The battery life is only shown in the Sonos app. If you're connected to Sonos with WiFi, the Sonos app will show you the battery charge by the name of the room in the room screen or at the bottom of the app. Over Bluetooth, if you open the Sonos app, when off of the wifi, you'll see a new screen with the product image, it's name, and the battery life left.
You'll also get an amber LED indicating when the unit is very low on power (15% or under), but no battery life is displayed outside the Sonos app.
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