Battery Backup on new battery

Guys I own a 160gb thin iPod classic, i changed the battery recently to a new one

The thing is battery indicator is not giving me correct position after 1 hr of playing it shows 90% and then after playing for a like 12 hrs or so it goes to red indicator and continues like that till its dead. i have tried caliberating battery, restoring it none of it works.

i got around 28 hrs continuous playback ( shuffle on , 320kbps mp3), there are no pending sectors or re allocations.

My question is isnt the 7g the most power efficient and should be giving over 40 hrs of playback.

Is there anything wrong with the hard drive. I used the HDTune Pro and scanned the sectors there are no errors and no errors in HD SmartData

What should i do ? Is it like everyone getting similar battery backup?