Before you buy a new battery, try reviving the old one.
1.Take the battery out of the casing and wrap it in newspapers (thick enough so no moisture will be in contact with the batteries).
2.Leave it in fridge for approximately a week.
3.Let it heat up to room temperature
4.Wipe off any traces of moisture.
5.Put the battery back in its casing.
6.Place in the laptop and charge. (Do not switch the laptop on!) Make sure it is 100% powered up.
if you follow this can not revive your battery,you should buy a new one.
suggest you buy replacement laptop battery,cheap and have one year warranty
such as http://www.laptop-battery-adapter.co.uk