I upgraded my Companion software from 8.4.5 to 8.5.0 Beta 2 today.  After the upgrade was completed, my unit kept reporting less than 5% battery and would shut down immediately upon boot.  When I began the upgrade, my battery was >90%, and before one of the shutdowns I was able to get onto a charger which reported my battery was full.  During boot, the HUD would report I had ~92% battery, and then immediately drop to somewhere around -10,000% battery before forcing a shutdown.  I was able to fix the problem on my end by disconnecting the Main Controller and battery while shut down, and then reattaching them.

Core unit: XSU Main Controller
Battery: NS Naninte-Assisted Metabolic Power Cell 13-0001-C