The battery that came with my Omnia exhibited the same weakness until I conditioned it by letting it almost fully discharge and then giving it full charge a couple of times. Worked wonders for the life. I confirmed the same behavior with two spare batteries I bought. I first charged them up and put in the phone. Battery life was much, much shorter than the original battery now has.
After conditioning them the same way, they are now on a par with the longer battery life. Try this, it really makes a difference.

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Wait until tomorrow to Procrastinate.