Stock G928A will not boot with battery connected

Long-time lurker, first time posting on this forum. I have a SM-G928A S6 Edge+ that is only a week or so old that ran down and will not boot again. No signs of life at all, and no amount or combination of buttons will get it to display anything. If I disconnect the battery (I repair cell phones professionally), and plug the device in, I get the normal charging screen, and the phone will even boot correctly and run for about 30 seconds before rebooting. I replaced the battery with a new one, and it does the same thing. With either battery connected, no button combo, charging, anything will get the display to turn on. Nothing displays - completely black. There is no accessing recovery, no logo, no safe mode; staring at a black screen. If I disconnect the battery, it operates almost completely normally. Bone stock phone. Any ideas anyone?
Thanks!

Update G928A to MM

Hello,
Just bought this phone and i dont know if its a factory unlocked or unlocked with a 3rd party tool and Currently my g928a is having lollipop 5.1.1.
I want to update to MM, Will the phone Still be unlocked ?
Thank you guys.

Anybody know where i can get the G928A firmware for marshmallow?

It rolled out recently and I can't do an OTA. I've been looking all over for a legitmate file but no luck as of yet.

Will be eternally grateful for any help!