However, for optimal performance, it's important to regularly test and replace the smoke detector 9V battery as needed to ensure reliable operation and uninterrupted protection against fire hazards.
To keep your smoke detector fully functioning to the best of its ability, here are recommended maintenance practices to follow. Test your alarm monthly. Replace batteries once every year.
Measuring at about 2.5 by 2.5 by 2.5 inches, the mini alarms sold in packs of two or three and came with lithium-ion batteries ... smoke alarms installed until receiving the replacement alarm.