My iPhone 5s could not go 12 hours without needing to be charged. And this was with very minor use.
The repair is a little tedious, but with patience and care, it can be done without hurting the phone.
The pulling part, when you pull the old battery adhesive strip off, be very slow and gentle, specially with the one near the outside of the case, it rips very easily, and then you have to carefully heat and pry the battery off. I did this repair on two phones and both ripped and had to be pried off, but I think next time I'll know how to prevent it.