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1. Introduction

In modern logistics and industrial production, forklifts, as important material handling equipment, play a crucial role. The lead-acid battery of the forklift, as its main power source, its performance and lifespan directly affect the operating efficiency and usage cost of the forklift. A thorough understanding of the charge and discharge cycles of lead-acid batteries in forklifts is of great significance for optimizing the use of forklifts, extending battery life and reducing operating costs.

2. Working Principle of Lead-Acid Batteries

(1) Basic Structure

Lead-acid batteries are mainly composed of positive plates, negative plates, separators, electrolyte and casings, etc. The active material on the positive plate is usually lead dioxide, while that on the negative plate is spongy lead. The electrolyte is generally a water solution of sulfuric acid.

(2) Chemical Reactions during the discharge process

When the lead-acid battery of a forklift discharges, a chemical reaction occurs inside the battery. The lead on the negative plate combines with the sulfate ions in the electrolyte to form lead sulfate and releases two electrons. Lead dioxide on the positive plate combines with sulfate ions under the action of sulfuric acid and electrons to form lead sulfate and water. During this process, sulfuric acid is consumed, the density of the electrolyte gradually decreases, and the voltage of the battery also drops accordingly, thereby providing electrical energy for the forklift to drive it to perform operations such as walking and lifting goods.

(3) Chemical reactions during the charging process

When charging, under the influence of an external power source, the chemical reactions inside the battery proceed in reverse. The lead sulfate on the negative plate is reduced to lead after gaining electrons and releases sulfate ions. The lead sulfate on the positive plate reacts with water to form lead dioxide and sulfuric acid, and loses two electrons. As charging proceeds, sulfuric acid is constantly generated, the density of the electrolyte gradually increases, the battery voltage rebounds, and electrical energy is re-stored in the form of chemical energy.

3. Important Parameters of Charge and Discharge Cycles

(1) Cycle life

Cycle life is one of the important indicators for evaluating the performance of forklift lead-acid batteries. It refers to the number of charge and discharge cycles that a battery can maintain a certain proportion (usually 80%) of its initial capacity under certain charge and discharge conditions. At present, the design life of mainstream lead-acid batteries for forklifts is generally 1,500 cycles, but the actual cycle life is affected by various factors, such as charging and discharging methods, usage environment, and maintenance conditions. For instance, frequent deep discharges and unreasonable charging methods can significantly shorten the battery's cycle life, while good usage and maintenance habits can help extend it.

(2) Depth of Discharge (DOD)

The depth of discharge refers to the percentage of electricity discharged by a battery during a single discharge process to its rated capacity. For instance, for a battery with a rated capacity of 100Ah, when it discharges 50Ah of electricity, the depth of discharge is 50%. The depth of discharge has a significant impact on the cycle life of batteries. Generally speaking, the deeper the discharge depth, the shorter the cycle life of the battery. This is because deep discharge will cause more active substances on the plates to participate in the reaction, making the structure and performance of the plates more vulnerable to damage. Long-term deep discharge may also cause problems such as sulfation of the negative plate, further reducing the battery's performance and lifespan. Therefore, when using lead-acid batteries for forklifts, the depth of discharge should be controlled as much as possible to avoid excessive discharge.

(3) Charging efficiency

Charging efficiency reflects the battery's ability to convert input electrical energy into chemical energy and store it during the charging process. The charging efficiency of lead-acid batteries is typically between 75% and 90%, which means that during the charging process, a portion of the electrical energy is lost in the form of heat and other factors. Charging efficiency is influenced by multiple factors, such as the performance of the charger, the magnitude of the charging current, the temperature of the battery, and the health condition of the battery, etc. An efficient charger can control the charging process more precisely, reduce energy loss and improve charging efficiency. An appropriate charging current can enable the chemical reactions inside the battery to proceed more fully and evenly, thereby enhancing the charging efficiency. Both excessively high and low battery temperatures can have adverse effects on charging efficiency.

