Any app on your phone that requires a wireless (Bluetooth) connection to upload data can use more of your phone's battery. The Pivot app uses Bluetooth to connect with your breath sensor and upload the data to the app. Both of these processes use your phone’s battery.
One way to minimize this battery usage is to close the Pivot app after each use (adding a cigarette, doing a lesson, etc.), and avoid connecting your sensor to your phone after each individual breath sample. Your sensor will store data for you until the next sync, so you don't have to sync after every breath test.