iPhone 5 Battery Drain Problem, U2 IC Problem ! strange problem with iphone

Hello Guys, I bought iPhone 5 few days back and I observed very less battery time roughly around 8 hours on standby.

I restored it using iTunes as a new phone the battery time ramained the same 8 to 9 hours Along without use even Along without wifi and SIM.

Thought it would be a battery problem then I replaced it Along with another one, the problem remains the same 8-9 hours battery stand by time.

In short, tried all software solution and changing the battery problem, is it related to U2 IC? My phone charging is working fine just the battery time problem.

The phone died once it was drained out then had to take the battery out charge it manually then worked.