These are a good product, I have had two for about ten years, they are linked together so when one goes into alarm mode, they both do. The battery change is simple enough once you get the hang of it. They last about a year. They have a sticker indicating the replacement date when you are expected to discard them. One of mine went faulty a few months before the date. I decided to purchase two of the same model to replace them. They have a 5 year guarentee and the manufacturer will aparently replace any one that developes a fault within five years of purchase. You do not need a reciept, they can tell when they were manufactured.