[MetroMPG]

SW...

I think it's more of a dummy-proofer. A boss of mine once told me "you can't make anything foolproof because fools are so ingenious."

I seriously think a kill switch would more worthy of your time and more likely to be appreciated by first responders. Another place for an interlock is on any opening compartment that exposes the traction wiring - but then again good insulating covers may be better overall. Those kind of interlocks too often get defeated while the thing is in "R&D."

Pretty much. Besides being a safety valve of sorts, it's also a way to disconnect parts of the circuit w/o putting switches everywhere. The potbox probably doesn't need a fuse because it's just a 'sense' circuit. The 5k pot varies from a dead short to 5,000 ohms and is just a voltage divider for the controller input. It shouldn't have much current flowing if all connections remain correct. Besides some controller internal malfunction, the only danger would come from some higher power wire coming in contact it. This is where the quality of your physical electrical connections will pay off in overall reliability.

It would be easier to wire if everything except 'power' was 12V but it depends on the parts themselves.

A 36 volt contactor coil energized with 12V won't pull more current because of the lower voltage (prob less) and thereby heat up, but there could be another problem with the lower V. If as you said, they came out of the forklilft as 36V devices, then 12V may not be enough to get them to close firmly. If they engage weakly, or worse chatter, then the power contacts will probably arc, self destruct, and otherwise ruin your day.

Also, if the correct V is 36V, then 48V into the contactor coils would entail more power draw and more heat, but I don't know if that's destructive. If it is and you still want to go to 48V or more, there are ways you could handle that with varying amounts of scope creep:

1. Just use a wire coming from the third battery (36V worth) for the contactor circuit. This one falls into the "if it's stupid and it works then it ain't stupid" category. It means assymetrical loading of the batteries but maybe only slightly.
2. Use an electronic regulator circuit to drop the 48V down to 36V for those coils (it's not as complicated as it sounds).
3. Change the contactors to 12V ones (prob the most expensive).

BTW, I'm pretty sure the small wire to the controller still has to be at full pack V.