In traditional battery-based PV systems, the components associated with battery charging are all connected together on the DC side of the system—the PV array connects to a charge controller which is connected to the battery bank. This has some cost advantages, such as only needing one inverter. These systems also have some limitations, particularly with the maximum PV system voltage and the distance between the PV array and the balance of system components (i.e., charge controller, inverter, battery bank, etc). The PV array voltage will be limited by the charge controller; most charge controllers can accept between 150 and 250 VDC from the PV array.
To overcome losses from long distances, the wire size between the PV array and battery bank can be increased— but that adds cost. Another option is to AC-couple the PV array. This means connecting the PV array to a batteryless inverter, which can accept much higher voltage (up to 600 VDC), which reduces the array current, so that smaller-diameter wire can be used for long wire runs. The AC output from this inverter is then connected to a backup subpanel. A second, battery-based inverter is also connected to this subpanel. The battery-based inverter provides the stable AC power source that the batteryless inverter needs to operate. It is then up to the battery-based inverter to properly connect to the utility grid and charge the batteries—in essence it manages the batteryless inverter.
This solution can be used both in utility-interactive and stand-alone systems (see HP133, page 43, for an example of an off-grid AC-coupled system schematic). When used on the utility grid, it is critical that the battery-based inverter is properly listed to UL1741 standards for utility interconnection. While it is technically possible to connect nearly any batteryless inverter to a properly listed battery backup inverter, there is only one inverter designed to fully function in this capacity—SMA America’s Sunny Island. The Sunny Island allows the battery-based inverter to directly communicate with the batteryless inverter and properly manages the energy flow. Other configurations require the use of external relays and controls to ensure the battery bank is properly protected from overcharging.