I changed the battery in my iPhone 6S (after about 14 months) and in my 11 Pro (after about 2 years). Both times instead of buying a new phone.
With the 6S I tried a local Apple retailer chain and in the 11 Pro I let it change at an Apple Store. I prefer the Apple Store experience, they cleaned it throughfully as well, the iPhone looked (and worked) as good as new afterwards.
Both were a bit over 80%, probably around 82-84%.