2012 MBP Retina 15inch screen black at 30% battery

Check your battery's condition using System Profiler and clicking on the Power entry in the sidebar. I suspect your battery needs to be replaced.

That shows a good battery, so you may wish to check the charger. I would also suggest taking it in for service as something is not working properly. Nevertheless, I would like to see what this looks like when the charger is not connected.

From what I see there is nothing wrong with your battery. However, the Full Charge and Charge Remaining numbers are the same. That's a bit unusual for a battery with 589 cycles. So, it is possible the battery has a problem, but it doesn't show it in System Profiler. I suggest you take it in for service with the battery run down close to the 40% level so you can demonstrate what happens when it reaches 30%. That is truly odd.

Have you tried reinstalling Yosemite? A dysfunctional upgrade might cause this to happen.

Reinstalling OS X Without Erasing the Drive

Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Reinstalling OS X Without Erasing the Drive

Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the Macintosh HD entry from the the left side list. Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Continue button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

Alternatively, see:

Reinstall OS X Without Erasing the Drive

Choose the version you have installed now:

OS X Yosemite- Reinstall OS X

OS X Mavericks- Reinstall OS X

OS X Mountain Lion- Reinstall OS X

OS X Lion- Reinstall Mac OS X

Note: You will need an active Internet connection. I suggest using Ethernet

if possible because it is three times faster than wireless.

Check out what shows in Activity Monitor.

Open Activity Monitor in the Utilities folder. Select All Processes from the View menu. Click on the CPU tab in the toolbar. Click twice on the CPU% column header to display in descending order. If you find a process using a large amount of CPU time (>=70,) then select the process and click on the Quit icon in the toolbar. Click on the Force Quit button to kill the process. See if that helps. Be sure to note the name of the runaway process so you can track down the cause of the problem.

Using Activity Monitor to read System Memory and determine how much RAM is being used (OS X Mountain Lion and earlier)

How to use Activity Monitor - Apple Support

Runaway applications can shorten battery runtime, affect performance, and increase heat and fan acti…