* Since these are fairly technical issues, we will break them down by category for you below. The structure of this guide is as follows:
issue type, specific issue description, cause of the issue, and solution. Please don't be confused.
Note that a single issue may have multiple causes, which we will address one by one.
Voltage-Related Faults
Issues 1 : Abnormal Cell Voltage Difference: Causes and Solutions for Charging Cutoff and Power Loss
During the operation of a lithium-ion forklift battery, a voltage discrepancy may arise between a particular cell and the others, manifesting as an abnormally large dynamic or static voltage difference.
During charging, we may observe that the voltage of a specific cell quickly reaches the full-charge cutoff voltage limit, triggering the Battery Management System's overcharge protection mechanism and causing the charging process to stop before the battery is fully charged.
Alternatively, when we press the accelerator, the voltage of one or more cells may drop much faster than that of the other, normal cells; conversely, when we apply the brakes, the voltage of these problematic cells rises faster than that of the normal cells.
Simply put, this indicates cell inconsistency. We need to further determine whether the issue stems from the cells themselves or from other factors. Let's analyze this together.
Cause 1 of the malfunction: The connection between the battery cell busbar and the terminal post has become loose (loose bolts/nuts)
This refers to battery cells in a lithium-ion forklift battery pack not being properly connected in series. The battery cells in a lithium-ion forklift battery pack or assembly are connected via copper busbars, which are secured to the battery cell terminals to form a stable series circuit.
This is where the problem arises: when the connecting bolts or nuts become loose, the contact pressure between the copper busbar and the battery cell terminals decreases, reducing the contact area and thereby increasing the contact resistance at the connection point.
Under normal conditions, current flows through the copper busbars with virtually no loss, and voltage changes across cells in each string remain largely consistent. However, once a connection point becomes loose, during high-current operating conditions-such as when the forklift starts, accelerates, or performs regenerative braking-additional voltage loss occurs at that connection point, causing the voltage change of the corresponding cell to deviate significantly from that of the other cells in the string.
As mentioned earlier, during discharge, the voltage of this string drops faster than that of other cells due to increased connection impedance; conversely, during charging or regenerative braking, the voltage of this string may rise rapidly. This causes the Battery Management System to detect abnormal cell voltage, triggering measures such as premature termination of charging, reduced power output, or protective alarms.
Therefore, the problem does not lie with the forklift battery cells themselves, but rather at the connection points between the copper busbars and the terminals-such as loose bolts, oxidation of contact surfaces, contamination, ablation, or poor contact.
Solution: If such issues are detected, first inspect the connections between the battery cells inside the battery pack to verify whether there is any looseness, oxidation, burn damage, or abnormal contact between the copper busbars and the cell terminals.
If poor contact is caused by loose bolts or nuts, retighten the fasteners according to standard torque specifications to ensure sufficient contact pressure between the copper busbars and the cell terminals. If the contact surfaces have oxidation, dust, electrolyte residue, or other contaminants, clean the contact surfaces and, if necessary, replace damaged copper busbars or bolts.
After completing the connection repair, the battery pack must undergo charge-discharge testing to monitor voltage changes in each cell. The BMS's active and passive balancing functions can be used to resolve inconsistencies in cell parameters caused by prolonged connection issues.
If the connection points have developed serious problems, contact CoPow to replace the copper busbars, bolts, or even the entire forklift battery. We offer a wide range of replacement parts and can also customize brand-new lithium-ion forklift batteries according to your needs.
Cause 2 of the malfunction: After charging, if left for a period of time, the cells in a forklift battery may show low or high voltage, resulting in a voltage difference.
We have already provided a detailed analysis and overview of this issue in the article "BMS Shows Low Cell Voltage: Causes, Checks & Fixes" which you can view by clicking here.
The article points out that a low cell voltage does not necessarily indicate cell damage. The cause of the low voltage may be an issue with the data acquisition line-specifically, a problem with the Lithium Battery Monitoring Unit (LMU)-which causes the recorded voltage data to be higher or lower than the actual value. In addition, the article provides detailed instructions on how to diagnose and resolve the issue.
