Question about smoke dectector

Since our smoke detector is wired directly into the electrical system I have never replaced battery or do any maintenance. Yesterday I observed both units are flashing red light at interval in stead of green as I remember it should. Did I remember wrong or something is wrong with the units? Thanks!

It's been years since I looked at my smoke detectors so don't take my word for it, but could it be that the units have a backup battery in case the power goes out that are depleted?

I had trouble with the detector in my living room (Executive Plan - high ceiling) and it needs replacement. Your post provided the impetus for me investigate and reply. My unit is BRK Electronics Model No. 1839N. I could not find it on the BRK website. The unit does not come apart so that convinces me that there is no battery backup. The unit has a warning label that says: replace the detector by the year 2003.

BRK's customer service phone no. is 1-800-323-9005. They are closed today for President's Day. I will call tomorrow and get a replacement model. I think it would make sense to get a combo unit that is both smoke and carbon monoxide. With my high ceiling I don't want a unit with a backup battery if the unit alerts when the battery needs replacement but that maybe a requirement. I will post when I find out more.

Note: If existing units have a limited lifespan that required replacement back in 2003 we all may not have the protection we think we have!

I talked to BRK Electronics this morning. They recommended model no. 9120 which is an ion smoke detector 120V without battery that has a 10 yr warranty. I found it on Amazon for under $10. BRK and Kidde recommend having both ion and photo smoke detectors that can be separate units or combined in one unit. New construction requires smoke and carbon monoxide detectors.

It makes sense to me to replace my defective 1839N with the the 9120 and then decide if I want additional protection that can be battery operated in other rooms. With the 9120 I hopefully won't have to worry about it for 10 years and it will do the same job as the original.