I did exactly that, replacing the old battery of a well-used iPhone 6 to give it a new lease on life. I was pleased to find the phone on eBay for only £75 (about $100 or AU$140) back in 2020.

For maximum performance, the battery health should show above 80%. If your battery health is under that, then it's time to replace your battery before you begin to see permanent damage to your iPhone.