Why wont my iPhone charge beyond 50%?

I am the owner of an iPhone 3G which I've had for around 25 months now.

The iPhone was unable to charge past 50%, i tried all methods, docking stations, USB, Ca Chargers, Wall Socket, nothing, the very most it would go to is 65% but it would quickly go back to 50%. Even after being on a Wall Charger for several days the battery remained no more than 50%. I did some research and learned that iPhone batteries tend to last between 12-16 months, and so i replaced the battery last week. When i got the new battery and put it in it was fully charged and i thought the problem was solved, but after putting it back on the charger it again would not go beyond 50%.

I have tried restoring the phone from iTunes many times, and holding in the power and home buttons to reset it, but still nothing...

Can someone please help me as alot of my everyday schedule is stored on my iPhone, and i used it to read my work invoices.

Thanks.