Pre-charge Resistor

Connecting a discharged capacitor directly to a DC source produces an unbounded inrush current limited only by parasitic resistance and contactor wear — easily 10 kA peaks in EV traction systems. The contactor contacts arc and weld, the bus traces vaporise, and the capacitors can fail. The pre-charge resistor inserts a controlled R in series with the bus so the inrush is bounded by V_bus / R and the capacitors charge along an RC time constant.

The sequence: (1) auxiliary contactor closes, current flows V/R through the resistor into the capacitors; (2) capacitor voltage rises toward V_bus over 5τ ≈ 5 × RC; (3) controller monitors bus voltage and, when within 5 % of source, closes the main contactor in parallel — bypassing the resistor; (4) auxiliary contactor opens, pre-charge circuit goes idle until next startup.

Sizing: choose R so the peak current V/R does not exceed contactor or source rating; choose energy capacity ½CV² (which all flows into the resistor regardless of R value) and power rating compatible with worst-case re-cycling. For a 400 V / 5 mF EV battery system, a 50 Ω / 200 W aluminum-housed resistor sees 8 A peak, charges to 99 % in 1.25 s and absorbs 400 J per cycle. Pre-charge failure is a leading cause of contactor weld in EVs and BESS — the resistor is a critical safety component, not an afterthought.