I've been watching this as well, and it's frustrating to see the number of people playing engineers on forums on a system that they didn't design and don't intimately understand. I can't speak for Ford, Nissan, etc, but I do have a decade history of working on electric vehicles, robotics, and currently battery-powered consumer electronics, so I'm somewhat familiar with a variety of battery charging systems & techniques.
In almost all cases, the battery charging systems are very smart - they will make sure you don't overcharge or undercharge the battery. For larger battery systems, they have heating/cooling systems to quickly and efficiency charge and discharge batteries.
It's hard to imagine that cars with so much testing and so many sophisticated systems onboard would allow us, as drivers, to cause damage to our batteries. Again, I'm only guessing here, but I would imagine that Ford (and Nissan, Tesla, etc) have mechanisms in place to protect the batteries from the user's abusive driving & charging behaviors, to ensure the best range for the longest possible time. I know for a fact on that previous hybrids from Ford, they've been very clear that a full charge on a hybrid was not 100% of battery capacity, and that "running out of battery" was not actually 0%. They floated the Escape Hybrid between 40% and 60% of the capacity (so to the driver, 40% was "empty" and 60% was "fully charged").
Only a Ford or Nissan engineer would truly know the values behind the scenes, but I very much doubt that when we are at 100% charge on a Focus Electric, that you are using the full 24kWh battery pack at 100%. I would also bet that Ford put in some overhead to account for degradation (for example, call 18kWh as 100% so that as the battery pack degrades, your computed range does not for several years).