You would have to ask a tax attorney. The problem is that the "placed in service" language is used for business property, it is not used for the home solar credit. The home solar credit says "when the original installation of the item is completed."
To me, placed in service means installed, inspected, and fully operational. "Completed" might or might not mean the same thing. If you can use the battery (it powers the house but is not connected to the grid) even though it is not inspected, that might be enough. If you can't use it yourself, that might not be enough. But I am not an attorney and can't tell you if there is any important difference between "completed" and "placed in service." It would be your risk to explain if you were audited.
