Working Principle of Undervoltage Protection Function for Electric Forklift Batteries
The undervoltage
protection of electric forklift batteries is a two-level protection mechanism jointly implemented by the Battery Management System (BMS) and the Vehicle Electronic Control Unit (VECU). Its core is to prevent irreversible damage caused by deep battery discharge and ensure operational safety through real-time voltage monitoring, threshold judgment, and graded intervention. The following is a separate explanation for lead-acid battery and lithium battery systems:
I. Core Components and Responsibilities
| Component Name | Functional Responsibilities | Execution Frequency |
|---|
| Voltage Acquisition Module | Collects real-time voltage of battery cells/battery packs; for lead-acid batteries, it collects pack voltage + cell sampling; for lithium batteries, it collects voltage of each cell string | 100ms/time |
| Battery Management System (BMS) | Sets undervoltage thresholds, judges voltage status, and outputs protection commands | 200ms/time |
| Vehicle Electronic Control Unit (VECU) | Receives BMS signals and executes graded power-off of the power/hydraulic systems | 50ms/time |
| Acoustic and Optical Alarm Module | Receives protection commands and triggers indicator light + buzzer alarm | Synchronous response |
II. Working Process of Undervoltage Protection for Lead-Acid Batteries
Lead-acid batteries do not have cell-level monitoring (some high-end models are equipped with sampling monitoring), so the battery pack voltage threshold is used as the core judgment basis, divided into three levels of protection:
Early-Warning Protection (Threshold: 90% of Rated Voltage)
- Trigger Condition: The battery pack voltage remains below 90% of the rated voltage for 3 consecutive seconds (e.g., below 43.2V for a 48V battery).
- Execution Actions: The undervoltage indicator light on the dashboard flashes, and the buzzer alarms intermittently; the BMS limits the maximum output current (reduced to 70% of the rated current), resulting in weakened forklift power and slowed hydraulic lifting/lowering speed.
- Purpose: Remind the operator to park and charge the forklift in a timely manner to avoid continued high-current discharge.
Forced Power-Limiting Protection (Threshold: 85% of Rated Voltage)
- Trigger Condition: The battery pack voltage remains below 85% of the rated voltage for 5 consecutive seconds (e.g., below 40.8V for a 48V battery), and the operator does not stop the operation.
- Execution Actions: The undervoltage indicator light stays on, and the buzzer alarms continuously; the VECU cuts off the high-current circuit of the drive motor and only retains the minimum power supply for the hydraulic system (allowing the forks to descend slowly); the forklift is prohibited from traveling and heavy-load lifting/lowering.
- Abnormal Handling: If the hydraulic system jams, manual pressure relief is required; forced operation is strictly prohibited.
Emergency Power-Off Protection (Threshold: Cell Voltage < 1.75V)
- Trigger Condition: Sampling detection shows that the cell voltage is less than 1.75V, or the battery pack voltage is below 80% of the rated voltage for 2 consecutive seconds.
- Execution Actions: The BMS sends an emergency power-off signal, and the VECU cuts off all power circuits of the forklift (including the hydraulic system); the forklift shuts down completely, and only the dashboard power supply is retained.
- Safety Requirement: After power-off, place warning signs 5 meters in front of and behind the forklift; restarting is prohibited.
III. Working Process of Undervoltage Protection for Lithium Batteries
Lithium batteries rely on precise cell-level voltage monitoring (BMS collects voltage of each string one by one), with more refined protection logic divided into four levels of protection and featuring self-recovery characteristics:
Cell-Level Early-Warning (Threshold: Cell Voltage 3.0V)
- Trigger Condition: The voltage of any cell string is less than 3.0V for 3 consecutive seconds.
- Execution Actions: The BMS records the number of the abnormal cell string, and the dashboard displays "Cell Undervoltage Warning"; peak current output is limited, and sudden acceleration and heavy-load climbing are prohibited.
Pack-Level Power-Limiting (Threshold: Cell Voltage 2.9V)
- Trigger Condition: The voltage of ≥20% of the cell strings is less than 2.9V for 5 consecutive seconds.
- Execution Actions: The acoustic and optical alarm is activated; the VECU reduces the driving power to 50% of the rated power and limits the hydraulic system pressure to 60% of the rated value; only no-load driving to a safe area is allowed.
Forced Shutdown (Threshold: Cell Voltage 2.8V)
- Trigger Condition: The voltage of any cell string is less than 2.8V for 2 consecutive seconds.
- Execution Actions: Power supply to the drive motor and hydraulic system is cut off, only the BMS power supply is retained; the forklift cannot be started, and the lifting mechanism cannot be operated.
- Self-Recovery Condition: The protection is automatically released after charging until the cell voltage reaches ≥3.2V, without manual reset.
Over-Discharge Lockout (Threshold: Cell Voltage < 2.5V)
- Trigger Condition: The cell voltage drops deeply below 2.5V (extreme undervoltage).
- Execution Actions: The BMS activates forced lockout, prohibiting charging and discharging; the lockout can only be released and the battery recovered by connecting a dedicated activation device to boost the cell voltage to above 3.0V.
- Hazard Warning: After triggering this level of protection, the cell capacity will experience irreversible attenuation of ≥20%.
IV. Environmental Logic Correction for Undervoltage Protection in Low-Temperature Winter Conditions
In low temperatures, battery capacity attenuates, so the protection thresholds are dynamically adjusted to avoid false triggering:
- When the ambient temperature is < 5℃, the early-warning threshold for lead-acid batteries is increased to 92% of the rated voltage, and the cell-level early-warning threshold for lithium batteries is increased to 3.1V.
- When the temperature is < -10℃, the BMS automatically limits the discharge current to ≤50% of the rated current and triggers power-limiting protection in advance to reduce the risk of undervoltage.
V. Key Points for Troubleshooting Protection Function Failure
- 1.If the undervoltage protection is not triggered, prioritize checking whether the wiring harness of the voltage acquisition module is loose or oxidized (replace it if the contact resistance exceeds 10mΩ).
- 2.An outdated BMS program version for lithium batteries will cause threshold judgment deviations; firmware upgrade must be performed once a quarter.
- 3.If the sampling monitoring point of lead-acid batteries fails, replace the cell voltage sensor; the replacement frequency is once a month.
