Guys, read more carefully, Simran said a different thing: "do not leave you laptop for 5-10 days unused at a low battery".
That is: the battery is not broken, but getting old, be more careful about the discharge state, or you will shorten the battery life dramatically.
Why: Batteries for laptops are made of several elements which typically age differently from each other. The battery will last as long as its weakest element will. As the battery has lost a part of its capacity, leaving it discharge too much is more likely to completely drain one of the elements.
Normally the battery control circuit prevents this fully discharged state, forcing a soft or hard shutdown, but this may eventually go undetected and happen if one element is significantly weaker than the other ones, especially if the battery is slowly discharged while the computer is in sleep mode.
Lithium based battery elements, if left completely discharged for some time, they get irreversibly damaged. IF this happens, even only to the weakest element, the battery must be replaced (worst case scenario, it won't hold charge even for 10 minutes).
Advice: don't let the Mac rest with very low battery (the weakest element may get damaged in just a few days) and don't expect that the battery will hold charge for several days of sleep (it will appear to loose 10% or more of its charge each day, instead of 2-3% like when new). Apart from that, you can still use it as long as the weakest element doesn't age so much that it won't hold charge. Plan to replace the battery in the next future (some months, one year? depends on your usage pattern), do not wait until the last minute if you cannot afford to stay without your laptop.