Since the iOS 9 update, you have the functionality Energy saving. It reduces the energy used by the device, thus making the battery last longer. According to Apple, this mode can increase your battery life by up to 3 hours.
It allows you to stop the automatic fetching of your emails (Fetch), the Hey Siri function, the background refresh of apps, automatic downloads and certain visual effects.
It is triggered automatically only when you have 20% remaining autonomy. You will be alerted by a message which will offer you to activate the Energy saving mode, then the battery icon will then turn orange.
This is a feature that must be activated manually.
In fact, to activate the Energy saving mode of your smartphone: go to Settings → Drums, then activate Energy saving.
When your battery reaches 80% energy, this mode turns off without you needing to touch anything.