There are various 12v battery drain issues on the C-max. I also had problems and had a heck of a time getting info on the issues. Most dealerships and techs are not familiar with these cars and rarely if ever have tried to tackle these problems. There can be chafed wires near the trunk and a couple of other areas that can cause the battery drain. Mine happened to be the 12v water pump and 12v vacuum pump connectors. They would get water in them and, since there is a constant 12v supply at the pumps, cause them to run when the HVB was charging. After replacing the water pump, connector pigtail and re-splicing the vacuum pump wires, I have had no problems. Let me know if you're interested in info on checking some of these items.
