If OS X isn't giving any warnings about battery when you click on the battery icon next to spotlight it's probably the charger. Owning the same laptop, I have replaced the charger not once, not twice but three times in the last 4.5 years I owned it.
The L-Mag Safe adapter could be one of the products with worst quality Apple has ever made.
And no, I don't use it harsh but on the contrary I use it very neatly.