Over-discharge protection of lithium battery packs is a core technology for ensuring battery safety, extending service life and maintaining stable performance. It needs to achieve systematic protection through five major links: voltage monitoring, protection threshold setting, response mechanism design, recovery strategy optimization and redundant design. The following are the specific technical points and key implementation paths:

First, voltage monitoring technology

High-precision sampling circuit

It adopts 16-bit or higher ADC chips and is combined with low-noise operational amplifier circuits to control the single-cell voltage sampling error within ±5mV, avoiding false triggering caused by voltage drift.

Transient voltage fluctuations during the charging and discharging process are suppressed by combining hardware filter capacitors (10μF-100μF) with digital filtering algorithms (such as moving average filtering).

Case: A certain energy storage system reduced the over-discharge misjudgment rate from 0.5% to 0.01% by optimizing the sampling circuit, thus avoiding system shutdown caused by false protection.

Multi-level voltage monitoring

Dual redundant monitoring of individual cell voltage and total voltage is implemented. When the individual cell voltage is lower than the protection threshold or the total voltage is lower than the theoretical lower limit, the protection action is triggered.

For the series battery pack, a Daisy chain voltage sampling topology is adopted to reduce the number of sampling cables and lower the voltage error caused by cable voltage drop.

Case: Through multi-level monitoring, a battery pack of a certain unmanned aerial vehicle successfully identified over-discharge of a single cell (2.3V), while the total voltage remained normal, thus avoiding the failure of the entire pack.

Second, setting the protection threshold

Dynamic threshold adjustment

Different thresholds are set according to the battery type (such as lithium iron phosphate and ternary lithium). The over-discharge protection voltage for lithium iron phosphate batteries is usually set at 2.5V±0.05V, and for ternary lithium batteries, it is set at 3.0V±0.05V.

Combined with the environmental temperature compensation algorithm, the threshold rises by 2% to 5% at low temperatures (such as -10℃) and drops by 1% to 3% at high temperatures (such as 45℃), avoiding false triggering caused by voltage offset due to temperature.

Case: A certain electric vehicle, in an environment of -20℃, raised the over-discharge protection voltage from 2.5V to 2.6V through dynamic threshold adjustment to prevent false protection caused by increased internal resistance at low temperatures.

Hierarchical protection strategy

First-level warning: When the voltage of a single cell drops below the threshold by 5%, a warning signal is sent via the CAN bus to prompt the user to reduce the load or stop discharging.

Secondary protection: When the voltage of a single cell drops below 10% of the threshold, the main discharge circuit is cut off, and the power supply to the BMS self-check circuit is retained to prevent data loss due to complete power failure.

Case: A certain energy storage system, through hierarchical protection, triggers a warning when the cell voltage drops to 2.38V and cuts off the discharge when it drops to 2.25V, thereby extending the battery life by 20%.

Third, design of response mechanism

Hardware-level rapid cut-off

It adopts a dual protection circuit of MOSFET+ fuse. The MOSFET cuts off the discharge circuit within 10μs, and the fuse melts when there is a continuous overcurrent (such as 5 times the rated current), preventing the spread of thermal runaway.

Through the design of a voltage comparator and a delay circuit, false disconnection caused by transient voltage drops (such as sudden load changes) is avoided.

Case: The battery pack of a certain power tool was cut off at the hardware level, responding to over-discharge within 0.5ms, preventing the cell voltage from dropping below 2.0V, and the capacity recovery rate was increased to 95%.

Software latch and reset

The BMS records over-discharge events and latches the protection status. The protection needs to be lifted through a dual condition of manual reset and charging activation to prevent the recurrence of faults.

When charging and activating, first pre-charge with a small current (such as 0.1C) to a safe voltage (such as 2.8V), and then resume the normal charging process.

Case: The battery pack of a certain robot was locked by software to prevent repeated discharge without repairing faults, reducing the damage rate of battery cells by 80%.

Fourth, optimization of recovery strategies

Small current pre-charge activation

After over-discharge, pre-charge at a current of 0.05C-0.1C for 10-30 minutes to repair the damage to the SEI film inside the battery cell and avoid lithium plating caused by direct high current charging.

During the pre-charging process, the voltage recovery rate is monitored in real time. If the voltage stops or rises slowly, the battery cell is determined to be faulty and isolated.

Case: A certain energy storage battery pack successfully restored 85% of the capacity of over-discharged cells through low-current pre-charging, with a 30% increase in recovery rate compared to direct high-current charging.

Capacity balancing and calibration

After recovery, the battery pack is equalized charged to eliminate individual voltage differences, and the SOC (State of Charge) estimation accuracy is calibrated through OCV (Open Circuit Voltage) testing.

Capacity calibration should be carried out on multiple over-discharged cells. If the capacity is less than 80% of the rated value, they should be marked as retired cells and isolated.

Case: A certain electric vehicle battery pack has reduced the voltage difference between packs from 200mV to 50mV through capacity balancing, and its cycle life has been extended by 15%.

Fifth, redundant design

Multi-channel voltage sampling redundancy

For key cells (such as the first cell, the last cell, and the middle cell), dual sampling channels are adopted. When the voltage difference between the two channels exceeds the threshold (such as 20mV), a fault alarm is triggered and the standby channel is switched.

The sampling circuit is powered by an isolated power supply to prevent the entire sampling group from failing due to a single point of failure.

Case: A battery pack of a certain medical device successfully identified a single-channel sampling fault through dual sampling channels, avoiding equipment shutdown caused by voltage misjudgment.

Redundant protection circuits

The main protection circuit and the backup protection circuit adopt different topological structures (such as MOSFETs for the main circuit and relays for the backup circuit) to avoid double failure caused by common-mode faults.

The backup circuit conducts regular self-checks. If the main circuit fails, it will seamlessly switch to the backup circuit within 100ms.

Case: In a certain data center, the UPS battery pack has a redundant protection circuit. When the main circuit MOSFET is short-circuited, the backup circuit takes over within 80ms to ensure that the power supply is not interrupted.

Sixth, optimization of typical application scenarios

Electric vehicle

Low-temperature start-up optimization: In an environment of -20℃, the over-discharge protection voltage rises to 2.7V to prevent false protection caused by increased internal resistance at low temperatures. At the same time, activate the heating film to preheat the battery to 0℃ before discharging.

Case: A certain electric vehicle reduced its range degradation from 40% to 25% through low-temperature optimization in an environment of -15℃.

Energy storage system

Long-term float charging management: In scenarios where long-term shallow discharge (DOD < 10%) is caused by power grid dispatching, a deep discharge to 20% SOC is conducted once a month, followed by balanced charging to prevent false capacity overcharging.

Case: A certain energy storage power station has increased the battery pack capacity retention rate from 85% to 92% through float charging management.

drone

High-rate discharge protection: When discharging at a high rate of 5C-10C, the over-discharge protection voltage drops to 2.4V to prevent voltage sags caused by polarization effects. Simultaneously monitor the temperature of the battery cells in real time and limit the discharge current when it exceeds 60℃.

Case: A certain unmanned aerial vehicle (UAV) reduced the voltage drop from 0.8V to 0.3V during 8C discharge through high-rate protection, and its endurance was extended by 12%.