The Elro FS4610 Duo Pack (10 years) consists of two extra-thin smoke detectors with a long-lasting battery. They feature a built-in lithium battery with a lifespan of 10 years. Thanks to the slim design, you can install the smoke detectors anywhere without them being very noticeable. The LED light on the detectors turns off in the dark, so you won't suffer from unnecessary light in your bedroom. When the optical sensor detects smoke or fire, the detectors emit a loud signal of 85 dB. With the battery indicator, you will know exactly when the battery is running low. Replace it in time and regularly test to ensure that the smoke detector is still functioning properly.