Updated by Bill Lofton 12/30/15
Courtesy of Elithion
Precharge circuits are used to precharge the load capacitors in motor controllers before the contactor is closed to fully energize the motor controller. Without a precharge circuit, upon closing the contactor, the resistance of the circuit connecting batteries and load capacitors is so low that a near short-circuit condition occurs.
The resulting in-rushing current is well above the peak current rating of the installed batteries, contactor, and load capacitors. Bad thing!
How To Precharge Capacitors