4. Factors Affecting Charge and Discharge Cycles

(1) Charging method

Constant current charging: During the charging process, the charging current remains constant. The advantage of this charging method is that it is simple to operate and easy to implement, enabling the battery to accept a larger charging current in a relatively short time and accelerating the charging speed. However, if the charging current is too large, it may cause the battery to heat up severely, accelerate water loss and plate aging of the battery, and shorten the battery life. Moreover, in the later stage of constant current charging, when the battery voltage gradually rises and approaches full charge, excessive charging current will cause the battery to overcharge, further damaging the battery.

Constant voltage charging: During the charging process, the charging voltage remains constant. As charging proceeds, the battery voltage gradually rises and the charging current gradually decreases. Constant voltage charging can prevent overcharging of the battery, but the initial charging current is too large, which may cause an impact on the battery. Moreover, the charging time is relatively long, which may lead to insufficient battery charging.

Segmented charging: Combining the advantages of constant current charging and constant voltage charging, a larger constant current charging is adopted at the initial stage of charging to enable the battery to quickly absorb electrical energy. When the battery voltage rises to a certain level, it switches to constant voltage charging to prevent overcharging of the battery. This charging method can ensure the charging speed while improving the charging efficiency and reducing the damage to the battery. It is a relatively ideal charging method.

(2) Discharge current

The magnitude of the discharge current has a significant impact on the charge and discharge cycles of forklift lead-acid batteries. A larger discharge current will accelerate the chemical reaction rate inside the battery, causing the battery voltage to drop rapidly and the actual amount of electricity that can be discharged to decrease. At the same time, it will also accelerate the corrosion of the plates and the shedding of active substances.

(3) Temperature

High-temperature impact: When the ambient temperature is too high, the chemical reaction rate inside the battery accelerates, the evaporation of the electrolyte intensifies, water loss increases, and the concentration of the electrolyte rises. This will accelerate the corrosion of the plates and shorten the battery's lifespan. At the same time, high temperatures will also accelerate the self-discharge rate of batteries, making the stored electricity more likely to be lost. For instance, in the high-temperature environment of summer, if the lead-acid batteries of forklifts are exposed to sunlight for a long time or placed in poorly ventilated high-temperature areas, their performance and lifespan will be severely affected.

Low-temperature impact: In a low-temperature environment, the viscosity of the electrolyte increases, the diffusion rate of ions slows down, and the internal resistance of the battery increases, resulting in a decline in the battery's discharge performance, a reduction in the actual amount of electricity that can be discharged, and making charging more difficult. Moreover, at low temperatures, the battery may experience over-discharge, further damaging the battery. For instance, forklifts used in low-temperature environments such as cold storage facilities need to take special insulation or heating measures for their lead-acid batteries to ensure normal charging and discharging performance.

(4) Battery maintenance

Electrolyte replenishment: During the charging and discharging process of lead-acid batteries, the water content in the electrolyte gradually decreases due to electrolysis and evaporation. If water is not replenished in time, the concentration of the electrolyte will increase, accelerating the sulfation and corrosion of the plates, and reducing the battery capacity and cycle life. Therefore, it is necessary to regularly check the electrolyte level and replenish distilled water or dedicated supplementary solution in a timely manner.

Plate cleaning: During the use of the battery, some impurities and deposits may adhere to the surface of the plates. These substances can affect the activity of the plates and the performance of the battery. Regular cleaning of the plates and removal of surface impurities can ensure the normal progress of internal chemical reactions in the battery and extend its lifespan.

Equalization charging: In a battery pack, due to the certain differences in the characteristics of each individual battery, after multiple charge and discharge cycles, there may be a situation where the voltage and capacity of some batteries are inconsistent, which is known as a "lagging battery". Regular equalization charging can ensure that each individual battery reaches a fully charged state, preventing the further deterioration of "lagging batteries" and enhancing the performance and lifespan of the entire battery pack.

5. Key Points of Charge and Discharge Cycle Operation

(1) Charging operation

Charging environment selection: Charging should be carried out in a dry, well-ventilated, and temperature-appropriate environment (generally 10-30 °C). Charging should be avoided in damp, high-temperature, or open flame locations. At the same time, the charging area should be kept away from flammable and explosive items to ensure charging safety.

