In the past few days my galaxy S9 has started to restart itself and then power down by itself. I have went through the settings and have also booted into safe mode, wiped the cache partition and factory rese . I haven't seen it power down while plugged into the charger. There haven't been any drops, falls or anything that could damage it to cause this and even though the phone is waterproof/resistant it hasn't been in or near water. is there a way to test the battery or troubleshoot further on my own or is it something I can't fix by mysel .
I'm sorry to see your S9 has been power cycling on it's own. You have already done quite a bit to resolve this, and I would be glad to troubleshoot further with you. I would recommend running a Device Health Check http://spr.ly/6608166sC to see if there any any alerts.
To clarify, did it still power down when you had it in Safe Mode?