AutoVoltix
EV NewsHybrid & PHEVBatteries & ChargingAutonomous & SoftwareClassic CarsICE VehiclesIndustry & MarketsStock & Financial MarketsMicromobilityElectric AviationHydrogen & Alt Fuels

← Back to all lessons

BMSLEVEL 2Reading time: 18 min
Learning Objectives
  • Explain the roles of HV contactors and the pre-charge circuit.
  • Explain why the DC-link capacitor is pre-charged, and what happens if it isn't.
  • Build a contactor open/close sequence using state-machine logic.
  • Identify pre-charge failure modes (timeout, welded contactor).
  • Calculate pre-charge duration from the RC time constant.

BMS-06 — High Voltage Contactors and the Pre-charge System

1. Contactors, and a Split-Second Wrong Decision

A contactor is a switch that opens and closes the HV circuit electromechanically. A typical pack has three: the positive main contactor, the negative main contactor, and the pre-charge contactor. Having them open means the HV circuit is electrically isolated — which is essential for safe servicing and fault response.

Sitting at the inverter’s input is a large DC-link capacitor (typically hundreds of microfarads to a few millifarads). Connect this capacitor, empty, directly to the pack voltage, and you get a massive inrush current limited only by cable and contactor resistance:

I_inrush(t=0) = V_pack / R_parasitic

With V_pack = 355 V and a parasitic resistance of ~10 mΩ, I_inrush ≈ 35,500 A — far beyond what the contactor and cabling can handle. That current can weld the contactor contacts shut, needlessly trip the fuse, or shorten component life. Pre-charge exists for exactly one reason: to pull that current down to a safe level.

2. How Pre-charge Works

A pre-charge contactor and pre-charge resistor, wired in parallel with the main positive contactor, limit the capacitor’s charging current. The RC time constant sets the charging speed:

V_C(t) = V_pack × (1 - e^(-t/τ)),  τ = R_pre × C_dc

With R_pre = 50 Ω and C_dc = 500 µF, τ = 25 ms; the capacitor is practically considered “full” after ~5τ = 125 ms. During that window, peak current is limited to I(t=0) = V_pack/R_pre = 7.1 A — a safe level compared to the 35,500 A in the resistor-free scenario.

ASSUMPTION — Real pre-charge resistor and DC-link capacitance values are vehicle-specific; the figures here are only representative, meant to illustrate the RC time-constant concept.

Once the capacitor voltage gets close enough to pack voltage, the main positive contactor closes and the pre-charge contactor opens. A typical closing sequence:

  1. The negative main contactor closes.
  2. The pre-charge contactor closes — current charges the DC-link through R_pre.
  3. Once DC-link voltage reaches the threshold, the positive main contactor closes.
  4. The pre-charge contactor opens.

The opening sequence is usually the reverse: load current is cut first, then the contactors open. This sequencing avoids arcing and unwanted current paths.

3. When Does the BMS Turn on HV?

The BMS only activates HV once all of these hold: isolation resistance is adequate, HVIL is closed, cell voltages/temperatures are within limits, pre-charge has completed, and there’s no active fault. These conditions are exactly the state machine’s (BMS-14) PRECHARGE → HV ACTIVE transition criteria.

4. Three Typical Faults

Fault What happens How the BMS detects it
Pre-charge timeout The capacitor doesn’t reach the threshold in the expected time (a load is drawing current, the resistor has failed, or the contactor hasn’t closed) Pre-charge is aborted, HV activation stops
Welded contactor The contact stays conductive despite an open command Voltage is still present on the affected line
Stuck-open contactor The contact fails to become conductive despite a close command DC-link voltage doesn’t rise

All three are safety-critical. The BMS never simply assumes a contactor did what it was told — it confirms with independent feedback (an auxiliary contact or a voltage measurement). This is a textbook example of a “safety mechanism” in ISO 26262 terms (BMS-17).

5. How It’s Verified in Production

Pre-charge duration and timeout thresholds are verified against the real DC-link capacitance during EOL testing. HIL testing injects scenarios like a welded contactor or an open/short pre-charge resistor to confirm the BMS detects the fault correctly and transitions to a safe state (BMS-19, BMS-20).

6. How It Connects to Other Systems

Pre-charge and contactor sequencing are managed by the state machine (BMS-14); isolation monitoring and HVIL (BMS-07) feed it as a safety precondition; and safely connecting HV loads like the inverter and charger also depends on it (BMS-07).

Interactive Tool

Enter a pre-charge resistance and DC-link capacitance into the calculator below to compute the RC time constant and the estimated pre-charge duration.

Summary

  • Pre-charge limits inrush current by charging the DC-link capacitor in a controlled way.
  • τ = R × C sets the charging speed; practical pre-charge duration is ~5τ.
  • Contactor sequencing reduces arcing and damage risk.
  • The BMS ties HV activation to safety conditions and confirms contactor state with independent feedback.

Sources

  • ISO 26262 — contactor control safety context.
  • UNECE R100 — HV safety.
  • Contactor manufacturer application notes — general concept.

Technical Diagrams

A four-step sequence diagram showing the negative contactor closing, then the pre-charge contactor, then the positive contactor, and finally the pre-charge contactor opening.
Contactor Closing Sequence — The negative → pre-charge → positive contactor sequence and the transition to HV ACTIVE (BMS-06).
A contactor cross-section drawing showing the coil, moving armature, fixed/moving contacts, and HV terminals.
HV Contactor — Cross-Section Diagram — The internal structure of an electromechanical contactor made of a coil, armature, and contacts (BMS-06). Original drawing; contains no copyrighted material.

Interactive Calculator

Pre-charge Calculator

Pre-charge direnci, DC-link kapasitansı ve paket gerilimini girerek RC zaman sabitini ve tahmini pre-charge süresini hesaplayın (bkz. BMS-06).

Formüller: τ = R_pre × C_dc · Pratik dolma süresi ≈ 5τ · I_inrush(t=0) = V_pack / R_pre

Quiz

Basic

Why is a pre-charge circuit needed?

Basic

Which formula gives the RC time constant?

Intermediate

For R_pre=50Ω, C_dc=500µF, what's τ in ms and the practical pre-charge duration (~5τ)?

Intermediate

How does the BMS detect a welded-contactor fault?

Advanced

What's the correct order in a typical contactor closing sequence?

Advanced

Why is pre-charge timeout an important safety mechanism?

Glossary

English TermDefinition
DC-link CapacitorThe large capacitor at the inverter's input; creates a high inrush current if not charged in a controlled way via pre-charge.
Inrush CurrentThe sudden, high current pulse produced by directly energizing an empty capacitor.
BJB (Battery Junction Box)The box housing HV switching/protection elements like the main contactors, pre-charge circuit, fuse, and current sensor.
HV ContactorA switching element that electromechanically opens and closes the high-voltage circuit.
Pre-charge ResistorA resistor, in series with the pre-charge contactor, that limits the DC-link capacitor's charging current to a safe level.
Contactor SequencingThe logic defining the order in which the negative, pre-charge, and positive contactors close and open.
Welded ContactorA contactor fault where the contacts stay conductive despite an open command; detected via independent voltage feedback.
RC Time Constantτ = R × C; the time constant that sets the DC-link capacitor's charging speed in a pre-charge circuit.
Fast Charging (DC)A charging method where the station supplies DC power directly, aiming for a significant SOC gain in a short time via high current/voltage; limited by temperature and lithium-plating risk.