Pre-charge Circuit — V_bus RC Rise Curve

Why a resistor is needed before closing the main contactor — and how the BMS decides it's safe

Pack voltage (V) 400 V
Pre-charge resistance (Ω) 50 Ω
Inverter bus capacitance (mF) 3.0 mF
Threshold (% of V_pack) 95%
—
τ = R×C (ms)
—
Time to threshold (ms)
—
Peak inrush (A)
—
Peak R_pre power (W)
Without pre-charge At key-ON, inverter bus caps = 0V.
Main contactor closes → instant short:
I_inrush = V_pack / R_pack_int (could be kA!)
Arc welds contacts shut. Pack fault. Fire risk.
With pre-charge resistor R_pre limits inrush to V/R ≈ 400/50 = 8A.
V_bus rises as RC exponential.
BMS polls V_bus every 10ms via sensor.
When V_bus ≥ 95% V_pack → close main, open R_pre.
R_pre sizing Min: limit inrush below contactor arc current.
Max: τ must be short enough for fast startup.
Typical: 30–100 Ω, rated for V²/R watts for 5τ.
Wirewound or thick-film power resistor.
BMS detection V_bus sensor reads every 10 ms.
Timeout: if V_bus < threshold after 3×τ → fault.
Could mean: cap short, R_pre open, bad sensor.
Fault logged: PRECHARGE_TIMEOUT.