But by disconnecting the fender end of the negative ground wire, you are not disconnecting the battery. The battery is still grounded by 3 wires coming from the BMS (Battery Management System) module still attached at the negative end of the battery and therefore to your car with 3 seperate wires in a shielded cover. I believe these go to the engine, car body and alt. The module that they come from has letters and numbers stamped on it like Fomoco C95Y0 SW 2 21 and
AV6N 10C679 88.
May be why there was a hot smell looming the first time out. Just a thought...
Maybe Wolfe 10 can help us out here with some info on this module and 3 wires on the negative battery post end of the battery cable on the 2012 ford focus. Purpose, function and where they go and why.