Why is it difficult to charge from a generator?

A dual section "Variac", 7 kVA maximum.  For use at 110 volt 12 amps, the minimum safe power capacity required is about 2 kVA.

I heard from other Mini E drivers that charging the car from a generator was usually unsuccessful. I never had occasion to try it, but it seems like a valuable backup plan if you could count on using a generator. So I wondered why it doesn't work. The story goes that large diesel generators sometimes work, but the Mini E always refuses to charge from a small gasoline generator.

I believe the reason is actually a safety feature built into the circuitry from AC Propulsion. The car measures the supply voltage before it starts charging, and then if the voltage drops too much as the car starts to draw power, it will stop charging. It figures you are using an extension cord that is too long, too thin, or has corroded connections or is somehow otherwise a fire hazard.  This is why Mini explicitly says not to use an extension cord.

I noticed this safety mechanism once when I was trying to charge from a 110 volt outlet at a considerable distance. I had some very heavy cable (10 gauge, rated at 30 amps) but even though I was only drawing 12 amps, the car would refuse to charge through this very long extension.  So after a long dry spell, I drove on the grass to get closer to the outlet and found that I could charge if I plugged in directly, or even if I used 100 feet of my heavy cord. But at 180 feet, it would refuse. If I recall correctly, the limit was about 8 volts of drop from no load to full load. More than 8 volts drop and the car refused to charge.

I had a large variable transformer for other reasons, so eventually I tried using it to correct for the voltage drop.  It worked, and I was able to charge the car even using 180 feet of heavy cord.  I would set the "Variac" (as it is called) so that the output voltage was equal to the input, which was about 110 volts. Then as the Mini E started drawing current, I would turn up the knob so that the output voltage stayed at 110 volts, more or less. And the car kept charging.

A Variac usually can adjust the output voltage over a range from +20% to -100% of the input

So my theory is that a small generator would have a significant voltage drop under load. A Variac could compensate for this drop. If someone who is still driving the Mini E has a small generator and would like to test this theory, leave a comment and I will get back to you.  Maybe we can try it sometime.

It might not work, since a small generator would also change its frequency with load. I don't know if the Mini E cares about power line frequency, but it might. And frequency is harder to compensate for than voltage is. The large diesel generators that have been reported to sometimes work for charging the Mini E would likely have less variation in both frequency and voltage than a small generator.

If you are curious about the theory behind this voltage drop measurement, see the technical discussion on Wikipedia about Thévenin's theorem.