable battery voltage would be starting from 1/3 – 1/4 of the inverter DC bus voltage and the system would see an economy as regards installation costs (cables of reduced sections and switches).
The advantageous battery
The producers of residential storage systems consider advantageous the use of that kind of cells that are spreading also for other use, especially in the car industry, that makes the price competitive in terms of €/kWh.
This leads to the use of cells with a capacity in the range from 40-60Ah and voltages of about 48V.
High voltage systems contain lower voltage batteries (around 100V) and a DC/DC converter that raises the voltage up to the rated voltage.
Compared to this type of configuration, high voltage modular systems do not need an additional converter and potentially have a higher efficiency.
Battery standard configuration and conversion system
It should be pointed out that today the standard configuration battery and conversion system has not yet been consolidated: 48V batteries have economic advantages, those in high voltage have potentially higher return.