Temperature-Related Faults
Issues 1 : High Temperature Protection Caused by Forklift Battery Overheating
If a forklift battery feels very hot to the touch, this will immediately trigger the temperature protection feature of the battery management system during charging, causing the battery to stop charging.
More precisely, it is not the forklift battery itself that is generating heat, but rather one or more of the temperature sensors among the multiple temperature sensing points installed inside the battery that have detected abnormally high temperatures.
For example, during normal operation or charging, the temperature of most battery cells may remain between 30°C and 40°C, but the temperature at a specific location may suddenly rise to a preset alarm threshold (such as 45°C or 55°C, depending on the BMS parameters). In this case, the BMS system will determine that there is a risk of battery overheating, thereby triggering an alarm and limiting the charging current to halt the charging and discharging processes.
Cause 1 of the malfunction: Temperature Sensor Failure
The temperature sensors (NTCs) inside the battery pack monitor the temperature of the battery cells and modules in real time and transmit the data to the Battery Management System.
If a temperature sensor is damaged, a wire is broken, a connector is loose, or the sampling signal is abnormal, the BMS may receive incorrect temperature data, causing the system to incorrectly determine that the battery temperature is too high.
For example, if the actual battery temperature is only 30°C, but due to a sensor failure, the BMS displays a temperature of 60°C at a certain point, this will trigger the high-temperature protection function.
Solution: If a temperature sensor is suspected of malfunctioning, first use an external temperature measurement device (such as an infrared thermometer or thermocouple) to measure the actual temperature inside the battery pack and compare it with the temperature data displayed by the Battery Management System.
If the actual temperature is significantly lower than the temperature displayed by the BMS (for example, the actual temperature is 30°C, while the BMS displays 60°C), this indicates that there may be an issue with the temperature sensing system.
Next, inspect the temperature sensor itself, its resistance value, and the connecting wiring. Since lithium-ion battery packs typically use NTC temperature sensors, their resistance values change with temperature.
Therefore, use a multimeter to measure the NTC resistance and determine whether the sensor is functioning properly based on the corresponding temperature curve. If the measured value does not match the standard resistance value, this indicates that the temperature sensor is damaged and must be replaced with a new NTC sensor.
If the sensor is functioning properly, proceed to inspect the temperature sensing harness and connectors to check for issues such as broken wires, loose connections, loose pins, oxidation, or poor contact. Wiring problems can also cause the BMS to receive erroneous signals; for example, if there is an open circuit in the signal path, the BMS may display an extremely high temperature. After repairing the wiring or reseating the connectors, power the system back on to verify whether the BMS temperature display has returned to normal.
If both the temperature sensor and wiring are confirmed to be in good condition but the BMS still displays abnormal temperatures, there may be a fault in the BMS's internal temperature sensing channel. In this case, the LMU (Local Monitoring Unit) must be calibrated or replaced entirely.
The final troubleshooting process can be summarized as follows: Verify the actual temperature → Test the temperature sensor → Inspect the sampling wiring harness → Calibrate the LMU → Replace the faulty component.
Cause 2 of the malfunction: LMU Failure Causes Abnormal Temperature Readings
LMU not only collects cell voltage data but also collects cell temperature data. If a problem occurs, just as incorrect voltage data might be recorded, incorrect temperature data might also be recorded.
Solution: The method is actually the same as for testing voltage. First, use external specialized equipment to check for abnormal temperatures, then compare the measured temperatures with the issues displayed by the BMS. The rest of the process is the same as the testing procedure mentioned earlier.
Cause 3 of the malfunction: The battery cell is not properly connected.
As we mentioned earlier, improper connection of battery cells can lead to increased resistance. Increased resistance not only causes voltage fluctuations but also results in a rise in temperature.
