iPhone Battery Sudden Drains

Hi guys.. I am now facing the battery problem of my iPhone 4. That problem is quite weird.

Everytime when my Phone battery is 23 or 22%, battery suddenly went down directly to 19%.

I experienced this problem on every Phone that I poccessed. I meant this is my 2nd exchange phone. My first iPhone got that problem too.

I switched off all services excpet wifi. I set up a couple of mails in built in mail app.

I thought there was something wrong in my backup data. Then I set up my phone as new iPhone and I installed some of my favourtie games.

The problem is still there. Battery status showed me 23% driectly went down to 19%.

I expreienced that problem not only on iOS 5 but also on iOS 4.3 and above.

I have no idea why this is happening..

Anybody facing that kind of problem too... or any solution??

iPhone 4 , iOS 5.0.1