Q: Macbook Air battery dies

Hi Apple support!

I bought a Macbook Air from 2011 and it has worked perfectly fine till about late 2015 when the battery began to trouble. It wasn't a huge problem, since the warranty was expired and I just had to replace the battery with a new one. But now, a year after, the battery is performing peculiar.

When the battery is fully charged and i plug out the charger, it runs fine for about 20-40min but then strangely turns off immediately. It is only if I plug the charger back in, my macbook starts from scratch, but the battery shows that it has only been drained to about 80-70%. Someone who knows what the problem is, and do I have to buy a new battery for the Macbook to work?

Thank you in advance

Kind Regards,

Frederik Petersen