I put it in my phone, turned it off and fully charged it before turning it back on. This is day 2.
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OK. I've read and been told that with a brand new battery, in order to "condition" it, for the first 5 or 6 cycles, let the battery drain all the way down to zero (or as close to zero as you can). Fully charge it back up, then do the same thing with each cycle until you've done it at least 5 times. It's a pain sometimes, when you'd rather just plug & charge, but it really does help the performance of the battery; at least for me it did.
However, if you find that that doesn't help, you may need to get it replaced.