A number of Philips Hue devices have a battery that must be replaced. How often depends on the device and the usage - a motion sensor drains it's battery much quicker than a Hue wall switch module (which battery life should be more than 5 years according to the manufacturer).
While you can see the battery level for some devices in the offical Hue app one at a time, you can't see for all (the dimmer switch as an example).
Using Hue Log, you get a complete overview of the battery status of all your Hue devices.
In the sidebar, under the Status section, click Battery.
Levels are indicated with color and percentage. If it is red, the device may fail at any moment. If it is yellow the battery level is low and some features may stop working (for example, software updates of the device's firmware may be skipped).