What shall the relief vent in a battery venting situation be at minimum?

The minimum requirement for the relief vent in a battery venting situation is that it must be the diameter of the vent serving the battery vented system. This specification is crucial because the relief vent needs to effectively handle the air flow for equalization and prevent any pressure build-up that could compromise the drainage system's effectiveness. By using the diameter of the existing vent, you ensure that the relief vent can properly discharge air, maintaining the system's overall efficiency and venting capability.

The other options do not meet the necessary criteria for appropriate vent sizing. For instance, tying it to the diameter of the soil or waste branch may not provide sufficient air flow capacity, while halving the diameter of the building drain or specifying a fixed size like 1 inch could lead to inadequate venting. Such mismatches in sizing can lead to potential issues like siphoning or backup, which the plumbing code aims to mitigate through established venting standards.