Default [Q] Extended battery issues

My other half's standard battery pretty well gave up so I bought her a Extended Battery With Battery Cover For Samsung Galaxy S3 ( 3500mAH, 3.7V ) and it doesn't seem to have made any difference at all to what she was originally getting out of the standard battery

I was wondering if maybe just putting the battery in and using it like a standard battery isn't correct? Her phone is rooted and running Omega rom.

On the site where I ordered the battery from it says the battery should last up 70 hours but 24 seems to be the go

Any help appreciated