That's why dragging a car back to the pits at a racetrack probably won't cause any problems. OTOH, towing it on four wheels behind your RV @ highway speed would be disastrous.I found the towing report. Seems like maybe best to keep it below 7mph so there's no regen. Post #6 reports no adverse effects:
Well, it happened. What I thought might be just a close call turned into a depleted traction battery. I went to visit a friend today. If I start out with a full battery, I usually have just a bit over 50% remaining after the round trip, so I figured I'd be OK starting out with 50% in the tank. I...www.fiat500owners.com
In a nutshell, the PMSM will be generating some voltage, depending on the speed it's spinning. Every EV with an AC drive system (induction motors or PMSM motors) has an inverter that provides the 3 phase current for the motor. But, when things are run in reverse, like towing, it gets interesting. An induction motor requires a 3 phase signal for the rotor to make a rotating magnetic field. A PMSM will make a rotating magnetic field simply because it has magnets built into the rotor. That rotating magnetic field creates currents in the stator, which is electrically connected to the inverter. Every inverter has diodes that will rectify the 3 phase current into DC at some voltage. This is BEMF. If that voltage exceeds the battery voltage it will charge the battery. This uncontrolled charging of a lithium battery can be disastrous. That's the reason behind the manual's rules in Iceman's post# 34.
When I first purchased my 500e, I looked into this. I towed it using the cheepo tow dollies that support the front wheels and drag the rear wheels. As the manual says, that's safe.