There is nothing special about the 12V system. Any 12V auto charger will work. Keep in mind that the faster the charger the more wear and tear it puts on the battery. Also if your FFE is misbehaving and something is not turning off like it's supposed to you could have a drain on your 12V battery that is killing it and if you just put a charger on it you can end up toasting the charger too. I learned that the hard way. If you're still on the original battery, it might be at it's end of life. Either they don't make batteries like they used to (I remember getting 7-10 years out of a battery) or all the electronic crap they have on cars now shortens the life of the battery as a battery in an FFE tends to last 3-4 years.
Unless you are letting the car sit for weeks there is no need for a 12V charger. The DC-DC converter performs the same function as an alternator in a traditional car. It provides ~14V DC to run all the 12V car systems and charges the 12V battery back up. When the car is completely off the 12V battery powers the module looking for your key fob and the telematics unit (TCU). It also provides power when the car "wakes up" and to energize the HV battery contactors. Once the HV battery contactors are energized the DC-DC converter takes over and starts charging the 12V back up.
I would suggest getting the 12V battery tested to see if it is going bad. I also suggest carrying a portable 12V jumpstarter so that you can jumpstart if necessary and not be stranded (make sure headlights are off when using it). It also might be worth the money to have the dealer perform a draw down test on the 12V before replacing the battery. This test checks to make sure the drain on the battery is less than 50mA when everything is completely shut down. If you have something causing excess drain on the battery (greater than 50mA) you will likely prematurely wear out a new battery and potentially have to replace it again in as little as a few months.
There are multiple threads on here about 12V issues you can read thru. Hopefully in your situation your battery has just reached the end of its useful life.
2014 Blue Candy FFE
http://www.fuelly.com/car/ford/focus/2014/triangles/303811 (since this forum doesn't allow BBcode in sigs)