A smoke detector typically uses a 9-volt battery as its power source. However, some newer models may use AA or AAA batteries. The battery is responsible for powering the smoke detector and ensuring that it functions properly. When the battery in a smoke detector is running low, the device will emit a chirping sound to alert the homeowner that the battery needs to be replaced. This chirping sound is usually a short, high-pitched beep that occurs at regular intervals, such as every 30 seconds or so. It is important to replace the battery in a smoke detector as soon as it starts chirping to ensure that the device continues to operate effectively. Regularly checking and replacing the battery in your smoke detector is crucial for maintaining the safety of your home and alerting you to potential fires.