How do I fix "Battery is not charging" for 2012 macbook pro retina? I have a Macbook Pro, and it's battery is not charging. 

If I switch off the power adapter, the computer instantly switches off. No shut down sequence. I have to plug in the power adapter to switch it on. 

On each switch-on, I am presented with the screen showing Macintosh HD. And a network selection option at the bottom. On pressing the arrow under the HD icon, the macbook boots up. Takes quite long. About a couple of minutes. Before this problem started, it used to startup in a few seconds.

Mackbook details:



Battery remains at 0%



Battery status:
Battery appears normal and macbook detects it.



Things I tried: 


Reboot. 
Reset SMC. 
Clean magsafe port. 
Disconnected, and reconnected the battery plug by opening the back.


None of this worked.

I also tried to find the answer in other questions here, like 
Macbook Pro (Mid 2012) Battery Not Charging
and 
Macbook Pro Retina – &quot;Battery Is Not Charging&quot;

But nothing has changed the battery status.

How do I troubleshoot this?
Troubleshooting is a process of elimination and often requires patience. 

In this situation, if you've already reset the SMC (and it may be worth doing again, ensuring you've followed the correct steps for your model), you'd normally want to isolate the different components by testing them separately (e.g. testing the battery in another 2012 MBP, testing the AC charger with another MBP, and so on).

Of course, when you're trying to do things yourself, that isn't so easy.

Even though you've reset the SMC, I'd do that again first and ensure you refer to How to reset the System Management Controller (SMC) on your Mac on Apple's website. Read through this page carefully as the instructions differ for different models of Mac.

Once you've done that, and assuming it has no effect, you'd want to run Apple Hardware Test.

Run Apple Hardware Test

Your model MBP uses Apple Hardware Test. To use this, follow these steps:


Shut down your MacBook Pro 
Restart your MacBook Pro
Press and hold the D key before the gray startup screen appears.
After a while, Apple Hardware Test (AHT) will start.
When prompted, select your language and click the right arrow. 
When the AHT console appears, you can choose to run Basic tests by clicking the Test button. However, I suggest you select the "Perform extended testing" checkbox before you click the Test button.
Your test results will appear in the window in the bottom-right of the console.


Note 1: that the extended test will take some time. Take a note of the results and report back. 

Note 2: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete.

Once you've done that, let me know what happens.

Your battery

Based on your question, your battery has only done 27 cycles, so I'm assuming you've only recently bought this battery. If so, I would be speaking to Apple (or the 3rd party vendor) from whom you bought the battery as it is possible it's an EEOL (Early End Of Life) situation. Any reputable retailer will be willing to test/replace the battery in this situation.