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Introduction

In the field of industrial logistics, forklifts, as important handling equipment, the performance of their power sources is directly related to operational efficiency and costs. Lead-acid batteries have become a commonly used power choice for forklifts due to their advantages such as mature technology, low cost, and good high-current discharge performance. However, in the actual operation of forklifts, the working conditions are complex and the discharge rate changes frequently. Different discharge rates have multiple impacts on the performance of lead-acid batteries. In-depth exploration of these impacts is of great significance for optimizing the power system of forklifts, improving operational efficiency, and extending the service life of batteries.

A brief description of the working principle of lead-acid batteries

Lead-acid batteries achieve the mutual conversion of electrical energy and chemical energy through internal chemical reactions. During charging, under the influence of an external power source, lead dioxide (PbO_2) at the positive electrode and lead (Pb) at the negative electrode react with the sulfuric acid electrolyte and are respectively transformed into lead sulfate (PbSO_4), while the concentration of sulfuric acid increases. During discharge, the process proceeds in reverse. Lead sulfate is converted back into lead dioxide and lead. Sulfuric acid is consumed and its concentration decreases, thereby releasing electrical energy. The basic chemical reaction equation is: \(PbO_2 + Pb + 2H_2SO_4 \underset{charging}{\overset{discharging}{\rightleftharpoons}} 2PbSO_4 + 2H_2O\). This reversible chemical reaction is the basis for the operation of lead-acid batteries, and changes in the discharge rate will affect the rate and extent of the reaction, thereby influencing the battery performance.

The influence of different discharge rates on the capacity of storage batteries

Definition and representation method of discharge rate

The discharge rate is used to measure the extent of the discharge current in a battery and is usually expressed in two ways: time rate and current rate. Time rate refers to the length of time it takes for a battery to discharge to the terminal voltage under specific discharge conditions. According to IEC standards, common discharge rates include 20-hour rate, 10-hour rate, 5-hour rate, 3-hour rate, 2-hour rate, 1-hour rate, 0.5-hour rate, etc. The current rate is expressed as a multiple of the rated capacity of the battery to represent the discharge current. For example, 0.2C indicates discharge at a current value 0.2 times the rated capacity (C is the rated capacity of the battery).

Capacity attenuation mechanism under high discharge rate

When the lead-acid battery of a forklift is in a state of high discharge rate (high current discharge), the battery capacity will significantly decrease. This is mainly due to the electrochemical process changes on the surface of the plates. When a large current is discharged, the sulfuric acid in the pores of the plates is consumed rapidly. The diffusion rate of the sulfuric acid is difficult to meet the reaction requirements, resulting in a rapid decrease in the concentration of the electrolyte in the pores of the plates. According to the Nernst equation, the electrode potential is related to the concentration of the electrolyte. A decrease in concentration leads to a drop in the electrode potential, a reduction in the electromotive force of the battery, and thus a decrease in the output electricity. Meanwhile, the large amount of heat generated by high-current discharge will accelerate the shedding of the active material on the plates, further reducing the battery capacity. For instance, under the condition of 1-hour rate discharge, the actual discharge capacity of the battery may only be 50% to 60% of its rated capacity, which is far lower than its performance at low discharge rates.

Capacity characteristics at low discharge rate

When the discharge rate is low (small current discharge), lead-acid batteries can make more full use of the active material on the plates, and the battery capacity performance is closer to the rated capacity. During the low-current discharge process, sulfuric acid has sufficient time to diffuse into the interior of the plates to participate in the reaction. The active substances on the plates react uniformly and fully, making the battery output voltage more stable and enabling a higher discharge capacity. Under the condition of 20-hour rate discharge, the battery capacity can reach or even slightly exceed the rated capacity. This is because the concentration of the electrolyte in the pores of the plates can be maintained at a relatively high level, ensuring the continuous and efficient progress of the chemical reaction.

