Credits to digdat0, copied his message.

I rolled back firmware from 1.04.0000 to .700 .. all was fine. Was trying to flash battery and bring it down to .700 in DJI Go 4 and it hung at 95% and then showed failed. Middle two lights stayed on. 

The solution is to pop that battery out, put a different one and update the ac back to the latest firmware. Then pull out the battery, put in the 2 led battery and load assistant, let it re-update the firmware (inconsitent battery message) and it will fix it. The key is to get it re-updated before the battery dies, otherwise, its dead. Not sure if i could fix this without a second battery.