I have a 2013 C-Max. I replaced the 12v battery after 4 years because batteries usually die at 5 years. The new battery lasted just about a year. This March, the battery kept draining to where I had to jump start several times. I take it to the dealer - they replace the battery at no charge under warranty. Several weeks ago (June 2018), battery dies again. Dealer replaces battery under warranty.
1) Either the batteries that Ford sells are hit and miss in quality or
2) Ford suspects (or knows) something more serious is occurring but it's cheaper to replace batteries than to fix the problem.
Note: We've had a lot of rain this year. There is a reference about some component that might rust, causing a drain on the battery. Certainly a possibility.
In either case, by carrying my little lithium charger, it's not that big of problem to jump start and as long as Ford continues to replace the battery every so often.
I can live with it until the new Ford Escape hybrid arrives. Hopefully, it won't have the same problem.