The influence of different discharge rates on the lifespan of storage batteries

The reasons for the shortened lifespan due to high discharge rate

High discharge rate has a serious negative impact on the service life of lead-acid batteries in forklifts. On the one hand, the high current discharge causes a sharp increase in the surface temperature of the plates, accelerating the aging and shedding of the active substances. Under high-temperature conditions, the growth rate of lead sulfate crystals on the plates accelerates, and the crystal structure becomes coarser. These coarser crystals are more likely to fall off the plates during charge and discharge cycles, reducing the total amount of active substances involved in the reaction, causing irreversible decline in battery capacity and shortening battery life. On the other hand, when discharging at a high current, the polarization phenomenon inside the battery intensifies, leading to an increase in the battery's internal resistance, additional consumption of electrical energy and generation of more heat. This vicious cycle further damages the battery's performance and reduces its cycle life. Studies show that compared with the normal discharge rate, the cycle life of lead-acid batteries that are in a high discharge rate condition for a long time may be shortened by 30% to 50%.

The advantage of lifespan at low discharge rates

Under low discharge rate conditions, lead-acid batteries have a relatively longer lifespan. When discharging with a small current, the reaction of the active material on the plates is mild and uniform. The contraction and expansion of the plates are relatively small, and the bonding force between the active material and the plate base is maintained well, making it less likely to fall off. Meanwhile, at a low discharge rate, the internal polarization degree of the battery is low, and the internal resistance is stable. This reduces the energy loss and heat generation caused by the increase in internal resistance, which is conducive to maintaining the stability of the internal chemical environment of the battery and thereby extending the battery's cycle life. For instance, under the light-load and slow-running conditions of forklifts, the battery operates at a low discharge rate, and its service life can be extended by 1 to 2 times compared to that under high discharge rate conditions.

The influence of different discharge rates on the internal resistance of storage batteries

The physical process by which the discharge rate changes the internal resistance

The internal resistance of lead-acid batteries is composed of plate resistance, electrolyte resistance, and interface resistance between plates and electrolyte, etc. When the discharge rate changes, the internal resistance will change accordingly. At a high discharge rate, the migration speed of ions in the electrolyte increases, and the interaction between ions intensifies, resulting in a slight increase in the resistance of the electrolyte. Meanwhile, the high-current discharge causes the lead sulfate generated on the surface of the plates to increase rapidly, blocking some pores. The contact area between the active material and the electrolyte decreases, and the interface resistance increases significantly. Under the combined effect, the internal resistance of the battery increases. At a low discharge rate, the ion migration rate is moderate, the formation rate of lead sulfate on the plate surface is slow, the plate pores remain unobstructed, and the active material has good contact with the electrolyte. Therefore, the internal resistance is relatively stable and small.

The chain reaction of internal resistance changes on battery performance

The change in internal resistance has a chain reaction on the performance of lead-acid batteries. When the internal resistance increases, the internal voltage drop of the battery during the discharge process increases, resulting in a decrease in output voltage, which affects the power performance of the forklift, causing the lifting speed to slow down and the operating power to be insufficient. The increase in internal resistance will also cause the battery to heat up more intensely during the charging process. Excessively high temperatures not only accelerate the evaporation of the electrolyte but may also damage the internal structure of the battery, further reducing its lifespan. For instance, when the internal resistance of the battery increases by 20% to 30% due to a high discharge rate, the voltage of the forklift when it is fully loaded may drop by 5% to 10%, seriously affecting the normal operation of the forklift.

The voltage characteristics of storage batteries at different discharge rates

Characteristics of voltage drop at high discharge rates

Under high discharge rates, the voltage of lead-acid batteries drops rapidly and significantly. In the early stage of discharge, due to the instantaneous intensification of the polarization phenomenon caused by the large current discharge, the terminal voltage of the battery will drop rapidly. As the discharge proceeds, the concentration of the electrolyte in the pores of the plates decreases rapidly, the electromotive force of the battery continues to decrease, and the terminal voltage further drops. When the battery is discharged at a high discharge rate of 1 hour close to the terminal voltage, the terminal voltage may rapidly drop from the initial approximately 2.1V to below 1.7V, seriously affecting the power supply stability of the forklift to the load, and may cause the forklift control system to malfunction or fail to drive the load normally.