You'll notice that whenever you drive a forklift uphill or perform operations that involve high-current discharge, the battery heats up, triggering the BMS's overheating protection feature and causing the forklift to suddenly shut down.
Solution: First of all, stop operating the forklift. Start by inspecting the connections of each cell in the battery pack: Are any bolts loose? Are there any issues with the connections? Is there any dirt or debris? Are the copper busbars deformed? Even check for signs of burning.
Address any problems you find with targeted repairs: tighten loose bolts, wipe away dirt, and replace deformed or burnt-out copper busbars. In short, make sure all connections are secure.
Cause 4 of the malfunction: Overcharging
Both lead-acid and lithium-ion batteries can overheat due to overcharging. However, lithium-ion batteries often do not overheat because they are protected by a Battery Management System and specialized lithium-ion battery chargers.
Chargers have fixed output voltage limits. For example, the fully charged voltage of a LiFePO4 cell is 3.65V, and the fully charged voltage of a 16-cell battery pack (51.2V) is 58.4V. Therefore, chargers are typically set to a voltage value close to the cut-off voltage. Once the set voltage is reached, the charger stops outputting power or enters a constant-voltage phase, thereby preventing overheating caused by overcharging.
Heat generation in lithium-ion batteries is generally caused by issues with the BMS, such as the temperature sensor failure and LMU failure we mentioned earlier.
In addition, there is a new type of failure: the failure of the BMS's balancing function, combined with capacity degradation in one or more cells, leading to a rapid rise in voltage during charging and premature overcharging. This is still a result of cell inconsistency.
Solution: Please use a professional lithium-ion forklift battery charger and regularly check the operational status of the battery management system. If you discover any issues, please report them to the battery manufacturer promptly.
Cause 5 of the malfunction: The heat dissipation system was not properly designed.
It is not acceptable for the internal space of a forklift battery to be too cramped. When manufacturing lithium-ion forklift batteries, we always reserve space for heat dissipation, install fans, and strategically position the exhaust vents.
Charging-Related Faults
Issues 1 : Charging Start Failure & SOC Calibration Error
The forklift battery cannot be charged, and the remaining charge cannot be calibrated correctly after a full charge.
Cause 1 of the malfunction: Various issues with forklift batteries trigger the charging protection mechanisms of the battery management system.
For example, if the internal resistance of a particular cell increases, its voltage will rise much faster than that of other cells during charging, directly triggering the BMS's overvoltage protection threshold. Since the BMS will interpret continued charging as potentially damaging to the cell, it will immediately terminate the charging process.
Abnormal temperatures can also prevent charging from starting. For example, when charging in a low-temperature environment, LFP cells may be at risk of lithium plating. If the temperature falls below 0 degrees Celsius, the BMS will prohibit charging to prevent battery damage.
Solution: Use the BMS's active and passive balancing functions regularly to restore consistency in parameters such as capacity, impedance, and discharge rate among the battery cells.
Before charging in low-temperature environments, we recommend equipping your lithium-ion forklift with a preheating module in advance so that the battery can be automatically heated in low-temperature conditions, thereby ensuring normal charging and discharging.
Cause 2 of the malfunction: BMU Malfunction (Charging Module or Charging CAN Anomaly)
The BMU (Battery Management Unit) is primarily responsible for managing the battery's status and exchanging information with the charger.
The charging process for forklift lithium-ion batteries does not simply involve plugging in the charging gun and immediately supplying current; instead, it requires communication and confirmation between the charger and the BMU.
During charging, the charger uses CAN communication to obtain the battery's current status, including battery voltage, allowable charging current, charging demand, and whether charging is permitted.
If the internal charging control module of the BMU malfunctions, or if there is an issue with the CAN communication line (such as an open circuit, short circuit, poor contact, or communication protocol error), the charger will be unable to receive the correct charging instructions, preventing the charging process from starting.
This fault typically manifests as follows: both battery voltage and temperature are normal, but the charger does not respond after the charging gun is inserted, or charging stops shortly after it begins.
Solution:
