2.4 Battery Pack Service, Main Contactors, Pre-Charge Relay & SMR Diagnostics
Key Takeaways
The System Main Relay (SMR) or Primary Relay Assembly (PRA) consists of three high-voltage contactors: SMR-B (positive main), SMR-G (negative main), and SMR-P (pre-charge) with a series pre-charge resistor (20–50 ohms, 20–50W).
The pre-charge sequence prevents catastrophic inrush current into uncharged inverter bulk smoothing capacitors by closing SMR-G and SMR-P first, waiting for bus voltage to reach 80%–90%+ of pack voltage, closing SMR-B, and finally opening SMR-P.
Contactor failure modes include welded contacts (contacts remain stuck closed when de-energized), open operating coils, and high-resistance contacts causing excessive voltage drop and localized heat under load.
The BMS monitors contactor integrity using dedicated auxiliary feedback switches or voltage sensing circuits across contactor terminals to detect stuck-closed (welded) contacts prior to relay energization.
High-voltage manual service disconnects (MSDs) incorporate integrated High-Voltage Interlock Loop (HVIL) pins that mechanically disconnect low-voltage logic first, commanding contactors open before physical high-voltage busbar separation occurs.
Battery Pack Service, Main Contactors, Pre-Charge Relay & SMR Diagnostics
High-voltage contactors—designated by manufacturers as System Main Relays (SMR) or the Primary Relay Assembly (PRA)—act as the electromechanical gatekeepers between the high-voltage battery pack and the vehicle's high-voltage bus (inverter, DC-DC converter, and A/C compressor). Understanding their operational sequence, diagnostic feedback mechanisms, and failure modes is fundamental to ASE L3 powertrain troubleshooting.
1. System Main Relay (SMR / PRA) Architecture
The contactor assembly is housed inside the sealed battery enclosure or an integrated high-voltage junction box. It contains three heavy-duty electromagnetic contactors featuring hermetically sealed, inert-gas-filled (hydrogen or nitrogen) arc-quenching chambers with silver-alloy contacts capable of switching 400V to 800V DC and interrupting thousands of fault amperes.
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| SYSTEM MAIN RELAY (SMR) SCHEMATIC |
| |
| (+) HIGH-VOLTAGE BATTERY TERMINAL |
| | |
| +------------------[ SMR-B: Positive Main Contactor ]-----------------+ |
| | | |
| +---[ Pre-Charge Resistor: 20-50 Ohm ]---[ SMR-P: Pre-Charge Relay ]--+ |
| | |
| v |
| [ INVERTER POSITIVE BUS ] |
| | |
| +------------------+ |
| | Inverter Bulk | |
| | Smoothing Cap | |
| | (500-2000 µF) | |
| +------------------+ |
| | |
| v |
| (-) HIGH-VOLTAGE BATTERY TERMINAL [ INVERTER NEGATIVE BUS ] |
| | | |
| +------------------[ SMR-G: Negative Main Contactor ]-----------------+ |
+-------------------------------------------------------------------------------------------------+
The Three System Relays
- SMR-B (System Main Relay Positive / Contactor Positive): Connects the positive terminal of the high-voltage pack to the positive DC bus of the inverter.
- SMR-G (System Main Relay Negative / Ground / Contactor Negative): Connects the negative terminal of the high-voltage pack to the negative DC bus of the inverter.
- SMR-P (System Main Relay Pre-Charge / Pre-Charge Contactor): Connected in parallel with SMR-B, wired in series with a ceramic-encased, wire-wound Pre-Charge Resistor (typically 20 to 50 , rated at 20W to 50W).
2. The Step-by-Step Pre-Charge Power-Up Sequence
Connecting a 200V–800V battery directly across the discharged bulk smoothing capacitors (, typically 500 to 2,000 ) in the inverter would result in an extreme, instantaneous inrush current according to Ohm's law (). With negligible circuit resistance (), inrush current would reach thousands of amperes within microseconds, causing catastrophic contact welding, capacitor dielectric puncture, and fuse opening.
To prevent this, the BECM executes a strictly timed, multi-stage pre-charge sequence during vehicle power-up (key-on to READY state):
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| STEP-BY-STEP PRE-CHARGE TIMING SEQUENCE |
| |
| STEP 1: PRE-CHECK & SMR-G CLOSURE |
| - Power Management ECU verifies High-Voltage Interlock Loop (HVIL) closed. |
| - Contactor voltage feedback circuits verify NO contactors are welded shut. |
| - SMR-G (Negative Contactor) is ENERGIZED (Closed). |
| | |
| v |
| STEP 2: PRE-CHARGE INITIATION (SMR-P CLOSURE) |
| - SMR-P (Pre-Charge Contactor) is ENERGIZED (Closed). |
| - Current flows from Battery (+) ---> Resistor (20-50 Ohm) ---> Inverter Cap ---> SMR-G (-) |
| - Current is safely limited: I_max = V_pack / R_precharge (e.g., 300V / 30 Ohm = 10A). |
| - Inverter capacitor charges exponentially: V_cap(t) = V_pack * (1 - e^(-t / RC)). |
| | |
| v |
| STEP 3: BUS VOLTAGE VERIFICATION & SMR-B CLOSURE |
| - BECM monitors Inverter Bus Voltage via internal voltage sensor. |
| - When Inverter Bus Voltage reaches 80% to 90%+ of total pack voltage (typically 50-300 ms): |
| - SMR-B (Positive Main Contactor) is ENERGIZED (Closed). |
| - Inrush current across SMR-B contacts is virtually zero (Delta-V across contacts < 10-20V). |
| | |
| v |
| STEP 4: PRE-CHARGE SHUTDOWN & READY STATE |
| - SMR-P (Pre-Charge Contactor) is DE-ENERGIZED (Opened). |
| - Prevents continuous current dissipation and thermal overload of the pre-charge resistor. |
| - Vehicle reaches full READY state; primary drive power delivered through SMR-B and SMR-G. |
+-------------------------------------------------------------------------------------------------+
3. Contactor Diagnostics, Feedback Circuits & Failure Modes
Modern BECMs perform active self-tests of contactor states during every power-up and power-down cycle using voltage feedback sensing across the inverter bus and dedicated contactor auxiliary feedback switches.
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| CONTACTOR FAILURE MODES & SCAN TOOL CODES |
+------------------------+------------------------------------+-----------------------------------+
| Failure Mode | Root Cause | Diagnostic Trouble Code (DTC) |
+------------------------+------------------------------------+-----------------------------------+
| Welded Positive | High inrush current, contact arcing| P0AA1 (Hybrid Battery Positive |
| Contactor (SMR-B) | or mechanical binding of contacts. | Contactor Circuit Stuck Closed) |
| Welded Negative | Arcing during emergency shutdown | P0AA4 (Hybrid Battery Negative |
| Contactor (SMR-G) | under full load. | Contactor Circuit Stuck Closed) |
| Welded Pre-Charge | Resistor overheated, welded relay | OEM Pre-Charge Contactor Circuit |
| Contactor (SMR-P) | contacts from repeated key-cycles. | DTC (Mfr-Defined / INF Sub-Code) |
| Open Pre-Charge | Thermal burnout of ceramic wire- | OEM Pre-Charge Timeout / Voltage |
| Resistor | wound resistor from stalled starts | Rise Performance DTC (Mfr-Defined)|
| High Resistance Main | Pitted, oxidized, or eroded | P0AA0 (Hybrid Battery Positive |
| Contactor Contacts | contact surfaces ($R > 0.05\ \Omega$). | Contactor Circuit Resistance) |
+------------------------+------------------------------------+-----------------------------------+
Welded Contactor Detection Logic
- Pre-Power-Up Check: Before commanding any contactors closed, the BECM samples the voltage on the inverter side of SMR-B and SMR-G. If voltage is present while contactors are commanded off, a welded contactor is detected. The BECM immediately locks out the system, sets a DTC (e.g., P0AA1 or P0AA2), and prohibits power-up to prevent a dead short or unintended high-voltage exposure.
- Open Pre-Charge Resistor Diagnosis: If the pre-charge resistor burns open, the inverter bus voltage fails to rise to of pack voltage within the calibrated timeout window (typically 300 to 500 ms). The BECM aborts the sequence, opens SMR-P and SMR-G, and sets a pre-charge circuit performance DTC.
4. Battery Pack Service, Manual Disconnects & Module Replacement
Servicing high-voltage battery enclosures requires stringent procedural adherence to ensure technician safety and prevent pack damage.
