Which tablet do you have?
sorry kobo aura
Question? Did you let battery drain to 15% then charge then let it drain again then charge again?
no the battery was almost flat when i recharged but it only lasts 2 days on all bought books but lasts for the whole reading of library book up to 20 days the kobo is over one year old
okay can you let it drain to 15% then charge it again, then let it drain again to 15% and charge it again. See if that will bring up the battery for your purchased books. Make sure nothing is running extra on your Kobo that will drain battery also. If that doesn't do it, it sounds like you might have to return it because it might be defective. Not sure if it is exactly a year or you could still be under warranty.
having your wifi on is the biggest drain of your battery. you have to realize that the books in your library are already in the queue or downloaded to that will not drain battery that much (sort of like an archive) but when you purchase books that is a draining the battery a bit.
You can also try a reset: