Yesterday I performed "open heart surgery" on my phone to replace its battery (phone doesn't have a user replaceable battery). Actually went pretty well following a ifixit guide (ifixit.com/Guide/Samsung+Galax…).
Just letting my phone to discharge to 0% and I'll do a full charge to measure its capacity. Only then will I glue the back back on !
Fun huh (no pics available because, well, my phone was off at the time)