I have a MacBook Air 13" High Sierra, early 2015. The computer is only a year old. Battery life is good, amperage good, holding a charge. The problem is, the very instant I unplug the Magsafe adapter it shuts down. Goes black. Dead. I've reset the SMC to no avail. I'm attaching images to show that the battery is indeed fine. Before I take this to the Apple store I thought I would check in with this bright, enlightened community.
I have a MacBook Pro 2009 with the exact same problem. I followed many internet threads. None of the advices worked, even after replacing the battery with a new one. Finally I changed the power setting to "better battery life" in the system preferences and the problem went away. Previously I had it on "higher performance"