2013 Macbook Air suddenly shuts down

Hi guys.

My 2013 Macbook Air just suddenly shuts down automatically even if the battery is at 90%. Most of the time though it suddenly shuts down when the battery is around 70%. It doesn't do this though if it is plugged in to a power source.

I brought it to PowerMac and they did a diagnostic check and found nothing wrong with the hardware and internals.

Hoping anyone here has experienced this or knows how to resolve this problem.

Thanks.