Many battery life guides suggest quitting apps, but this doesn't save battery life and can in fact drain more battery. Closing out an app removes it from the iPhone's RAM, requiring a reload when it reopens, which uses more battery.
Don't close apps using the App Switcher
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Place iPhone face down
When you're not using your iPhone, place the handset face down.
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13 iPhone Battery Saving Tips to Extend Battery Life
How to improve iPhone battery life with low power mode, background app refresh, location services, and more settings