You don't need to monitor your battery level. it will take care of it for you.
Play a sound when your device's battery reaches a certain level or status, such as when it's fully charged or low.

Key features :
• Configure your sound notifications based on battery status (full charged, low battery or custom battery percentage)
• Custom sound notification: You can choose any audio file
• Run in background and automatically start after you boot up your device or update it
• Free, simple, and easy to use