I have had 4 Nest Protect units installed for the past 3 years and never had a problem. A few days ago, they started chirping once or twice a minute and checked the app where I noticed one of them said "batteries very low" and when I went to the unit is smelled like something was burning. I removed the batteries (all original batteries provided with unit) and found one of them (top on left) was very, very hot and the other 5 seemed normal. I placed the hot battery in the kitchen sink to observe and it took more than an hour to become somewhat normal temperature.
I see others have posted the same issue but have not seen the resolution. Are these units safe / could they start a fire? Is the Nest unit defective or the Energizer battery?
Update - After contacting Google nest support, they started an investigation collecting a lot of information. They do not believe the other units are at risk but asked me not to use the defective unit again. They sent me a replacement unit and return label so I could provide the defective unit/batteries for further analysis.