iPhone 5: Charges to 100% but still says it's charging

I've noticed in the past few days that my iPhone 5 is doing something that I hadn't noticed before.

While it is plugged in to the lightning cable to charge the battery displays the lightning bolt withing the battery icon. But once it hits 100% charged (even after hours of sitting at 100%) it still says charging. Previously It would display the icon for being plugged in but not still charging. Now the lightning bolt won't go away.

So what's going on here? Is this going to kill my battery?

I think I may take it

Thanks