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Introduction

In modern logistics and industrial production, forklifts, as key material handling equipment, play a crucial role. As the main power source of forklifts, the performance of lead-acid batteries is directly related to the operational efficiency, reliability and service life of forklifts. Internal resistance, as a key internal parameter of lead-acid batteries, precise measurement and in-depth analysis of it are of great significance for assessing the battery's health status, optimizing the forklift power system, and ensuring the continuity of production operations.

An overview of the Working Principle of Forklift Lead-acid Batteries

The working process of lead-acid batteries is based on complex electrochemical reactions. As charging proceeds, the concentration of sulfuric acid increases, and the battery stores electrical energy. During discharge, the reaction proceeds in reverse, converting chemical energy into electrical energy to power the forklift, and the concentration of sulfuric acid decreases. During this process, there are various factors inside the battery that hinder the conduction of current. These factors combined constitute the internal resistance of the battery.

The composition and influencing factors of internal resistance

Ohm resistor

Ohm resistance is an important component of internal resistance, mainly derived from the conductive materials inside the battery. This includes the hindrance to the current caused by electrode materials, electrolytes, and connecting components, etc. Among them, the resistance of the electrolyte is closely related to the concentration of sulfuric acid and temperature. Between -20 ℃ and 30℃, there exists a concentration range corresponding to the minimum resistivity of sulfuric acid solution, which is approximately 27% - 31.5%. When the concentration deviates from this range, the resistance of the electrolyte will increase significantly. In addition, a decrease in temperature will increase the viscosity of the sulfuric acid solution, making ion migration difficult and leading to an increase in the resistance of the electrolyte. For instance, when the ambient temperature drops from 30℃ to 0℃, the resistance of the electrolyte may increase several times. The resistance of the electrode material itself should not be ignored either. Factors such as the material, thickness and manufacturing process of the electrode plate will all affect its resistance value. Poor-quality plate materials or unreasonable manufacturing processes may lead to an increase in plate resistance, thereby raising the overall ohmic resistance of the battery.

Polarization resistance

Polarization resistance includes electrochemical polarization resistance and concentration polarization resistance. Electrochemical polarization is caused by the sluggishness of electrochemical reactions on the electrode surface. During the charging and discharging process, the reaction on the electrode surface requires a certain activation energy, which causes the electrode potential to deviate from its equilibrium potential, generating electrochemical polarization resistance. Concentration polarization is caused by the uneven distribution of ion concentrations in the electrolyte. When the battery is charged and discharged, the ions on the electrode surface participate in the reaction, causing the ion concentration near the electrode to differ from that in the electrolyte body, thus forming a concentration polarization resistance. In the later stage of discharge, as the active substance is consumed, the concentration polarization will become increasingly severe, and the concentration polarization resistance will also increase accordingly.

Other influencing factors

In addition to the above main factors, the internal resistance of a battery is also affected by many other factors. For example, the service life of the battery and the number of charge and discharge cycles. As the usage time increases and the number of cycles grows, the plates inside the battery will gradually corrode and age, and the active substances will fall off, resulting in an increase in internal resistance. The installation method and connection status of the battery will also have an impact on the internal resistance. If the terminals are not firmly connected, the contact resistance will increase, which will cause the internal resistance of the entire battery to rise. In addition, the state of charge of the battery is also related to its internal resistance. Generally speaking, at a low SOC state, the internal resistance of the battery is relatively large.

Analysis of Internal Resistance Measurement Methods

Direct current discharge method

Principle

The measurement principle of the DC discharge method is based on Ohm's Law. By applying a large instantaneous current to the battery for discharge and precisely measuring the voltage drop across the battery terminals at the moment of discharge, the internal resistance of the battery can be calculated given the discharge current.

Advantages and disadvantages

The advantage of this method is that it has a relatively high measurement accuracy. Under ideal circumstances, the measurement accuracy error can be controlled within 0.1%. However, its shortcomings are also very obvious. On the one hand, high-current discharge will cause a significant current impact on the battery in a short period of time, which may lead to severe polarization of the internal electrodes of the battery, accelerate the aging and failure of the battery, and shorten the service life of the battery. On the other hand, since the internal resistance of the battery is dynamically changing, in order to accurately grasp the real-time situation of the battery's internal resistance, periodic measurements are required. This undoubtedly causes frequent damage to the battery. Therefore, this method is more suitable for battery research and development testing or prototype trial production in a laboratory environment. However, in actual forklift usage scenarios, It is not realistic to frequently use the direct current discharge method to measure the internal resistance.

Alternating current impedance method

Principle

The alternating current impedance method involves applying an alternating current of a specific frequency to the battery under test. Under the action of alternating current, components such as ohmic resistors, polarization resistors and capacitors inside a battery will respond differently to the current. Based on Ohm's law and capacitance law, the phase difference, amplitude relationship and other parameters between current and voltage are comprehensively analyzed to calculate the internal resistance of the battery. For instance, when an alternating current with a frequency of 1kHz is applied, by measuring the phase difference and amplitude between the voltage and current at both ends of the battery, and using complex mathematical models and circuit theories, the internal resistance of the battery can be calculated.

