If you have nothing to declare for a particular declaration period, you will need to enter a timelyzero declaration.
To do so, go to https://mybatbase.bebat.be and log in.
Then click “Declarations” - “Enter declaration” - “Declaration”. Then click the relevant declaration period and click “Submit” at the bottom. You then reply “Yes” to the question “Are you sure you want to declare 0 batteries"?