Right. My cousin recommended me to charge the phone whenever it comes near to dying. He was wrong, luckily I searched on the YouTube and the guy in the video suggested to charge the phone whenever you are not using it, don't let it drain all the way then charge.
I bought my iPhone 6S 64GB (used) at 79% battery health, it's still there after nearly 1 year of use, however, I have to get the battery replaced. I've been also thinking of upgrading to iPhone 7S plus for the dual-camera setup & better battery life.