Voltage stability at low discharge rates

When the discharge rate is low, the voltage drop of lead-acid batteries is relatively gentle and stable. Low current discharge causes a slow reaction on the surface of the plates, uniform change in the concentration of the electrolyte, weak polarization phenomenon, stable electromotive force of the battery, and a slow rate of terminal voltage drop. During the 20-hour low-discharge rate discharge process, the battery terminal voltage drops slowly from the initial value until it approaches the terminal voltage at the end of the discharge. The voltage fluctuation is relatively small throughout the discharge process, providing good power supply conditions for the stable operation of the forklift and facilitating the precise control of load lifting, handling and other operations by the forklift.

Considerations and suggestions in practical applications

The operating conditions of the forklift match the discharge rate

The actual working conditions of forklifts are complex and diverse, including heavy-load handling, light-load short-distance transportation, and frequent lifting, etc. Under heavy-load handling conditions, forklifts need to output a large current instantaneously. At this time, a high discharge rate is inevitable, but the duration of the high discharge rate should be shortened as much as possible to avoid excessive battery wear. The light-load short-distance transportation condition enables the battery to operate in a state of low discharge rate, which is conducive to extending the battery life and improving the capacity utilization rate. Enterprises should, based on the specific operation scenarios of forklifts, rationally plan the tasks of forklifts, try to match the battery discharge rate with the working conditions, and optimize the battery performance.

Reference discharge rate factors for battery selection

When choosing lead-acid batteries for forklifts, the factor of discharge rate should be fully considered. If forklifts are mainly used for frequent lifting, heavy load handling and other conditions that require high discharge rates, batteries with large capacity and good high discharge rate performance should be selected, such as those with special plate processes and high-conductivity electrolytes, to meet the demand for instantaneous large currents and reduce capacity attenuation. For forklifts that mainly operate under light loads and for long periods of continuous work, it is advisable to focus on choosing batteries with high capacity retention at low discharge rates and long cycle lives to reduce long-term usage costs.

Maintenance measures are aimed at adjusting the discharge rate

Maintenance measures should also be adjusted accordingly based on the different discharge rates of lead-acid batteries in forklifts. After operating at a high discharge rate, it is necessary to focus on checking the condition of the battery plates, such as whether there is any shedding of active materials or deformation of the plates. Remove the shed materials in time to prevent short circuits. At the same time, close attention should be paid to the liquid level and density of the electrolyte. A high discharge rate accelerates the consumption of the electrolyte, and distilled water or electrolyte that meets the requirements should be replenished in a timely manner. For batteries operating at low discharge rates, the maintenance focus lies in regularly checking whether the battery connection parts are loose to ensure good electrical conductivity and maintain a stable working state of the battery.

Conclusion

The discharge rate of forklift lead-acid batteries has a significant impact on their performance such as capacity, lifespan, internal resistance and voltage characteristics. Although a high discharge rate can meet the instantaneous large power demand of forklifts, it will lead to rapid attenuation of battery capacity, shortened lifespan, increased internal resistance and unstable voltage. A low discharge rate is conducive to the full utilization of battery capacity, extending service life, maintaining stable voltage output and low internal resistance. In practical applications, by reasonably matching the working conditions of forklifts with the discharge rate, scientifically selecting models based on the discharge rate, and making targeted adjustments to maintenance measures, the performance of lead-acid batteries in forklifts can be effectively enhanced, operating costs can be reduced, and the efficiency of industrial logistics operations can be improved. In the future, as the requirements for forklift operation efficiency and environmental protection continue to rise, in-depth research on the performance optimization strategies of lead-acid batteries at different discharge rates will be of great significance for promoting the sustainable development of the forklift industry.