Sounds like your battery is dying/dead. Recommended you replace it and see how it goes from there. At some point the laptop may not boot up at all because the dead battery will try to draw too much power to even start up. If and when this happens take the battery out and operate the laptop while plugged in, bear in mind to keep the power adapter securely in the laptop, if you pull it out mistakenly your laptop will instantly turn off and this could damage the internals.
I agree. Your battery needs to be replaced. If the laptop is under warranty, contact the manufacturer. If it isn't, then you can either try doing it yourself, or take it to a local tech. Just be sure you get an OEM one. Aftermarket batteries are not worth it as they can cause all kinds of issues.