Currently, I have three ionisation-type smoke alarms in my home. They are just the normal cheap type that come with most homes. They are mains powered and also battery backed (as required by Australian standards).
When the 9v batteries approach 9 volts (always at 0230 in the morning when it's the coldest), they let out a single chirp periodically to let you know the battery is low. But it's not a soft chirp, it's the same type and volume as the full alarm sound, so naturally it scares the shit out of you when it goes off. It's not just the one alarm that goes off either, since they are all interconnected, they all activate.
I'm looking to replace them with something a bit more sensible. Even at full alarm, they don't need to be that loud since there are multiple alarms throughout the house. I have looked at the Nest alarms however what I don't like is that they have built-in microphones that form part of the self-test function. Other than that, I do like how they function and provide non-startling audio feedback.
Does anyone know of something similar to the Nest alarms but don't pose a cyber security/privacy risk?