BYD Song PLUS EV Traction Battery Pack and BMC Fault Diagnosis Guide

Fault Background

The Song PLUS EV's SA3EB traction battery pack has a systematic fault diagnosis method: 7 fault modes on the battery pack side (over-temperature, SOC jump, leakage, capacity calibration error, impact, water ingress, others) + 6 fault modes on the battery management controller (BMC) side (voltage sampling, temperature sampling, fuse blown, CAN communication loss, signal acquisition abnormality, others), with an ED400 diagnostic code table that directly maps DTCs to components to be inspected. Two rules are particularly noteworthy: the battery management controller is installed inside the battery pack and must be removed/installed by the battery pack supplier; leakage detection uses the "parallel resistance method" to calculate insulation resistance, and leakage is determined when it is below 500Ω/V. This article organizes the fault modes, diagnosis methods, diagnostic code table, and removal/installation specifications into a field guide.

Battery Pack Fault Modes and Diagnosis

Leakage Detection: Parallel Resistance Method

1. Wear insulating gloves and insulating shoes, ensure safety, then use a multimeter to measure battery pack data: positive terminal to body voltage V1, negative terminal to ground voltage V2, total voltage;
2. If V1 > V2: connect a known resistor R (50kΩ, 100kΩ, 110kΩ, 150kΩ, preferably around 400kΩ or 500kΩ) in parallel to the positive terminal, then measure voltage to ground V3, then Rinsulation = (V1 − V3) / V3 × R;
3. If V1 < V2: connect resistor R in parallel to the negative terminal, then measure voltage to ground V4, then Rinsulation = (V2 − V4) / V4 × R;
4. If Rinsulation < 500Ω/V, determine battery pack leakage, remove and hand over to manufacturer for professional repair.

Capacity Calibration Recovery Procedure

Method 1 (with charging cabinet): Use charging cabinet to discharge vehicle until it automatically cuts off power → charge to SOC 100%, at around SOC 90% connect to upper computer via front compartment power CAN port, open battery management controller monitoring system, collect the charging capacity of this charge when vehicle reaches SOC 100% → recalibrate this charging capacity corresponding to SOC 100% into the battery management controller → restore vehicle power-on.

Method 2 (without charging cabinet): With SOC as low as possible, park vehicle at charging spot, turn on PTC heating to discharge until power automatically cuts off → charge to 100%, at around SOC 90% follow Method 1 to collect and recalibrate.

Method 3 (backup): Battery management controller has built-in repair function; after multiple charge/discharge cycles, capacity is automatically corrected to near actual capacity—but this method may cause driver to misjudge driving range and lead to breakdown.

Battery Management Controller Fault Modes

Removal and installation of battery management controller: inside the battery pack, only by the battery pack supplier—the shop must not disassemble/install BMC privately.

ED400 Diagnostic Code Table (ISO15031-6)

Removal/Installation and Torque

Battery pack removal procedure: Lift vehicle → disconnect high-voltage and low-voltage connectors → collect coolant → lower battery pack. Installation procedure: Lift battery pack into alignment → connect low-voltage connectors, ground wire, underbody shield, coolant, and bolt fasteners.

• Place traction battery pack assembly on lift platform or simple support platform (height 1.2~1.4m, load capacity below 1000kg, platform surface 1800mm×800mm; lifting equipment load capacity above 300kg), align mounting holes and tighten fixing bolts, torque 135N·m;
• Protective equipment: cloth gloves (for screw removal and handling protection), standard electrical insulating gloves (for operations at traction battery leads, service disconnect switch, signal wire interfaces), protective face shield (for face protection when removing pressure-relief traction battery);
• Precautions: Yellow wiring connections or components with high-voltage labels on the traction battery must not be disassembled by non-authorized BYD service store personnel; immediately disconnect the battery pack service disconnect switch before removal and cover the switch socket with insulation protection; power output outlet socket must be insulated and covered to prevent foreign objects from falling in; sampling wires must not be pulled forcefully or bent excessively; power copper busbar connections and module connection positions must be cleaned of dust and contaminants before assembly; pay attention to component labels to prevent omission or incorrect installation; after installation, apply torque marks to fasteners; prohibit violent disassembly, dropping, impact, module tilting, heavy pressure on modules, excessive pulling of sampling wires, and intentional short circuits.

Safety Warnings

• This article is for technical reference; traction battery repair must be performed by certified personnel according to OEM specifications. Readers unfamiliar with the procedures are strictly prohibited from performing operations themselves.
• Traction batteries are high-voltage components; improper operation can cause personal injury or death; do not touch or operate the traction battery without appropriate protective equipment.
• The battery management controller (BMC) is installed inside the battery pack and can only be removed/installed by the battery pack supplier; in-shop repairs are limited to fuse replacement, ED400 diagnosis, and whole battery pack return to factory.
• Leakage detection must be performed with insulating gloves and insulating shoes; after confirming Rinsulation <500Ω/V, immediately stop use and remove the pack for factory return.
• Battery pack removal must follow the procedure sequence: disconnect service disconnect switch and insulate → disconnect high/low voltage connectors → collect coolant → lower battery pack; fixing bolts at 135N·m and apply torque marks.

Repair Insights

The Song PLUS EV battery pack diagnosis manual clearly defines "what the shop can do" and "what the shop cannot do": the shop's role is "diagnosis + whole pack replacement", and all in-pack repairs (including BMC removal/installation) belong to the supplier. Therefore, the three things to master on-site are: First, the parallel resistance method to calculate insulation—compare V1/V2 to decide whether to connect the resistor in parallel to the positive or negative terminal, Rinsulation = (V1−V3)/V3×R, and if below 500Ω/V, hand over the pack; this arithmetic is cheaper than any insulation tester and consistent with OEM standards; Second, the "90% collection point" for capacity calibration—must connect to the upper computer via the front compartment power CAN port at around SOC 90%, collect the charging capacity of this charge up to 100%, and recalibrate into the BMC; collecting earlier or later will not capture the full-charge capacity; Third, the "component column" of the ED400 code table—for P1A3400 pre-charge failure, six objects must be checked simultaneously: traction battery, high-voltage distribution box, motor controller, OBC, compressor, and PTC; only by checking column by column can you avoid "fault persists after replacing battery pack".

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【This article is for reference only】