+-------------------------------------------------------------------------------------------------+
| MANUAL SERVICE DISCONNECT (MSD) OPERATION |
| |
| [High-Voltage Pack Section A] ----[ MSD Male Plug / Fuse ]---- [High-Voltage Pack Section B] |
| | |
| [ HVIL Interlock Pins ] |
| | |
| Physical Removal Sequence: v |
| 1. Lift locking lever ---> Short HVIL pins open FIRST ---> BECM immediately commands SMRs OPEN |
| 2. Pivot lever 90° ---> Physical separation of high-voltage male busbar pins |
| 3. Pull plug straight out ---> Pack is electrically split into two isolated, lower-voltage halves|
+-------------------------------------------------------------------------------------------------+
Manual Service Disconnect (MSD) & HVIL
The MSD serves two critical safety roles:
- Electrical Pack Splitting: The MSD is physically situated in the middle of the series cell string. Removing the MSD splits a 350V pack into two isolated 175V sections, eliminating high voltage at the external pack terminals.
- Integrated HVIL Interlock Pins: The MSD incorporates short auxiliary low-voltage pins wired into the High-Voltage Interlock Loop (HVIL). When a technician lifts the MSD lever, the HVIL circuit breaks first, commanding the BECM to open the high-voltage contactors before the high-voltage power terminals disengage, preventing destructive electrical arcing.
Cell & Module Replacement Rules
When replacing individual modules in a serviced battery pack (e.g., rebuilt or remanufactured NiMH/Li-ion packs):
- Voltage Balancing: Replacement module voltages must be within 0.05V (50 mV) of all other pack modules prior to reassembly. Installing a module with an unbalance will immediately trigger imbalance DTCs and uneven thermal loading.
- Capacity & Internal Resistance Matching: Never mix modules of disparate capacities (Ah) or internal resistances. A replacement module must undergo a 3-cycle charge/discharge capacity test to verify it matches the average SOH of the host pack.
- Terminal Fastener Torquing: Always use insulated torque tools to tighten terminal nuts to exact OEM specifications (e.g., 4.5 to 5.5 N·m / 40 to 49 in-lb). Under-torquing leads to contact resistance, heating, and voltage drop faults; over-torquing cracks cell terminal seals.
Caution
After removing the Manual Service Disconnect (MSD), always observe the manufacturer-mandated capacitor discharge wait time (typically 5 to 10 minutes) and perform a three-point voltage verification with a CAT III/CAT IV 1000V rated DMM before touching high-voltage components.
During hybrid vehicle power-up, which of the following describes the correct operational closing sequence for the System Main Relays (SMR)?
SMR-B (positive main) closes first, followed by SMR-P (pre-charge), and finally SMR-G (negative main).
SMR-G (negative main) closes first, then SMR-P (pre-charge) closes through a resistor to charge inverter capacitors, followed by SMR-B (positive main), after which SMR-P opens.
SMR-P (pre-charge) and SMR-B (positive main) close simultaneously, followed 500 ms later by SMR-G (negative main).
SMR-B (positive main) and SMR-G (negative main) close simultaneously while SMR-P remains energized continuously during vehicle operation.
What is the primary function of the High-Voltage Interlock Loop (HVIL) sub-circuit pins built into a Manual Service Disconnect (MSD) lever assembly?
To measure the internal resistance of the high-voltage safety fuse located inside the service plug.
To provide a ground path for static electricity accumulation on the technician's insulated safety gloves.
To open the low-voltage logic circuit first during removal, commanding the BMS to de-energize high-voltage contactors before the main high-voltage pins physically separate.
To trigger an automatic discharge circuit that instantly shorts the high-voltage battery terminals to vehicle chassis ground.
A hybrid vehicle sets DTC P0AA1 (Hybrid Battery Positive Contactor Circuit Stuck Closed). What is the potential safety and operational hazard if this vehicle is placed into service without repair?
The 12V auxiliary battery will immediately reverse polarity and destroy the engine control module.
The pre-charge resistor will remain permanently disconnected, preventing the vehicle from starting.
The cooling fan will run continuously at maximum speed, draining the high-voltage battery below 1.0V per cell.
High voltage remains present on the inverter positive terminal and underhood cables even with the ignition OFF, and closing the remaining contactors creates a severe inrush hazard.
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