First, make sure you have the latest version of iOS 9 installed. Then go to Settings > Battery and toggle on Low Power Mode. When Low Power Mode is enabled, the battery indicator will turn yellow. Low Power Mode will provide a notification when the battery is below 20 percent.
You can make additional modifications to apps within the Battery settings such as disabling notifications on an app by app basis before Lower Power Mode kicks in. Apple recently confirmed that closing running apps does not improve battery life. In fact, it can negatively affect the performance. However, disabling features such as Location services and App Refresh can help you get more out of your battery life.
Launch Settings > General > Background App Refresh and then toggle off the apps you don’t need to check for new information on Wi-Fi or Cellular Data.
Location services located under Privacy uses GPS, Bluetooth, and crowd-sourced Wi-Fi Hotspot to determine your location. In addition to being a privacy concern, having this enabled eats up battery life. You can adjust access to Location services on an app by app basis.
Other quick adjustments you can make to extend battery life include; reducing screen brightness, disabling background services, not in use such as Wi-Fi and Bluetooth.
Overall, Apple has added significant improvements to iOS 9 that make it more efficient at managing battery life. Using Low Power Mode can help squeeze out a couple more hours before you need to plug it in.