There can be multiple reasons for why the tracker (suddenly) runs out of battery more quickly than expected. Please check the list below for common scenarios.
1. Movement activation is enabled and the tracker is moving.
When movement activation is enabled and the tracker detects movement, the tracker will switch to real-time tracking for 5 minutes, which means that for the following 5 minutes a location update will be sent every 3-5 seconds. This will drain the battery more quickly, depending on how much the device moves. For example if the device is placed in a moving vehicle or object (e.g. a bag), then it will likely trigger its movement activation very often. If you do not want the battery to drain very quickly, try turning off movement activation in the app (Settings --> Device "Tracker" --> Location updates).
2. Live tracking override is (accidentally) enabled in the live tracking screen.
When you open the live tracking screen in the app (which you can do by clicking on the tracker on the map or by clicking the live track button in the bottom right corner), there is an option to temporary switch the tracker to live tracking. This is done by clicking on the text in the top-right corner. When the text says "delayed" and you click on it, you get the option to switch to live tracking. When this is enabled the normal location updates settings are ignored and the device enters real-time tracking mode. This option stays enabled until you disable the option by clicking on the text again, or until the device is manually turned off. If the tracker runs out of battery very quickly, then check if this override has been set.