I believe the HVB will probably last as long as the car. With 136K mi. I'm getting as good of MPG's as I ever have. I think I'm using only about 10% of the HVB (38 to 48%) most of the time. IIRC I have seen a couple of times where actual SOC was less than 40% and the CMAX wouldn't go into EV+ Mode, not sure why. In EV+ I think I can get down to 32% SOC before ICE kicks in. When SOC gets below 34% the ICE jerks when starting.
There is a thread around here that describes four C-Maxes which where monitored to the max by some government agency. As I recall, after about 100K miles there was around a 5% reduction in some HVB measurement.
I do believe leaving the battery overnight at a lower state of charge will increase longevity, it probably isn't going to be that big a deal. (There is also a thread about the reasons for that.)
I also noticed on ebay C-Max hybrid HV Batteries for sale in the 500-600 range. They don't seem to be wearing out as fast as spares are becoming available. (Though people do buy used batteries to take apart)
Edited by obob, 19 August 2016 - 11:38 AM.