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Enable "airplane mode" when you're... on a plane
Again, your iPhone's battery life may decrease far quicker because it is constantly searching and "pinging" for a higher-quality cell site with better signal, especially when it's not there. If you're on a plane or on the subway with no connectivity, you may as well squeeze as much life out of your battery as possible by switching off all radios with a single button.
Under the swipe-up Control Center, tap the Airplane Mode button (first on the left). You can tap it again to switch all radios back on again.
Disable 4G (and LTE) connectivity
While 4G is faster than 3G cellular connectivity, it also uses a lot more battery power. Long-term evolution (LTE) uses an entirely different hardware radio and often drains the battery even quicker. When not using high-bandwidth applications, streaming videos, or tethered to a laptop or tablet, you can disable 4G and LTE connectivity altogether. You'll receive slower download speeds (albeit still rather fast by modern standards) but it will also help conserve your iPhone's battery throughout the day.
Go to Settings > Cellular, then switch off the Enable 4G (or Enable LTE on newer models) button.
Reduce auto-lock period to switch off backlight
Reducing how long it takes for your iPhone to switch its display off can help to considerably conserve battery life. When the smartphone is put down on a table, for example, it can take a minute or two — perhaps even longer — for the display to turn off. This means the display can quietly eat away at the device's battery life while it's not actively being used.
Go to Settings > General > Auto-Lock, then set this to 1 Minute or 2 Minutes, depending on your preferences. The shorter the time period, the greater the benefits to your iPhone's battery life.