Can I do this with my 8xl/9920?
 

I want to connect in a series 3 6-cell packs in a series, for 18-cell. I see the 9920 will support up to 20-cell. Will the motor hold up?

Thanks,
Nick

Assuming 1.2v/cell, that will make the total rpm a little over 40k. That's a little hot if you plan on running at full speed a lot. But for brief speed runs or an easy trigger finger at the track, it should be fine. Just watch temps.

YEs, 1.2v/cell.

Next question, what motor could handle this without getting too hot?

Like I said, the 8XL will be good if you don't run it at that speed all the time. If you do plan to run max speed a lot, then a 9XL or 10XL would be a better choice. You generally want your motor to hit a max rpm of about 30k-35k rpm for best efficiency.