Dell Vostro 1540 - Battery Issue

Hello guys!!

I've Dell Vostro 1540 and suddenly on my laptop, battery icon is showing a red cross and also says that consider replacing battery.

My laptop is year and half old and this mark is showing since 2 months now; also I've to keep laptop plugged in while using, when tried to use on battery it shuts down in 5 mins.

I'm planning to buy battery now but just wanted to confirm that my battery has drained out right? and there is no other issue in laptop?

Thank you. Waiting for your suggestions.