After using the Spotify app on my Nokia 920 (AT&T), I repeatedly have terrible battery life. A typical use case is that I will take my 100% charged Nokia on the subway to work and listen to an offline playlsit for 30 minutes. When I arrive at work, I will have aproximately 90-95% battery life reamining. If I fail to plug my phone into a charger, the battery will be around 5-10% by the time I leave work 8-10 hours later even if the phone was left idle the entire time. If I do not use the Spotify app, restart the phone, or close the Spotify app by pressing the back button repeatedly, this issues does not exist.
This problem can be reproduced 100% of the time by simply playing a track on Spotify, pausing the music, pressing the home button to get to the start screen and letting the phone power off. Spotify does not need to be in the foreground for this to happen.
I believe this happens when Spotify is in offline or online mode, but I always listen to downloaded playlists as I do not have data coverage in the subway.
I'm having the same problem on my Nokia Lumia 620 (Windows Phone 8 ver. 8.0.10211.204). I'm getting about 8 hours of battery life whenever Spotify is open in the background or foreground. It doesn't matter whether the app is in offline or online mode. I can easily play music for dozens of hours through the built-in music app, so I'm guessing the small battery in the Lumia 620 isn't to blame.