Precharge Diagram

The key second position is usually the start position, unless you are talking about the accessory and ignition positions. The start position is a momentary connection so that wouldn't work without adding a latching relay. What you have drawn looks like a pretty normal precharge system. It requires the driver to remember to wait a couple of seconds between the time they switch on the precharge relay and main contactor. Another consideration is that the relay used should be rated to switch 120 volt DC. There are not that many relays available with high voltage DC ratings. It is possible to use a 5 or 10 amp 240 vac rated relay with appropriate snubber.

My buggy runs a Curtis 1221B controller and a 120 volt system. On the buggy the coils of the relay and contactor are parallel to each other, they both turn on and off together. The relay provides power to the ksi terminal and the contactor to B+. A 7.5 watt, 120 vac, bulb is across the contacts of the main contactor (this is the little round bulb that is barely larger than its Edison base.) This system leaves the controller capacitors always precharged to about 4 volts less than the battery pack.