Pretty odd to need a new battery like that. My OnePlus One is 3 years old now and the battery is fine.
The wifi is giving issues though, hence the OnePlus 5 currently in transit.
Yeah it still lasts me at least a full day of heavy use, but there are days where it just drains itself, so I think it's on its way out. Tried all the other usual fixes and recommendations.
At $12-odd for a new battery it's cheap to replace, and if it really really isn't that, I'll look at a new phone, but seriously, this phone is still performing as well as it did the day I bought it, very happy with it.