Why is my battery indicator staying at 100%

I just installed a new iFixit battery in my iPhone 3GS. I gave it a full charge and let the battery rundown by playing videos. Throughout this process the battery stayed at 100% the number never decreased. After a few hours the phone eventually shutdown. No warning like the battery is at 20% like is typical with iPhones. I had to plug it into the charger before the phone would turn back on. I then gave it a full charge and repeated the video playback process. Same thing happened. 100% power until the phone shut off. I have even done a full factory restore and not restored my backup. Anyone have any ideas?