New battery charges very slowly

I've just got the new battery that I ordered. I put it in my iphone 4 and I turned it on, so far so good. I saw that the battery was already a little bit charged, so i decided to use it untill it was around 3%. I plugged it into the charger all the night and when I woke up the phone was still charging and the battery level was around 17% after all the night. Did I get a defective battery ? Did I install it not correctly ? What can I do to fix it ? Thanks to everybody and sorry for my bad english ;)