Apple is currently investigating a bug on the iPhone 6S that displays the wrong amount of battery life while you're travelling - but there is a fix for anyone who runs into the problem.
The error occurs when you switch time zones automatically or change the time on the device manually. Some users are finding the battery percentage is not updating even though the charge is dropping.
To fix the issue, the Apple Support page suggests you restart your iPhone and then, once it has booted up again go into Settings, General, Date and Time, and then make sure 'Set Automatically' is switched to on.
Stuck in time
If the problem persists without you changing time zones, the advice is to contact Apple support.
Apple said it's aware of the issue and is exploring ways to fix it – we'd hope for a fix to come in iOS 9.2.1, but we're not sure when we'll actually see that.
It's not the biggest issue for the vast majority of iPhone users, but it is particularly annoying if you travel a lot.