Understanding AC-Coupled Batteries

Understanding AC-Coupled Batteries


AC vs DC Coupled Battery Systems

DC-Coupled System

In a DC-coupled setup:

Solar panels generate DC electricity → this DC power directly charges the battery → then it is converted to AC by a hybrid inverter for use in the home or export to the grid.


AC-Coupled System

In an AC-coupled setup:

Solar panels generate DC electricity → a solar inverter converts this to AC → the AC electricity is used by the household and/or sent to a battery inverter, which re-converts AC to DC to charge the battery.

How Power Flow Monitoring Works

Accurate load calculation in a solar-plus-storage system depends on the inverter’s ability to monitor three key data points:

  1. Solar Production – the amount of power generated by the solar panels.

  2. Battery Power – positive when discharging, negative when charging.

  3. Grid Power – positive when importing power from the grid, negative when exporting.

By using these three values, the inverter can determine the actual load (i.e., how much power is being consumed by the home or site):

Load = Solar Power + Battery Power + Grid Power

Example 1:

  • Solar: 3000 W

  • Battery: -2000 W (charging)

  • Grid: 0 W

    → Load = 3000 - 2000 + 0 = 1000 W

Example 2:

  • Solar: 3000 W

  • Battery: 0 W (fully charged)

  • Grid: 2000 W (importing)

    → Load = 3000 + 0 + 2000 = 5000 W

Example 3:

  • Solar: 6000 W

  • Battery: -2000 W (charging)

  • Grid: -1000 W (exporting)

    → Load = 6000 - 2000 - 1000 = 3000 W

Methods for AC-Coupling a Battery System

There are several approaches to integrating an AC-coupled battery such as a SkyBox into an existing solar PV system. Below are three common configurations, each with its own capabilities and limitations.

1. AC-Coupling with a SkyBox-Deye System Without Modifying the Existing PV

In this setup, a SkyBox (Deye inverter) is added alongside an existing PV system without altering the existing solar inverter.

  • A CT (current transformer) clamp is installed on the grid connection and feeds data to the SkyBox.

  • How it works:

    • If the CT detects that the grid is supplying power to the household (i.e., importing energy), the SkyBox discharges to support the load.

                        

    1. If the CT detects that the grid is receiving excess energy (i.e., exporting), the SkyBox charges the battery using this surplus.
                        

  • Limitation:

    • Because the SkyBox does not directly measure PV production from the existing inverter, it must infer energy flows based on grid import/export readings.

    • As a result, the real-time power flow readings in the Deye Cloud monitoring app may not be fully accurate.

    • However, reviewing the historical graph data or the Power Profile typically provides a more reliable representation of system performance.

2. AC-Coupling Using a Matched External Inverter (Same Brand: Deye)

In this configuration, the existing solar inverter is also a Deye, matching the brand of the inverter inside the SkyBox.

                  

  • Advantage:

    • When both inverters are registered under the same plant in the monitoring platform, they will appear together in the monitoring interface even if they are not physically interconnected.

    • This allows for a more unified visual representation of the system, simplifying performance tracking and user experience.

3. Adding a Second CT Clamp on the Existing Solar PV Output

Another option is to install a second CT clamp on the output of the existing solar PV inverter, feeding that data into the SkyBox.

  • Benefit:

    • This allows the SkyBox to measure the actual PV generation independently of grid measurements.

    • It improves the accuracy of battery charging behavior and enhances monitoring reliability, even when the PV inverter is from a different brand.

                  

Advantages of the Victron SkyBox for AC-Coupled Monitoring

The Victron SkyBox offers superior accuracy in power flow monitoring compared to some other AC-coupled configurations. This is primarily due to its dedicated AC-coupled input port, which allows it to directly measure power from an existing solar inverter.












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