Alienware m17x R4 - Not charging.

The notification says - (plugged in, charging) but the battery is not charging and stays at the same level.

It will play the charging animation when I plug the AC adaptor in and turn the computer on, but disappears about 8 seconds afterwards. The adaptor is only 3 months old; I replaced the old one as it was intermittent due to wire damage.

The battery is 2 years old now but I very very rarely use the computer on battery, The laptop would be plugged in 98% of the time. I'm not sure if this is a factor in battery lifespan.

HWMonitor shows
Designed capacity - 93240mWh
Full charge capacity - 66700mWh
Current - 18440mWh
Wear level - 29%

I'm sure I just need a new battery, but wanted to be certain as they are quite expensive...