We all know that GPS is a battery killer. Our apps don't often run the GPS aside from 'warming up' when logging in, and attempting a lock during a form entry. The lock during form entry is dome to provide the default 'Completed At' meta data that is gathered with each entry.
If you want to turn this function off go the Settings page of your Form Screen. Under 'Advanced Options' you will see a 'Log User Location' field that you can unselect. From there, click on Save in the top right corner of the screen and it will turn off for that Form.
Note: Turning off the 'Log User Location' does not prevent you from having Location type fields in your Form. They only run when the user taps on the 'Capture GPS' button in their Form entry.
This approach is more efficient if you have many Form entries with few/no location fields being captured each day.
Aside from GPS, the amount of time the devices' screen is powered on will affect battery life. If the device is in constant use and the screen is on for a length of time it will consume more battery.
Set your brightness settings lower to reduce backlight power use, but this may not be feasible if the screen is not easily viewable as a result.
Lastly, the mobile device processor and its data networking chips consume power. If you have a lot of images being captured during the day it can affect battery life. This happens because images are processed by the app (cropped, resized etc) and then uploaded in the background. If you have many images the processor and network chips are running a lot. If your field information requires this, there is not much can be done.
Newer devices are becoming more power efficient. Using devices that have newer SoC (system on chip) hardware may be worth exploring.