Really? Searching rapid battery drain in iphone 5 after battery replacement does NOT bring up U2 chip? I'm shocked.
Con is right, that is definitely the most likely case. The only other possiblity is a short on the board from some old water damage that you are unaware of. Worth it to open it up and have a look. Let us know what you see. If everything looks good, I'm afraid it is U2 ic chip. It will be hard to find someone willing to replace that for you. Sometimes I give it a shot, but I don't offer it professionally as a service.
jessa