What causes an ESC to fry if batts are left connected?

[BrianG]

Maybe. The quiescent current being drawn has to be pretty small; I would assume under 0.5A. Even at 6s, that's around 12w (assuming 0.5A), and the heatsink/PCB should be able to handle that kind of thermal load. With the switch "on", the fan should auto kick on if the ESC got too hot.

The only way I can think of for the ESC to overheat in this situation would be if the switch is "off" (where the fan can't come on), but again, I would guess the HS should be able to handle it. Again, I am assuming a ~0.5A quiescent current draw - it could be higher or lower which would make a difference.

And, even if the HS did get hot, I would think that as temps climb, there would be a point where the rate of dissipation exceeds the rate of heat (due to differential between HS temp and ambient temp), and that temp point would be under the catastrophic failure threshold?