My Battery is dead (i think)

About three days ago my Windows 7 told me I need to consider replacing my laptops battery. "There is a problem with your battery, so your computer might shut down suddenly". I did the energy report thing and it said my battery's full charge was 37%.

The weird thing is, overnight my laptop went from lasting 2hr to 20min. It wasn't a gradual thing, I just don't understand?

Alright, my battery is a AS07B51 11.1V 4000mAh Li-ion Battery pack.

I had a look on the net and a new battery is $50 and I would just buy one but it seems a lot of people are having this problem with their Windows 7 Laptop.

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