Advantages and disadvantages

The prominent advantage of the AC impedance method is that it can effectively avoid the problem of battery life attenuation caused by high current discharge in the DC discharge method. Since an alternating current is applied and the amplitude of the current is relatively small, the damage to the battery is relatively small. However, the drawback of this method is that the measured value of internal resistance is greatly affected by the frequency of the alternating current. At different frequencies, the response characteristics of each component inside the battery are different, resulting in variations in the measured internal resistance results. For example, in the low-frequency band, the polarization effect of the battery is more obvious, and the measurement results may contain more polarization resistance components; In the high-frequency band, the influence of ohmic resistance is more prominent. Therefore, when using the AC impedance method for measurement, it is necessary to precisely control the frequency of the alternating current and select an appropriate frequency range based on the characteristics of the battery to ensure the accuracy of the measurement results.

Other measurement methods

Density method

The density method is mainly applied to open-type lead-acid batteries. The principle is to indirectly estimate the internal resistance of the battery by measuring the density of the battery electrolyte. Since the density of the electrolyte is related to the state of charge and internal resistance of the battery to a certain extent, the internal resistance can be inferred through the density value. However, this method has significant limitations. For sealed lead-acid batteries, the density of the electrolyte cannot be directly measured. Therefore, the application scope of this method is very narrow and it is almost inapplicable in the measurement of valve-regulated sealed lead-acid batteries commonly used in forklifts.

Open-circuit voltage method

The open-circuit voltage method estimates the internal resistance by measuring the open-circuit terminal voltage of the battery. However, the accuracy of this method is extremely poor and it may even lead to incorrect conclusions. Even if the battery capacity has dropped significantly, its open-circuit terminal voltage may still be within the normal range in some cases and cannot accurately reflect the true changes in the battery's internal resistance. Therefore, in the actual measurement of forklift battery internal resistance, the open-circuit voltage method is basically not adopted.

The significance of internal resistance measurement for forklift lead-acid batteries

Evaluate the health condition of the battery

Internal resistance is a key indicator reflecting the health status of a battery. As the battery is used and ages, its internal resistance will gradually increase. When the internal resistance exceeds a certain threshold, it indicates that problems such as plate corrosion, active material shedding, and electrolyte drying up may have occurred inside the battery, and the battery's performance has seriously declined. By regularly measuring the internal resistance, the changing trend of the battery's health condition can be detected in a timely manner. For instance, if the internal resistance of a battery shows a continuous upward trend over a period of time and the increase exceeds the normal range, it can be determined that the battery may have potential faults and needs to be further inspected or replaced in advance to avoid sudden power outages and other situations during forklift operations, which could affect the normal progress of production operations.

Predict the remaining capacity of the battery

Although there is no strict linear relationship between the internal resistance of a battery and its remaining capacity, generally speaking, as the remaining capacity of the battery decreases, the internal resistance will gradually increase. By establishing a corresponding model of internal resistance and remaining capacity and combining it with long-term internal resistance measurement data, the remaining capacity of the battery can be predicted relatively accurately. This is of great significance for the rational scheduling and use of forklifts. For instance, in a logistics warehouse, forklift drivers can rationally arrange the tasks of their forklifts based on the predicted remaining battery capacity, thus avoiding operation interruptions caused by insufficient battery power. Meanwhile, for warehouse managers, they can also plan the charging and replacement of batteries in advance based on the remaining capacity of the batteries, thereby enhancing the efficiency and management level of logistics operations.

Ensure the safe and stable operation of forklifts

During the operation of a forklift, the performance of the battery directly affects the safety and stability of the forklift. If the internal resistance of a battery is too high, a large amount of heat will be generated inside the battery during high-current discharge, causing the battery temperature to rise. This not only accelerates the aging and damage of the battery but may also lead to overheating or even fire and other safety accidents. In addition, excessive internal resistance can also lead to unstable battery output voltage, affecting the normal operation of the forklift motor, reducing the forklift's power performance, and causing problems such as difficult starting and weak driving. By monitoring the internal resistance of the battery in real time, once an abnormal increase in internal resistance is detected, corresponding measures should be taken promptly, such as stopping the forklift for charging, maintaining or replacing the battery, etc. This can effectively ensure the safe and stable operation of the forklift and reduce the risk of accidents.

Conclusion

In conclusion, the measurement of the internal resistance of forklift lead-acid batteries is of irreplaceable significance in battery performance evaluation, health monitoring, and ensuring the safe and stable operation of forklifts. Different methods for measuring internal resistance each have their advantages and disadvantages. In practical applications, it is necessary to select the appropriate measurement method based on specific circumstances. By accurately measuring the internal resistance, the health condition of the battery can be evaluated in a timely and effective manner, and the remaining capacity of the battery can be predicted, providing a scientific basis for the rational use and maintenance of forklifts. With the continuous development of technology, it is expected that more accurate, convenient and less damaging internal resistance measurement technologies to batteries will emerge in the future, further enhancing the management level and service life of forklift lead-acid batteries, and promoting the efficient development of the logistics and industrial production industries.