battery charger not charging battery

The most common cause is poor contact between the charger and the battery or wiring (loose/corroded plugs, terminals or wiring).

The troubleshooting steps are listed below in order from easiest to hardest to check (each step: phenomenon → possible cause → solution → precautions).

1) Check the AC power supply and socket.

• Symptom: The charger has no indicator lights, the fan does not turn, and there is no response at all.
• Possible causes: no power in the power socket, circuit breaker tripped, time delay switch not turned on, or power cord damaged.
• Solutions: Test the socket with other known working devices; check the circuit breaker or switch in the distribution box; replace it with a known working extension cord or try connecting it directly to another socket.
• Precautions: Disconnect the charger power plug before operation. If you are not familiar with the distribution box, please consult an electrician. Avoid operating in humid environments.

2) Check the charger indicator lights/display and error codes.

• Symptoms: The indicator light is unusual (red light is constantly on, flashing, or displays an error code), or it displays “FAULT”.
• Possible cause: The charger detected a fault during self-test (such as internal overheating, short circuit, communication failure, or BMS refusing to charge).
• Solutions: Consult the charger manual for the meaning of the corresponding instructions; perform a reset according to the code or wait for it to cool down before trying again; sometimes turning off the power for one minute and then turning it back on can clear the overheating/transient error.
• Precautions: If smoke, a burning smell, or obvious burnt marks appear, immediately disconnect the power and stop using the device. Send it for repair or replacement.

3) Check the fuses and circuit breakers (AC side and DC side).

• Symptoms: The charger has no output or works intermittently; there is no charging current in the vehicle.
• Possible cause: The input-side fuse or the output-side fuse has blown, triggering the vehicle body short-circuit protection.
• Solution: Check and replace the fuse (of the same specifications) according to the manufacturer’s instructions; check if the circuit breaker has tripped and reset; if the fuse blows repeatedly, there may be a short circuit, and further inspection of the wiring or battery is required.
• Important notes: When replacing fuses, you must use the same specification; frequent fuse blowouts indicate a deeper problem in the system, so avoid replacing them directly multiple times.

4) Check the plug, charging port, and cable contact (poor plug contact).

• Symptoms: Poor contact during charging, overheating, unstable charging current, or need to be plugged in and unplugged multiple times before it can work.
• Possible causes: loose plug/terminal, oxidation/corrosion, bent pins, or contaminated contact surfaces.
• Solution: Disconnect the power supply and check the terminals after disconnecting; clean the copper contacts (with alcohol or a special contact cleaner), tighten the terminals, straighten the bent pins, or replace the worn plug; observe whether stable charging is restored after plugging and unplugging several times.
• Precautions: Disconnect the power before cleaning and tightening; avoid excessive sanding to prevent metal damage; if the connector shows obvious signs of burning, replace the entire connector set.

5) Measure the battery terminal voltage and check for voltage mismatch (voltage mismatch).

• Symptoms: The charger displays “voltage error” or does not start charging at all; the charging current is 0.
• Possible causes: The charger’s output voltage does not match the battery’s nominal voltage (e.g., using a 24V charger to connect to a 36V battery pack), the battery pack voltage is too low (deep discharge), or the individual cell voltages differ too much.
• Solution: Use a multimeter to measure the static voltage at the battery pack terminals and check whether the charger output requirements are consistent with the battery’s nominal voltage. If the battery is over-discharged and below the minimum allowable voltage of the BMS, it may be necessary to wake it up with a constant current small current or to wake it up with a short-term parallel connection of an auxiliary charger of the same voltage (professional operation is recommended).
• Precautions: Different battery chemistry (lead-acid, lithium battery) has different charging voltages. Do not use an incompatible charger for long-term charging. Maintain correct polarity during operation and measurement to avoid short circuits.

6) Check the BMS protection/battery management system.

• Symptoms: The charger is connected normally but cannot charge the battery, or the charging is disconnected after a period of time; there is a communication error message between the charger and the battery.
• Possible causes: The BMS detects an imbalance in the voltage of individual cells, abnormal temperature, excessive discharge, or internal fault, and therefore cuts off the charging path; the BMS fails to communicate with the charger (some systems require a handshake to start charging).
• Troubleshooting: Check the BMS fault codes or indicator lights, and reset according to the BMS manual (such as balancing unit or resetting protection); if it is a communication problem, check the communication cable (CAN/RS485, etc.) and connectors; if necessary, use BMS-specific tools or contact after-sales service for reset and diagnosis.
• Precautions: Do not repeatedly charge batteries with severe imbalances in individual cells, as this will damage the battery; if you are unfamiliar with BMS-related operations, please have them handled by a professional repair technician.

7) Determine the impact of ambient temperature on cryogenic protection (cryoprotection)

• Symptom: In cold environments, the charger does not charge or the charging current is very low, and the charger displays a low temperature/charging prohibited warning.
• Possible cause: Most modern batteries or chargers have low-temperature protection to prevent charging below a certain temperature (commonly 0~5°C, higher for lithium batteries) to avoid polarization or damage.
• Solutions: Move the battery/charger to a warm environment to allow it to warm up before charging; use a system with low-temperature charging capabilities or a heating device; in extremely cold regions, consider installing a battery insulation box or heating belt.
• Precautions: Charging at extremely low temperatures may shorten battery life or cause safety issues. Always follow the battery manufacturer’s temperature requirements.

8) Check the charger’s output and internal faults (unstable voltage/current, circuit aging).

• Symptoms: The charger has an indicator but a stable DC output cannot be measured with a multimeter, or the output voltage is unstable/the current fluctuates, and the charger makes abnormal noise or overheats during operation.
• Possible causes: damage to internal components of the charger (rectifier module, electrolytic capacitor, switching components), output circuit failure or control board malfunction, or incompatibility between the charger’s output specifications and the battery (e.g., output is too low to trigger charging).
• Solution: Use a multimeter to measure the charger’s no-load and load outputs (follow the instructions in the manual); if the output is abnormal, it is recommended to send it to a professional repair shop for testing or replace the charger directly; do not replace the internal high-voltage components yourself (there is a risk of electric shock).
• Precautions: The charger contains high voltage, and repairs must be performed by professionals; if the charger is under warranty, please contact after-sales service for assistance.

When should the charger be replaced?

• The charger still has no DC output or frequently malfunctions and cannot be repaired after the above checks;
• The charger has obvious physical damage, a burnt smell, smoke, bulging components, or a malfunctioning fan causing overheating;
• The charger is old and the repair cost is close to or exceeds the cost of purchasing a new device, or the manufacturer has discontinued production and there are no spare parts available for repair;
• It needs to be replaced to obtain a charging protocol compatible with the new battery/new BMS or a higher level of protection (such as changing from no low temperature protection to having low temperature protection).

Three recommended criteria for choosing a charger: 1) Voltage and charging curve must match the battery type and voltage pack: The charger’s output voltage and maximum charging current must match the battery’s nominal voltage, capacity, and charging curve (lead-acid, lithium iron phosphate, lithium-ion, etc.) to avoid voltage/current mismatch leading to incomplete charging or battery damage. 2) Compatibility with BMS and safety protection: Choose a charger that supports communication with the BMS or can operate normally under BMS protection conditions (with reverse connection protection, over-temperature, over-voltage, short circuit, and low-temperature protection), and prioritize products with relevant safety certifications (such as UL/CE/CCC, etc.) and good heat dissipation design. 3) Appropriate charging current and quality assurance: The charging current (A) should meet the battery’s recommended C rate, enabling fast charging without damaging the battery; choose a charger from a reputable brand with warranty and after-sales service for easy long-term maintenance and troubleshooting.