Lots of excellent advice above.
Figure I'd add a little (heh):
Simple: Keep it out of the heat. Heat is the #1 enemy of a lithium ion battery. Don't leave it in a hot car in direct sunlight. Try not to do intensive games in direct sunlight outside.
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More Info (skip if not interested): If you are keeping your phone for just 2 years there's little you can do (minus leaving it outside in the sun all the time) to damage the battery through normal use. My wife uses her phone like most people - lets it sit on the charger all night, uses it till it gets down to 10%, then plugs it in again. I am OCD when it comes to batteries and I usually keep my battery between 80-100% MOST of the time because I sit at a desk for a job. Sometimes my phone will sit on a charger for days and never go below 100%.
My wife's iPhone 6+, after 1.5 years has 96-98% original capacity and hundreds of charge cycles. My iPhone 6+ after 1.5 years had 99% original capacity and hundreds of charge cycles. Wife had about 150+ more cycles than I did.
This told me that just USING the device is the best advice.
Like HEK said - unless you're planning on keeping the phone for 10 years - optimal battery % is 40-60%. Trust me, this gets old fast even an OCD person like myself. My 6+ probably spent MOST of its life on a charger, my wife's iPhone didn't and yet there was almost no difference in battery health at the end of 1.5 years.
So now, with my 6s+? I use it when I want, plug it in when I want, and I don't fret if it goes down below 80% anymore. I love carrying Anker external batteries with me so I can charge whenever I want (tho, more often than not, I'm charging other people's phones). In a few months we'll have had these phones for 1 year and battery health is showing similar results to our 6+ phones.
We already agreed to keep our 6s+ phones for another year and wait out all this iPhone 8, 7s, Edition stuff out. Even then, a year from now, I may just plop down $85? on a battery and just use the phone for another year or two (if it needs it (it probably won't need it but I like seeing 0 cycles)).
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Best advice really is just to use it as you normally would and not worry. Batteries will deplete naturally over time and usage. There's really very little you can do to ebb the flow.
Yep, this sums it up perfectly.
