Image credit: Apple
iOS 9 introduces a new feature to the iPhone called "Low Power Mode." Like Apollo 13 on the way to the Moon, it shuts down non-essential services in an effort to maintain an emergency lifeline. That is, enough juice to make and hold an emergency phone call, be located or utilize Wi-Fi. Here's how to activate it.
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Low Power Mode, according to Apple, temporarily reduces your iPhone's power consumption. Non-essential things like email fetching, "Hey Siri," background app refresh, automatic downloads and some visual effects are reduced or turned off. Screen brightness is dimmed and you'll drop into Auto-lock quickly even if your setting, otherwise, is Never.
When Apple's Senior Vice President Software Engineering, Craig Federighi, first introduced this feature at WWDC in June, he commented, roughly, that Low Power Mode knows how to throw power saving switches you never even knew about.
Activate it with Settings > Battery > Low Power Mode. You'll know you're in this mode because the battery icon turns yellow. You can also ask Siri to turn it on (or off).
Apple says you'll get about an extra hour of battery charge when Low Power Mode is turned on. Apple has also published some battery saving tips.
Alternatively, when you battery gets down to 20 percent charge, you'll be asked if you want to enable it.
Notably, among the things that are not disabled are Music playback, Wi-Fi and Bluetooth. Wi-Fi may be essential to navigation and assistance, and the Bluetooth maintains a connection to your Apple Watch, if you have one, or your car.
If you wanted to save even more battery power in an emergency, such as being trapped in an elevator, etc, you could conceivably turn off Wi-Fi and Bluetooth, leaving only the cellular radio powered up. Of course, Airplane mode would completely blind your iPhone to the outside world, and that wouldn't be of much help in an emergency.
If you have a choice on whether to use voice or data (via Wi-Fi) to communicate in an emergency, use Wi-Fi. It uses less power than a cellular connection that has to punch through walls to get to a cell tower, perhaps very far away.
In this era, with inexpensive USB batteries so very small and inexpensive ... and USB ports in our cars ... it's unlikely that one would run an iPhone battery dead. But in a natural disaster or other emergency, it's good to have iOS 9's Low Power Mode. It could literally be a life saver.