battery charger flashing red and green while charging

The most common cause is usually poor contact between the charger and the battery (loose plug/wiring or contact corrosion), which leads to abnormal voltage/communication feedback and triggers the red and green indicators to flash.

The following are eight troubleshooting steps arranged from easiest to most difficult, each step including: phenomenon → possible cause → solution → precautions.

1) Confirmation of on-site power supply and indicator light behavior

• Phenomenon: After plugging in the power, the charger flashes red and green alternately immediately or intermittently.
• Possible causes: No power in the wall socket, circuit breaker/GFCI tripped, or unstable power supply.
• Solutions: Replace the known working socket, reset the circuit breaker/residual current device, and avoid using long and thin extension cords.
• Precautions: Disconnect the charger power before unplugging and plugging it in; do not test on a damaged power board.

2) Check the external fuse of the charger.

• Symptoms: The indicator flashes or the charger has no output or works intermittently.
• Possible cause: The fuse on the charger or vehicle side is blown or has poor contact.
• Solution: Refer to the instruction manual to locate the fuse. After disconnecting the power, check and replace the fuse according to specifications.
• Precautions: Use only fuses of the same current/type; disconnect power to prevent electric shock or short circuit.

3) Inspect the main connector, clamps, and terminals (for poor plug contact).

• Phenomenon: There is no continuous charging when plugged in; the status changes after slightly moving the plug, or red and green flashing appears.
• Possible causes: loose connector, oxidation, corrosion, insufficient contact area, or broken wire.
• Solution: After powering off, clean the terminals (lightly polish with a wire brush or sandpaper), tighten the connectors, fully insert the plug and try again; if necessary, replace the damaged connector or cable.
• Precautions: Avoid cleaning the terminals while they are energized; after cleaning, apply a small amount of anti-oxidation lubricant (electrical grade) to improve contact.

4) Measure the battery pack voltage and verify it against the charger’s rated voltage (voltage mismatch).

• Symptom: The charger flashes after being connected, but the measured battery voltage is abnormally low or does not match the charger’s specifications.
• Possible causes: The battery pack voltage is too low, some individual battery cells are disconnected, or the charger’s rated voltage is incompatible with the battery pack.
• Solution: Use a multimeter to measure the total voltage and individual cell voltage to confirm the voltage range; if the total voltage is too low, check each individual cell separately and replace the faulty cell, or use a charger that matches the battery voltage.
• Precautions: Avoid short circuits during measurement; if you are not familiar with high-voltage operation, please hand it over to an experienced person.

5) Check the status of the Battery Management System/BMS (BMS protection).

• Symptom: Battery voltage is normal, but the charger is not charging or the indicator is flashing; vehicle display/alarm indicates BMS malfunction.
• Possible cause: The BMS detects over-discharge, overcharge, cell imbalance, or communication error and disables charging.
• Solution: Read the BMS fault code (if there is an interface or display), and try resetting the BMS according to the instructions (e.g., disconnect the main battery power for a few minutes and then reconnect it); if the individual cells are unbalanced, perform balancing charging or replace the damaged battery cell.
• Precautions: Do not bypass or short-circuit the BMS to force charging; BMS debugging or replacement is best handled by a professional.

6) Low-temperature protection check (low-temperature protection)

• Phenomenon: The charger fails to start or flashes red and green lights in cold environments, which is more noticeable when the room temperature is low.
• Possible cause: The charger or battery has low temperature protection, which will prevent charging if the temperature is below the safe charging temperature threshold.
• Solution: Move the battery/vehicle to a warm environment to reach the manufacturer’s recommended temperature (usually >5–10°C), or preheat the battery in a controlled environment before charging.
• Precautions: Forcing charging at low temperatures will damage battery life and safety performance; avoid heating directly with open flame or excessive heat.

7) Test charger output and troubleshoot internal faults.

• Symptom: All the above checks are normal, but the charger is still flashing and has no output or abnormal output.
• Possible cause: Failure of internal components of the charger (switching power supply module, relay, capacitor, control board).
• Solution: Use a multimeter or clamp meter to measure the charger’s output voltage/current. If there is no output or the output is unstable, send the charger for repair or replacement.
• Precautions: The charger involves high-voltage AC and rectifier components. Do not disassemble it if you are not a professional. Choose a qualified repair shop for repairs.

8) Compatibility, firmware, and communication protocol issues (advanced diagnostics)

• Symptom: The charger and battery pack are smart devices, but communication fails, and the display is abnormal or continuously flickering.
• Possible causes: Incompatibility between the charger and the battery/vehicle controller protocols, firmware malfunction, or damaged communication cable.
• Solution: Verify that the charger model is compatible with the vehicle/battery, check the communication lines (CAN/serial port, etc.), and contact the manufacturer to check for firmware updates or compatibility notes.
• Important Notes: Firmware updates must be performed strictly according to the manufacturer’s instructions; incompatible models may pose security risks, so avoid making temporary modifications.

When should the charger be replaced (and when should repairs be abandoned):

• If the problem persists after completing the above checks and the charger has no output or the output is unstable;
• The charger repeatedly exhibits the same internal malfunctions within a short period of time (such as relay jamming, component overheating, smoke, or a burnt smell).
• If the repair cost is close to or exceeds the cost of purchasing a new machine, or if the manufacturer has discontinued production and there are no replacement parts available,
  in any of these cases, direct replacement is usually more cost-effective and safer.

Three criteria for choosing a replacement charger (recommended)

1. Voltage/current matching: Ensure that the charger’s rated voltage is exactly the same as the battery pack’s nominal voltage (e.g., 48V/36V), and select an appropriate charging current (too high a current will affect the battery’s lifespan, too low a current will result in slow charging).
2. Intelligent protection and compatibility: Prioritizes support for BMS communication, low temperature protection, overcharge/short circuit/reverse connection protection and temperature compensation, and confirms compatibility with vehicle/battery manufacturers.
3. Quality and Certification: Choose brands/suppliers with safety certifications such as CE/UL, appropriate IP ratings (waterproof and dustproof for outdoor use), and warranties and manufacturer technical support.