My laptop shuts off at 40% battery without warning

My Windows 7 Inspiron 1545 always seems to shut down whenthe battery is only at 40%, it used to shut down when it got down to about 5% which is reasonable. But now when i'm not expecting it, the laptop just turns off. A notification dosen't even come up anymore telling me when the battery level is low. I'm not sure how to get it back the way it used to be, i've already set the power settings to default but it hasn;t made a difference. It seems i can hardly use this laptop without it being on charge the whole time.