This message is not necessarily indicating that an app has high battery usage but that it is using battery in the background. The Cannondale app needs to run in the background so that it is ready to connect to your sensor when you start riding your bike. We have put a lot or work into ensuring that the Cannondale app uses the minimum amount of power. When just running in the background you should not see any impact on your device’s battery life.
If you want to see what amount of your battery the Cannondale app is using, you can see that in Settings under Battery Details.
Here are some details on how to disable that notification.