Almost certainly bricked device, looking for advice I have a Galaxy S3 (d2att) which had been working fine on CyanogenMod 13 when one day it randomly shut off in my pocket and wouldn't turn on again. It was using a third-party battery, and afterwards, the battery appeared to be slightly inflated. That being said, it still wouldn't turn on even after trying two other batteries.
Other than that, here's a list of other random bits of info I found about the device in its current state:


The batteries are charged. Checked with a voltmeter.
The batteries are being charged through the phone. Voltmeter used again here.
If I take the battery out and then plug a usb into the phone, the notification light turns red for a few seconds before turning off, even if I put the battery in. This only works if the battery is out of the phone. For a period of time, putting the battery in before the light turned off would keep the light on until it was unplugged. Sometimes it would even display the Samsung logo before dying again.
When I plug the phone in without the battery as described above, "qualcomm hs-usb qdloader 9008" shows up as a device in the device manager. Until it dies, that is.
I've also tried using a USB jig. 
I also checked for a voltage across pins 4 and 5 of the usb while it was plugged into the phone. It displayed a voltage consistent with what would be expected. After using the JIG, this voltage would go away until the battery was removed. However, the phone did not boot into recovery, nor was it recognized by my computer.


So, my question is: is this phone dead for good? I imagine it is, but if it isn't then that wouldn't be so bad either.
Yep 100% sure it's hard bricked as the qloader 9008 shows up. 

The whole eMMC is damaged or just the boot partition. But as it was caused by a faulty battery, I guess it is damaged completely and would need to get replaced (eMmC chip).

There is 3 methods that can be used to recover the device: 


Repairing via an SD Card (for corrupted boot partition)
Repairing via JTAG (need JTAG interface, soldering in the main board, etc,)
Hardware replacement if the above didn't solve the problem.


After successfully completed the above, yoi should be able to flash a firmware normally.