I just went through this - a friend drove me around in their car so my 2013 C-MAX just sat there over the weekend, short 12 mile trips to and from work followed by the two coldest days in the last 10 years. 3 times now went I went out to clean off the snow and when I went to unlock the door with smart key the car was "dead" and I had to use the emergency key in the FOB to get it open. As soon as I started the engine everything was fine. I had just changed the FOB battery about 4 months ago and neither FOB worked so I suspect the car was going into a deep sleep mode to protect the battery.
As someone else said, you really shouldn't have to run the engine on the C-MAX to charge the battery since there is a 12V inverter that is powered from the high voltage battery. You just need enough juice from the 12V battery to have the computers boot and close the relay contacts on the high voltage battery - then the car will use power from the HVB to spin the crankshaft and start the car and the power goes from the HVB to the 12V inverter which charges the battery.
Just for good measure I went out on the cold mornings and started the car, I also hit the red MAX DEFROST button which will override all of the temperature controls, blast heat out the vents and ensure that the engine keeps running. I guess I sort of ignored my own advice about not needing the engine but I figured one thing the battery does need is heat, and it was 4F outside for a few hours.
I actually made a few trips yesterday and there were no issues this morning so I have to assume that it was indeed that the battery was low, probably old and not keeping a good charge due to the cold. I think these batteries also have less cold cranking amps because really they don't need it, they aren't trying to turn over an engine, that's the job of the HVB.
I think just to be safe I'm going to arrange with Ford service to have the 12V battery replaced as I still have the original which by the VIN that car was produced May 2012?? I have all of the software updates which you should visit the service shop and get if you don't have them already.
I think they are doing a special for $99 motorcraft battery. The 12V battery on its own ranges from $87-$150 depending on where you buy it from. Plus it looks like you'll need some Torx and metric sockets to get to it, pull up some panel covers, disconnect some vent ductwork. I did it with my Prius, they don't make them easy to get to because it's an item that seldom needs replaced. All things considered by the time I buy the battery and the tools and take the time it's probably just worth letting Ford Service take care of it as long as that special is still going on.
I know some folks who don't drive very often or have classic cars use battery tenders but I think it would be pain to hook one up in this car unless you do it at the jump points under the hood? I remember panicking a little when the guys in the garage across the street said "oh we work on hybrids!" then opened the hood on my Prius and were trying to figure out where to put jumper cables when they didn't see the battery. These cars have multiple expensive computer modules that don't like being shorted out.
Edited by jestevens, 08 January 2018 - 07:51 AM.