You're completely right of course, I have no idea why I thought that the internal diode, would in any way be across the motor. Brain fart I guess.
I will see if there some way I can measure the inductance, my meter does it, but when I turn on the oscillator in the meter (needed to test inductance) the meter shuts off, and won't turn on until I turn off the oscillator.
I guess it's back to the old function generator, oscope, and an RC circuit standby.
This page claims that I will need a main capacitor. I would have thought the battery (110AH deep cycle) could hold it's voltage, and clamp the reverse spike on it's own (as long as there was a flyback diode). I guess on microsecond time scales it may not, especially with the inductance of the wires.
Not sure what kind of capacitor I would need for that. EDIT: The Open Source Motor Control (OSMC), uses a 1200uF electrolytic. It can handle 160A so....
It also shows using another MOSFET as the flyback protection. Which is a though, if I am going to have to order more parts anyway.....
EDIT: I also can't for the life of me figure out, how he's claiming that circuit does regenerative breaking. Reverse current from the motor is shorted throught the flyback MOSFET and lost as heat, and forward voltage when the drive MOSFET is off, wouldn't do anything but develop voltage across the flyback mosfet, which is always off because the gate is grounded to source.
EDIT2:I think I finnally "get" the regenerative breaking. If the Drive FET is off a voltage greater than the supply will develop on the motor, and that would put the drain of the Drive FET negative with respect to ground, meaning power would flow through the reverse diode in the Drive FET, allowing the side of the motor attached to V+ to go higher than V+ due to reverse voltage across the Drive FET, charging the battery. Is that right?