I only have Fios Internet, and no landline so I don't want the battery connected in the BBU. I understand BBU needs to stay plugged into the wall outlet to provide the power to ONT. When I disconnect the battery in BBU, the red light for Replace Battery comes on on the BBU. Is this normal? Is it safe to have the BBU plugged into the wall outlet and keep using the Internet with this Replace Battery red light on?