The Albright SW200 contactor is a high-power relay that allows the battery pack to power the drive circuit and other devices.
This contactor is available in a variety of coil voltages. This build uses a 72V coil. The coil is energized by pack voltage, applied through a smaller relay with a 12V coil of its own, and with contacts rated for pack voltage (see wiring diagram). The reason for this additional relay is that the existing motorcycle switches on the handlebars are rated for low voltage and would be degraded through arcing and pitting if pack voltage was applied to them.
The reason I ordered a contactor with 72V coil is that I misunderstood the controller requirements at the time. There is a statement in the AXE controller Operator’s Manual that says, “[The contactor’s] coil voltage should also be rated for the pack voltage.” That is not actually the case. They probably meant to say, if you wire the system according to the Alltrax wiring diagram, the coil voltage should be rated for pack voltage. They depict the contactor coil being energized by pack voltage, but there is no reason you cannot use a coil that can be energized by your 12V system, and in fact that is preferred.
Using a contactor with a 12V coil would have alleviated my need for the additional relay with the 12V coil. It also would have allowed me to purchase a smaller, lighter, sealed relay such as the LEV200, EV200, or GV200 series.
The contactor must be used with a precharge resistor and flyback diode, wired according to the diagram.