If your NBN Co FTTP battery needs replacing you will recognise this from the audible alarm coming from the power supply unit. It is really annoying and catches your attention straight away. The lead acid battery inside the unit is to provide power to the FTTP NTU in case of power outages, problem is in a power outage your router wont have power. First thing you want to check that it is indeed complaining about the battery. Have a look at the NBN Co battery backup power supply warning lights, is the battery indicator on? If it is it means that your battery charge is below 50% indicating that the battery needs replacement. Silence the alarm by depressing the silence button for 30 seconds, thus will silence the alarm for an hour. If you want to replace the NBN Co power supply backup battery you can do a search for NBN battery replacement service or you can replace the battery yourself,