XPS M140 New Battery Won't Charge Past 50%

My old battery got old, and just ran out of power, so i got a new one, 9-Cell, and for a while it would charge all the way, giving me lots of time to run with out plugging in. but now the computer stops charging it at 50%. when i unplug from the wall the battery meter says 50%. also, now that i have this new battery, when it's "fully" charged the power indicator flashes orange until i unplug it.

is this a problem with the battery, computer, or the charger? and what can i do to fix it?

I'm running Windows XP Media Center