Battery

You can monitor your battery usage on IOS devices from Settings / Battery where percentage use of battery is shown, either for the last 24 hours or 7 days. If you click on one of the apps you see more detail of how much time is used to refresh the App in the background (when you aren't using it).

When out and about some apps are power hogs constantly checking for information such as location in maps. You can switch this background use off from Settings / General / Background Refresh in order to save some power. This might limit the app's functioning of course.

When out and about, the main way to limit mobile data use (ie: not wireless) and therefore save power, is to switch apps off for using  mobile data, in Settings / Mobile. When overseas this is particularly useful. Podcast and music are very greedy of data and power because audio files are large. They should be downloaded when in range of a wireless network.

Portable 'power-bank' backup for phones etc is readily available with a large number of small rechargeable batteries that can be carried around, particularly while travelling.