A red flashing battery icon on your iPhone means it's not charging properly. The reasons for this problem can vary. Perform some basic troubleshooting procedures to diagnose and fix the red flashing battery icon on iPhones running iOS 8.
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Why Your iPhone Shows a Red Battery Icon
A red battery icon flashing three times, indicates your iPhone's not receiving enough power to charge properly. The cause could be a malfunctioning USB port or a third-party charger. Try plugging the phone into another power source or attach it to a known good charger. If you're charging via your computer's USB port, try plugging the cable into another port, and make sure it's compatible -- USB 2.0 or 3.0.
A stationary empty red battery icon means the battery is so low on power, it doesn't have the juice yet to even turn itself on; it needs to reach its its minimum charge point.
Checking Your Connections
If you're sure the phone is connected to the right kind of power source, check your connections. Make sure the charging cable is inserted firmly into the port on the iPhone. Inspect the charging port to ensure it's not damaged or plugged with debris. If there is reddish or orange coloration on the port or charging pins, the iPhone may have water damage. Also examine the charging cable itself to make sure it's not damaged. Replace the cable, if necessary.
Understanding Third-Party Chargers
Not all third-party chargers work with iPhones running iOS 8 and later. If you have recently upgraded to the iOS 8 operating system on your iPhone, use a charging cable with MFi certification. Cables without this certification may not charge your phone properly and may produce the flashing red battery icon. Because this certification requirement is still relatively new, some third-party chargers may not meet it. If you have access to the original charger that came with your iPhone, use that to charge the device instead.
Replacing Your iPhone’s Battery
Sometimes a battery becomes damaged or is simply too old to charge properly anymore. If your iPhone is still under warranty, take it to an Apple store and request that the battery be replaced. If it's no longer under warranty, you may need to pay to have the battery replaced either at an Apple store or a licensed iPhone repair shop.
Third-Party Battery Monitoring Apps
A number of third-party battery monitoring apps are available for download for the iPhone from the iTunes and App stores. These apps bring different power management features to the device, including battery usage meters and warning sounds that indicate when the iPhone's battery is getting dangerously low.