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Public Service Announcement: You Should Not Force Quit Apps on iOS (daringfireball.net)
John Gruber, writing for DaringFireball: The single biggest misconception about iOS is that it's good digital hygiene to force quit apps that you aren't using. The idea is that apps in the background are locking up unnecessary RAM and consuming unnecessary CPU cycles, thus hurting performance and wasting battery life. That's not how iOS works. The iOS system is designed so that none of the above justifications for force quitting are true. Apps in the background are effectively "frozen", severely limiting what they can do in the background and freeing up the RAM they were using. iOS is really, really good at this. It is so good at this that unfreezing a frozen app takes up way less CPU (and energy) than relaunching an app that had been force quit. Not only does force quitting your apps not help, it actually hurts. Your battery life will be worse and it will take much longer to switch apps if you force quit apps in the background. [...] In fact, apps frozen in the background on iOS unfreeze so quickly that I think it actually helps perpetuate the myth that you should force quit them: if you're worried that background apps are draining your battery and you see how quickly they load from the background, it's a reasonable assumption to believe that they never stopped running. But they do. They really do get frozen, the RAM they were using really does get reclaimed by the system, and they really do unfreeze and come back to life that quickly.
I have an iphone 6s, and I see the location icon come on when I check the weather (using the native app), because it is set to reporting local weather. Mostly, it goes away in a bit - but I have seen the icon stay on many times even when I have moved away from the weather app. I have to manually kill the weather app to make the location tracking go away.
5 or so years ago, it was definitely clear that leaving apps open was causing battery drain, and I obsessively closed everything as soon as I was done.
About a year ago I heard someone from Apple suggest it wasn't necessary any longer, and for the most part I leave things up and it seems to be true that they're not tremendous battery hogs.
Except Google Maps, of course. That burns through something like 1% of my battery every minute, and definitely keeps working even in the background. I will look stuff up, kill it, and repeat as necessary many times over rather than leaving it open any longer than I need to.
The Quirkz Handbook of Self-Improvement for People Who Are Already Pretty Okay
settings->battery. Scroll to bottom for a report of how much battery each app has used. View it as last 24 hours, or as last 7 days. and FYI, (for me) Apple Mail, when connected to a corporate exchange server, sucks battery in the background. 30% overnight iphone 6s IOS 10.3