BYD Han DM-p Starter Battery (LBMS Lithium) Diagnosis and Replacement Guide

Fault Background

The Han DM-p starter battery is not an ordinary lead-acid battery, but a starter battery system composed of a lithium battery and a battery management system (LBMS), located on the left side of the trunk. It has five roles: constant power supply, auxiliary power supply when DC-DC output is insufficient, absorbing transient overvoltage, voltage/current/temperature monitoring and fault alarm, and sending an "intelligent charging request" to the vehicle control unit (VCU) when the charge is low—the VCU determines the conditions and allows the traction battery (high voltage) or starting the engine to recharge the starter battery. On-site identification of its abnormality is straightforward: the instrument fault indicator lamp is always on + displaying "Check low-voltage power supply system". The diagnostic threshold is the same as the entire vehicle low-voltage system: standard voltage 12~14V, below 12V charge or replace first. This article organizes the system functions, intelligent charging logic, fault codes, diagnostic procedures, and removal/installation specifications into a reference guide.

System Composition and Five Functions

Five functions:

1. Constant power supply: Provides constant power to electrical equipment when not in over-discharge protection or ultra-low power mode;
2. Auxiliary power supply: When DC-DC output is insufficient, the starter battery assists in supplying power to electrical equipment;
3. Voltage stabilization protection: Absorbs transient overvoltage in the circuit, maintains stable voltage in the electrical system, and protects electronic components;
4. Status monitoring and alarm: Monitors voltage, current, and temperature; triggers fault alarm when abnormal—instrument fault indicator lamp illuminates (always on), and displays "Check low-voltage power supply system";
5. Intelligent charging: When the charge is low, sends an intelligent charging request to the VCU; after the VCU determines the conditions are met, it allows high voltage (traction battery) or starting the engine to charge the starter battery, and ends when the exit conditions are met.

Intelligent Charging Mechanism

Starter battery charge low → LBMS sends intelligent charging request to VCU → VCU determines the intelligent charging conditions are met, then allows one of two recharge paths: traction battery (high voltage) stepped down via DC-DC converter, or starting the engine to recharge → ends when exit conditions are met. Therefore, for "starter battery low charge" type faults, the intelligent charging chain must be checked simultaneously: whether the LBMS request is sent, whether the VCU allows it, and whether the DC-DC output is normal.

Connectors and Wake-up Signal

• Two CAN communication lines: Used for information exchange between the starter battery (LBMS) and the vehicle;
• Wake-up signal: Connected to the left front door microswitch through the vehicle wiring harness—pressing the microswitch wakes the starter battery from ultra-low power mode (linked with the smart key system and left front door microswitch, constant current <1A);
• Voltage pin: Positive voltage sampling wire.

Fault Code Table

Diagnostic Procedure

1. Vehicle driven into repair bay, analyze the fault described by the user (ask about vehicle condition and fault occurrence environment);
2. Check battery voltage: standard 12~14V—if below 12V, charge or replace first, then proceed to next step;
3. Refer to the fault symptom table: if the fault is in the table, handle according to the table; if not, proceed to next step;
4. Comprehensive analysis and diagnosis: comprehensive function check → ECU terminal check → check with diagnostic scan tool;
5. Adjust, repair, or replace wiring or components;
6. Confirmation test: after the fault no longer occurs, simulate the conditions and environment of the first fault and perform another test.

Removal and Installation Procedure

Removal:

1. Power down the vehicle to OFF position;
2. Remove the trunk cover panel and left trim panel;
3. Disconnect the starter battery negative terminal and low-voltage connectors;
4. Open the positive terminal fuse box cover and remove the positive terminal fuse box;
5. Use a socket tool to remove the starter battery mounting foot nuts, then remove the starter battery.

Installation: Align the stud holes and assemble → use a socket and 3 M nuts to secure the starter battery → assemble the positive terminal fuse box and low-voltage connectors → connect the negative ground wire harness → install the trunk left trim panel and cover panel → replacement complete.

Safety Warnings

• This article is for technical reference only; starter battery inspection must be performed by certified personnel according to OEM specifications. Readers unfamiliar with the procedure are strictly prohibited from performing it themselves.
• Before removal/installation, the vehicle must be powered down to OFF position; disconnect the negative terminal first, then remove the positive terminal fuse box to prevent short circuits.
• The starter battery is a lithium battery; do not drop, crush, or puncture it; replacement must use the same specification lithium battery assembly.
• When the alarm displays "Check low-voltage power supply system", first measure the voltage per the 12~14V threshold, then check the intelligent charging chain; do not blindly replace the battery.

Maintenance Insights

The Han DM-p starter battery system reminds us of three things on-site. First, it is a lithium battery, not lead-acid—it has its own LBMS management, monitoring voltage, current, and temperature; fault codes directly fall into the BFCxxx/B1FBxxx series, so read codes by checking LBMS first, rather than the old experience that "starter batteries only have two states: good or bad". Second, for low charge, check the intelligent charging chain first—when the battery charge is low, it is a closed loop of "LBMS request → VCU judgment → high voltage or engine recharge"; if any step is broken, it will manifest as repeated low charge; replacing the battery without checking the chain will result in the new battery being low again within days. Third, do not ignore the wake-up signal—the left front door microswitch wakes the battery from ultra-low power mode; if the switch or wiring is abnormal, it can cause "dead sleep" type faults, which appear completely normal under ordinary voltage measurement; they must be diagnosed by checking the wake-up pin and CAN communication.

---

【This article is for reference only】