Charger matching: Use a charger that matches the specifications of the forklift's lead-acid battery. Different models and specifications of batteries require different parameters such as charging voltage and current. An unmatched charger may lead to insufficient charging or overcharging, damaging the battery.

Charging connection: When connecting the charger and the battery, first insert the charger plug into the battery socket, and then connect the charger power supply. After charging is completed, the power supply of the charger should be disconnected first, and then the charger plug should be unplugged. This can prevent the generation of electric sparks when plugging and unplugging the plug, which may cause safety accidents.

Charging process monitoring: During the charging process, pay close attention to the displayed parameters of the charger, such as charging voltage, current, charging time, etc., as well as the battery temperature and electrolyte level. If any abnormal conditions are detected, such as battery overheating, electrolyte overflow, or abnormal charger display, charging should be stopped immediately, and inspection and handling should be carried out.

(2) Discharge operation

Avoid excessive discharge: When using a forklift, pay close attention to the battery power display. When the power approaches the specified minimum discharge limit, stop the operation in time and charge it.

Reasonable control of discharge current: According to the actual operation requirements of the forklift, adjust the load reasonably to avoid long-term high-current discharge. When performing heavy-duty operations or climbing slopes and other operations that require significant power, the duration should be minimized as much as possible to reduce the damage caused by discharge current to the battery.

Charge promptly after discharging: After the forklift is used, the battery should be charged as soon as possible after discharging to avoid it remaining in a state of low charge for a long time. If the battery is not charged in time after discharge, the lead sulfate on the battery plates will gradually crystallize, forming irreversible sulfation, which leads to a decrease in battery capacity and a shortened lifespan.

6. The Relationship between Charge and Discharge Cycles and Battery Life

(1) Battery life under normal charge and discharge cycles

Under normal charge and discharge cycle conditions, that is, by following an appropriate charging method, controlling a reasonable depth of discharge and current, as well as a good usage and maintenance environment, forklift lead-acid batteries can achieve their designed cycle life.

(2) Damage to battery life caused by abnormal charge and discharge cycles

Overcharging: If the battery is fully charged but continues to be charged during the charging process, overcharging occurs. Overcharging can cause a large amount of gas to be generated inside the battery, leading to accelerated water loss, accelerated corrosion of the plates, and an increase in battery temperature. In severe cases, it may cause the battery to bulge, deform, or even explode. Long-term overcharging will significantly shorten the cycle life of a battery. A battery that could originally achieve 1,500 cycles may experience severe performance degradation after just a few hundred cycles under overcharging conditions.

Over-discharge: When a battery discharges beyond its specified minimum discharge limit, over-discharge occurs. Over-discharge will cause a large amount of lead sulfate to form on the plates and the crystals to become coarser, resulting in irreversible sulfation, blocking the micro-pores of the plates, leading to an increase in the internal resistance of the battery, a decrease in the charging acceptance capacity, and a significant reduction in capacity. Over-discharge can cause extremely serious damage to batteries. A single deep over-discharge may lead to irreparable damage to the battery and significantly shorten its service life.

Frequent shallow charging and discharging: Although shallow charging and discharging (i.e., the discharge depth is relatively shallow and the battery is not fully charged each time) can reduce the wear of the battery plates to a certain extent, if it is too frequent, the active substances on the battery plates will not be able to have sufficient chemical reactions, resulting in plate passivation and a gradual decrease in battery capacity. Frequent and prolonged shallow charging and discharging can also affect the battery's cycle life, preventing it from reaching its designed service life.

7. Conclusion

The charge and discharge cycle of forklift lead-acid batteries is a complex process, involving chemical reactions inside the battery, various parameters and numerous influencing factors. Understanding its charging and discharging principles, mastering important parameters, recognizing various factors that affect the charging and discharging cycle, and following the correct charging and discharging cycle operation points in actual operation are crucial for extending battery life, improving the usage efficiency of forklifts, and reducing operating costs.