Charge it up until it is 100%, disble windows from putting it into standby and run the laptop on the battery until it cuts out. Recharge fully, doing this once should recalibrate it but if not repeat again.
Note: Draining your battery to nothing can damage it but when it's not working that great anyway it's worth a try!
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Oh, also check in your bios as some laptops have a utility built in for recalibrating the battery. Mostly dells I have seen this on.
There are numerous tutorials available on the web.