MacBook turns off when charger is unplugged

Hi everybody,

So suddenly tonight my Mac turned off by itself after being unplugged from the charger, even though it says it is 100% charged. Last night this wasn't happening.

I have noticed over the past few months that the full charge lasts less and less, something like 5 hours or so. I usually just have it plugged in all the time. I admit I use it a lot, so it doesn't surprise me battery capacity is going down. But it was only tonight that the 'Replace Now' message popped out.

I downloaded Coconut and the stats aren't looking good at all, which makes me think it's absolutely time to replace the battery.

I don't mind using it only while plugged in while I replace the battery, but I was wondering if there is any chance it isn't the battery since again, my Mac insists the battery is charged.

I reset the SMC and the issue of it turning off by itself after being unplugged persists. It also may take two or three times of pressing the power button til it turns on.

Thank you!