You may be experiencing any of these issues:

• Your phone's battery drains quickly.
• Your phone's battery requires frequent charging.
• Your phone's battery level drops quickly without active use.
• Your phone's battery does not stay charged as long as it should.

Ensure the phone is fully updated, by following these steps:

1. Navigate to Settings > System Updates. The exact path will differ depending on your phone.
   These are the three typical paths:
   
   Settings > System updates > HTC software update
   Settings > Software update
   Settings > AT&T Software Update
    
2. Select HTC Software Update to check for new updates.
3. If an update is available, follow the on-screen prompts to download and install the update.
4. If no update is found, confirm the device is on the latest software version by navigating to Settings > About > Software Information. 

Most third-party app issues can be resolved by updating the app or the device. Follow these steps to update your apps:

1. Open the Google Play Store app.
2. Tap the Menu icon and select My apps & games.
3. Check which apps listed under Updates on the Installed tab have available updates for them.
   To update all apps, tap Update All to begin downloading and installing all available app updates.
   To individually update apps, tap the app, and then tap Update.

Some applications are not optimized for battery use as well as others, and may affect phone battery life. Follow these steps to uninstall applications that have been installed since the start of the issue:

1. Navigate to Settings > Apps.
2. Select an application that was installed after the issue began.
3. Select Uninstall.
4. Repeat these steps until it is determined recently installed applications are not the cause of the issue. 
   If there are many applications to uninstall, performing a factory data reset may be more efficient. See How to factory reset a device.

Battery optimization helps extend battery standby time. Follow these steps to optimize battery life:

• Install the latest software and application updates; updates sometimes include battery performance improvements.
• Uninstall or disable apps that you never use.
• Many apps run processes or sync data in the background even when you're not using them; if there are apps that you don't need anymore, close or uninstall them.
• Keep the screen off while charging.
• End running applications by tapping the Recent Apps button and then swiping applications up to close them.
• If using a Google Play Edition, swipe left or right to close the app.
• Ensure that GPS or Navigation applications are not active while using the phone indoors; this can cause the phone to drain the battery quickly or charge slowly since it will be using battery resources trying to connect to GPS.
• Make sure you are using the charger supplied with your phone.
• For the fastest possible charge, power the phone off prior to charging.

For additional tips, see I am having charging problems.