batteries for cell phones

I have an old nokia cell phone which I have not used for almost one month now because I was issued with a new cell phone by our company. Even though I didn’t use my old cell phone I still switch it on because from time to time I have to see some of my saved notes there and I like using it’s built it camera. I was just wondering because even though I did not use my old cell phone, the battery easily gets empty. Well, I guess I have to look for batteries for cell phones and have it replaced with a new one.