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Contactors need full rated coil voltage to latch closed, but once the contacts are shut, a smaller current is enough to hold them in contact. The hum you hear is from the controller applying PWM to the coil to reduce the power being dumped into the coil. Without it, they will get very hot, like 90C. I have main contactors in my battery box that get alarmingly hot. They seem to be doing okay, but it will probably reduce their lifespan. I suspect yours are doing just fine.
 

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I am not sure about your specific questions, but it does sound like the contactor you specified has a 48v coil, so it would have to be using the traction battery voltage. Honestly, I wouldnt mess with it. It might be worth revisiting if the contactor burns out and wont latch closed anymore.
 
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