I have similar battery issues with my 6s Plus, but I've found that the % indicator is way, way off from what the remaining charge actually is.
For the first 15 minutes or so, it sits comfortably at 100% during use. Then it starts counting down rapidly, practically free-falling to some number between 85 and 95 before pausing there for quite a while. You can watch the indicator losing 1% every 5 seconds or so. This plummet-levitate-plummet-levitate thing (the rapid drops come in 10-15% sized chunks) keeps going until it's down to 1%. Then it keeps running for hours on this supposed "1%" before eventually shutting down. I mean that quite literally; I've had it run for up to 4 hours with 1% left (4 hrs of usage, not standby). You'd think that after showing it the true scope of its battery capacity, the phone would go "OMG! It's THAT much more than I projected? Wow. Thanks for alerting me to the huge power stash hidden under 1%. I will calibrate accordingly." Except it doesn't learn anything. It will be exactly as clueless after a calibration as it was before it.
The "normal" battery wear level reported by Coconut Battery (and Battery Life) is around 12% (88% of original capacity) - the phone is 17 months old - but once the free-falling is in progress, Battery Life will report anything from "VERY BAD - 70% wear level" to "BAD - 23% wear level". Battery Life will keep fluctuating between crappy orange and disaster red, and I won't see lime green ("AVERAGE - 12% wear level") until it's down to 1% again. If I reboot the phone at a time when the battery level appears unreasonably low, I often find that it magically regains 20 or so percent after restarting. What was 43% is now suddenly 65%, then it starts another rapid countdown and settles somewhere around 60% within a minute or two.
None of my other iOS devices behave like this. I have an iPhone 5s and an iPad Air that are both 3 years old and they're both at over 90% of original capacity, and discharging is stable and predictable. So I took the 6s Plus to an Apple Premium Reseller and the guy there claimed that it was normal behavior for iPhone models 6 and later. I was told the exact same thing by another guy when I called a support line. Well, it can't be normal behavior because then the battery indicator is so useless you might as well replace it with an 8-ball. 4 hrs usage on 1% would mean 400 hrs of usage and over 500 days of standby.
