The length of time a CR123A lithium battery will last in an alarm sensor varies depending on the sensor that is being used. Some sensors will allow for a significantly longer battery life than others. On average, a CR123A lithium battery will offer a lifespan of three (3) to five (5) years. The CR123A battery size is known as a "camera battery". This is because these batteries are frequently known for their wide-spread use in photography equipment. These are cylindrical batteries that are slightly shorter and fatter than the more common AA size. But the CR123A size found a new use in the security industry, as many alarm manufacturers began using the size to power wireless sensors. Today, CR123A batteries represent one of the most commonly used battery types in security system equipment. Every wireless alarm sensor will rely on one or more batteries for power. This power is necessary for communicating with the alarm panel. Some sensors may also use this power for general operation. This is typically the case for sensors that commonly feature LED lights, such as motion sensors. This also applies to sensors that rely on a microphone for listening, such as glass break sensors. Without a charged battery, a wireless sensor will no longer be able to communicate with the panel, and it will no longer work properly. A battery will lose its charge with continued use. Once the voltage for a battery drops below a certain threshold, the sensor will usually report a low battery condition. This trouble condition will appear on the panel to let the user know that it's time for a replacement. Failure to replace the battery will soon result in the sensor no longer communicating with the system. When that happens, the system will eventually display a loss of RF supervision trouble condition to let the user know that the sensor is no longer being detected. This trouble condition will appear after the sensor has failed to check-in with the system after a certain period of time. Please note that an RF supervision condition can only appear if RF supervision is enabled for the zone associated with the wireless sensor. If RF supervision is disabled, then the condition won't appear. Battery life is an important specification for a wireless sensor. A sensor with a short battery life will require more battery replacements, which can add up over time. That is why many users will make sure to buy sensors with longer battery lifespans to avoid the need for frequent replacements. You can often find the expected battery life for a sensor in the device's installation manual. This information may also be listed on the product packaging or inside of the product's user guide or starter guide. When selecting batteries for use with alarm sensors, we strongly advise choosing lithium batteries, as opposed to alkaline batteries. Lithium batteries will provide the longest lifespan. Typically, alarm system sensors can go three (3) to five (5) years without battery replacements when lithium batteries are used. Alkaline batteries will generally provide a shorter useful life. But please be aware that some sensors will offer longer battery lifespans than the usual 3 to 5 year period, while others will require more frequent battery replacements. Again, it can vary significantly depending on the sensor model that is being used.