My 2017 C-Max Energi was completely DEAD (no LED of any sort lit-up) when we returned from a months-long trip to Arizona. The car was stored in our attached garage with the 110v OEM Ford charging cable plugged-in, as recommended in the Owner's Manual, 2017 C-Max, Page 253, as follows:
"12V Battery... We recommend the following options for your plug-in vehicle:
Leave your vehicle plugged in. The 12V battery maintains power if left plugged in. However, this periodically uses electricity from the household outlet."
This is exactly what I did. They lied. The 110v charging cable did NOT 'maintain' the 12V battery in any way. Consequently, the 12V battery was dead upon our return, and there was no response from the car. It was dead, dead, dead. Full stop.
When I connected my Schumacher digital battery charger to the posts (as identified and described in the Owner's Manual, page 199) the weirdest thing happened: A pair of 'relays' in the fuse box began making 'popcorn' sounds. They were really 'popping' loudly and continuously. I used a metal rod between the relays and my ear to identify WHICH relays were making the sounds. They are (according to the Owner's Manual, page 210) relays numbered as "R12" and "R14", which are identified as "Power Relay Cooling Fan" and "Engine Control Relay", respectively. I tried this three times, with the same result each time. Occasionally during this process various LED lights in the C-Max flashed and flickered, but there was no consistency to it. The 12V battery failed to charge, so I discontinued the experiment.
The Ford dealer, of course, said to "tow it in" and "we'll take a look at it" around the middle of next week. Uh huh.
I decided to follow the "jump start" procedure described in the Owner's Manual (page 198) using my F-150 as the power source. The C-Max 'started' immediately, as if there had never been a problem, It 'ran' as an EV (the ICE didn't start immediately), showing "100%" on the dashboard battery meter. Apparently, having the car plugged-in to household current for four months kept the high-voltage battery fully-charged (as expected), BUT contrary to what the Owner's Manual says, it did NOT 'maintain' the charge in the 12V battery.
I fear that the 12V battery may be damaged ("sulfated") because it was drawn-down to 0%. Time will tell. As things stand, I test-drove the C-Max for a dozen miles (enough to run-down the high-V battery) and the ICE engaged correctly. The C-Max is now sitting in the driveway with the Ford 110v charger plugged-in, and I'll check it later tonight to see if the car acts normally, or not. At the moment, I don't trust it.