iPhone turns itself off with lots of charge left

My iPhone turns itself off when it has a lot of battery charge left, sometimes when it's still at 70%. Then I can't turn it on at all unless I plug it in to a charger.... After I do that, the apple logo appears and it turns on as normal with lots of charge.

Why is this happening?

It appears your battery is faulty.

If you have an iPhone 5, check the iPhone 5 Battery Replacement Program to get a free replacement.

If you have an iPhone 6s, check the iPhone 6s Program for Unexpected Shutdown issues to get a free replacement.

If your phone is under warranty or AppleCare extended warranty, you may be eligible for a free battery replacement. Even if it is not under warranty, go to the Apple Support webpage, "Start a service request" and fill in your problem details so Apple can help you.

If you have a Mac, you can find information about your battery by plugging into your computer and installing the free CoconutBattery app to check its health.