John
You say you put the PTC in the motor lead. Why there? Why not between the pickup and the board?
Also, what about using a littlefuse (?1 1/4 or so long with silver ends). One could mount a small fuse holder upside down underneath a tender. This would allow for replacement without removing the shell and would allow for experimentation on the fuse size to more precisely determine the lowest fuse size that will hold in normal operation?
Bill