Probably too late to be of benefit:
The 12VDC battery installed in a Ford C-Max Hybrid is a small battery, it is not used to provide power to start the gasoline engine. The battery serves two purposes:
1. Provide battery to power peripheral systems, such as interior lights, radio, various computers, door locks. These are mostly what is called Ignition Off Discharge or IOD functions.
2. Primary and most important, the battery provides the power needed to initialize the High Voltage for running the Electric Motor, Air Conditioning, Power Steering, and the starting system for the gasoline engine.
When this battery is discharged, nothing works for you cannot unlock the doors, start the electric motor or the gasoline engine (you are dead in your spot).
I have not yet determined if the EPS (electric power steering system) is functional during flat towing. The ABS (Automatic Braking System) is functional and will prevent a full lockup of the wheels in the event of a panic stop or breakaway situation; therefore, it is important that the battery remain in a charged condition during towing.
I have a SMI Stay In Play braking system installed in my toad (2013 Ford C-Max Hybrid) which also uses battery provided energy to monitor breaking conditions whenever the RVs brakes are applied. The SMI system has a vacuum pump to provide vacuum for power brakes (not used on the C-Max, I have it plugged to prevent the pump from running during braking) and to supply power to activate the brake pedal. There is also a G force detector that requires 12 VDC to function and to light LED signal that the Aux. is providing braking to the towed.
I have installed a charge lead from the motorhome to the C-Max controlled by an RVI charge controller mounted on the C-Max. When stopping over night you should switch off the SMI system to prevent discharging the C-Max battery, also it is a check list item for the start up procedure before leaving the overnight.
I highly recommend that a Tire Pressure Monitoring system be installed that will monitor tire temp and pressure on the Towing and Towed vehicles.
I hope to find answers for the following:
Is the ESP powered up during flat towing?
Does flat towing miles impact the oil change and preventive maintenance schedule of the transmission.
Does flat towing braking adversely effect the wear of brake pads and/or rotors.
Has anyone installed a wireless camera and monitor to provide vision directly behind their towed?