Lithium battery sudden voltage drop to 0.5V

Has any body had this happen? the first time I had a long conversation with Sargent who remotely monitored the system and came to the conclusion it was the battery at fault. Checking the graph the first time the voltage steadily dropped from 13V to 0.5V and rose back to 13V, nothing running, just the control panel. Dealer replaced the battery with one that would not charge then another which did!!
Happened again last night with a call from Sargent, I'm getting suspicious about the quality of TN batteries