The idea was that as the power was shut off, all of the isolating contactors open and the NC pole would complete the discharge circuit. It relied on the precharge relay remaining on as long as the main contactor was on. On the first time I turned it off, there must have been an arc carry across, as the relay internals were vaporised, the trace was vaporised and the very expensive 200 A fuses in my battery pack blew. At least they did their job superbly. Easy fix, but expensive.
So this circuit was not going to work. I have since removed the discharge circuit (NC pole to the battery negative of the shunt) by grinding the trace off - net result is a bike that works safely and discharges fully in 2.5 minutes. Now in the exceedingly unlikely instance that the bike is crashed, the rider is pinned and the inverter's DC bus connection is doubly severed, there is a risk that the capacitors will still store a lethal jolt, for about 2 minutes anyway.
How should I safely discharge the capacitors in under 5 seconds when the E-stop is hit(cutting power to all contactors and the inverter) WITHOUT adding new and exciting failure modes involving fire and sparks?
A separate relay and resistor across the switched terminals of the main pack voltage could work, but it would rely on there being an NC pole to do this, and the relay would need to be held NO whenever the main contactor is closed. This won't help, however, as the precharge and main contactor relays are managed by the inverter. As the inverter tries to precharge, it now has a 50 W load on it, so it will never reach full charge on the caps and won't turn on.
I just don't like putting things across the terminals of a battery if they add to the risk of a short and fire. Any other ideas?