Depending on the age of your battery, it may be near the end of its usable life. After many charging cycles the parts of the battery that store power start to erode which reduces the flow of electricity through the battery to the laptop.
One thing to try before replacing your battery (hopefully using our guide on our troubleshooting page) is cooling the laptop down via a laptop stand. Some enthusiasts believe this helps the battery to stay cool and hold a better charge.