I got my iPhone battery replaced today. I found this bit of information that came from a flyer in the store very interesting (this is copied from Reddit btw).
The 40-80% rule is true. In truth, 20-80% would also work. The upper and lower extremes stress out the lithium ions. Think of a battery like a balloon. If you completely fill a balloon every day, eventually it won't be nearly as elastic and snappy. It will get saggy.
Charging only to 80% doesn't work for a lot of people. If you charge your phone to 100% and drain it in 1 day, it won't work for you. If you're like me and use 25-50% max in a single day, the 40-80% rule works. A lot of people will reply "If I charge to only 80%, I am already losing 20% capacity, so even if I charge 0-100% and lose 20% capacity over a year, I will still have 80% capacity after 1 year, which is what you are using on day 1." This is true, but a phone charged from 20 or 40% to 80% will last a lot longer. Accubattery claims charging to 80% extends the battery lifespan by something like 211%.
Lithium ion battery technology has not advanced at nearly the rate as screens and camera technology has. They still make lithium ion batteries the same as ever. The advancement has come in quick charging and wireless charging.
At the end of the day, living your life worrying about battery isn't worth it. A new battery installed is anywhere from $40-$70, and that's for an oem Samsung battery and they will maintain water resistance. I personally use about 35% in a single day on average. Charging from 40-80% works for me. I won't miss out on that 20% capacity, and my battery is kept at a lower voltage and lower strain than 100%. Going from 80-55% every day is much better than 100-65%. I'll never run out of battery, and I can extend the life of my phone a bit longer.
People always love to say "this information is outdated, and that batteries are "smart" now. Samsung knows fast charging ruins batteries much quicker than normal charging, but it's all about an 18-24 month lifespan. VERY few people bought their s7 and planned to keep it til 2019 or beyond. There are people on this subreddit who have gotten their free battery replacement from Samsung for losing 20% capacity in under a year. That right there tells you that these batteries are very inclined to rapid capacity loss.
An average lithium ion is rated for 300-500 full cycles (charging 100% capacity) Since I charge 35% per day, it'll take me 3 days to use 1 cycle. If you fully charge your phone every day, you will come close to the lifespan of the battery in a year. This 300-500 cycle lifespan is something that is impossible to find out about the s7. Samsung never specified how many cycles the battery is rated for. My guess is that it's in this window. If it were good for 700 cycles or so like some people claim, there's no way that so many people would have already lost 20% capacity in under 12 months.
Charge your phone however you want. A new battery is cheap, but if you send it in to Samsung, you will be without it for a week or 2. A local shop can do it in a day, but if they're not Samsung certified it may lose it's water resistance.