Battery icon, percentage not going down
iPhone 6, IOS 11.0.3.
Last night I had an 'odd' event, where I looked at my phone around 9pm, it showed I had about 45% charge. About an hour later I looked, it was completely dead. Pressing buttons I got the discharged battery warning (battery pic with thin red line). I plugged it in, then when I looked at it again about 20 minutes later it indicated 100% charge. Obviously something is not right. I restarted, no change, so I left it plugged in as I went to bed.
This morning it shows 100%, but neither the percentage nor the icon have dropped in any way, despite normal usage that should have caused it to.
Thoughts?
iPhone 6, iOS 11.0.3