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So, the main contactor is basically welded closed? When did that happen? It's possible that the resistance could be almost 0Ohm. My fluke multimeter sometimes says 0.1Ohm when it's really almost zero ohm. Fran told me once that the mosfets can fail shorted if the capacitor bank fills up too fast. I don't know the mechanism for why, but evidently he's seen that. So, do you have something like this:
+125v Battery Pack --------- 10 Ohm ---------- switch ------------ AC and DC cap +
+125v battery pack -------- 0.1Ohm (contactor) --------------- AC and DC cap +
+125v pack ground --------- AC and DC controllers ground
That would mean the precharge part isn't really doing anything, since all the current would be hogged by the 0.1Ohm part of the circuit. That could explain why the failure happened after closing the DC breaker, but before closing the main contactor. I wouldn't do anymore tests with the AC unless you remove the contactor from the circuit, and maybe fill up the cap manually by some other means. Maybe something like this:
permanently wire 10Ohm resistor from +125v pack to the AC cap + (well, permanently just for testing). No breaker in the way:
+125v battery pack ------- 10Ohm resistor ----------- AC cap +
Then, for the high current path:
+125v battery pack ------- dc breaker ---------------- AC cap +
+125v ground -------------- AC Cap minus.
Then, because the caps are always full, you can open and close the breaker any time you want, and don't have to worry. Careful about running the code with the breaker open, though, since I caught a precharge resistor on fire by doing that. haha
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