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In modern logistics and industrial production, as an important material handling equipment, the power source of forklifts is of vital importance. As one of the main power sources of forklifts, battery packs are often connected in series to meet the working voltage required by forklifts. However, the series connection of forklift battery packs is not a simple connection operation. It involves numerous matters that need attention, which directly affect the performance, safety of the forklift and the service life of the battery pack. Proper series connection of battery packs and subsequent maintenance are of great significance for ensuring the efficient and stable operation of forklifts and reducing operating costs.

1. Consistency in battery selection

(1) Battery type

When choosing forklift batteries for series connection, it is essential to ensure that all battery types are exactly the same. Currently, common forklift batteries on the market include lead-acid batteries, lithium batteries, etc. There are significant differences in the working principles, electrochemical reaction processes and performance characteristics among different types of batteries. For instance, lead-acid batteries achieve charging and discharging through the electrochemical reaction of lead and lead dioxide in a sulfuric acid electrolyte, while lithium batteries store and release electrical energy by the movement of lithium ions between the positive and negative electrodes. If batteries of different types are connected in series, during the charging and discharging process, due to the inconsistency of their electrochemical reactions, there will be significant differences in the charging and discharging rates, capacity attenuation rates, etc. of each battery in the battery pack, which seriously affects the overall performance of the battery pack and may even cause safety issues. Therefore, when building a forklift battery pack, a suitable type of battery should be selected first based on factors such as the forklift's usage scenario, work intensity, and budget, and then all batteries of this type should be connected in series.

(2) Specification Parameters

1. ** Voltage ** : The nominal voltage of each battery involved in series connection must be consistent.

2. ** Capacity ** : Consistency in battery capacity is equally crucial. Capacity refers to the amount of electrical energy a battery can store, usually expressed in amperes. Suppose a battery pack is composed of multiple batteries of different capacities connected in series. During the discharge process, the batteries with smaller capacities will be discharged first. If the discharge continues at this point, it will force the batteries with smaller capacities into an over-discharge state. Over-discharge causes significant damage to the batteries, leading to problems such as sulfation of the battery plates and shedding of active materials, thereby reducing the battery's capacity and lifespan. At the same time, due to the early discharge of small-capacity batteries, the actual available capacity of the entire battery pack will also be limited, and it cannot achieve the expected working time.

3. ** Internal Resistance ** : The internal resistance of a battery refers to the resistance encountered when current passes through the interior of the battery. The magnitude of internal resistance will affect the voltage drop and heat generation of the battery during the charging and discharging process. For battery packs connected in series, if the internal resistance of each battery is not the same, during charging, the battery with a higher internal resistance will consume more energy for heat generation, causing its temperature to rise rapidly and potentially leading to dangerous situations such as thermal runaway. During discharge, batteries with high internal resistance will have a relatively low output voltage, making the output voltage of the entire battery pack unstable and affecting the normal operation of the forklift motor. Therefore, when choosing series batteries, batteries with similar internal resistance should be selected as much as possible to ensure the stability and safety of the battery pack during the charging and discharging process.

(3) Degree of newness or oldness

It is strictly prohibited to mix and connect new and old batteries in series. The chemical reaction substances inside the new battery are relatively more, the electrode activity is high, its internal resistance is small, and the terminal voltage is relatively high. After long-term use and charging and discharging cycles, the internal chemical reaction substances of old batteries gradually consume, the electrode activity decreases, the internal resistance increases, and the terminal voltage will also decrease accordingly. When new and old batteries are connected in series, during the charging process, the new battery, due to its lower internal resistance, will have a relatively larger charging current and is prone to overcharging. However, old batteries, due to their high internal resistance and relatively low charging current, may not be fully charged. During the discharge process, the new battery outputs a larger current, which forces the old battery to discharge at a greater current, accelerating the aging and damage of the old battery. At the same time, due to the decline in performance of old batteries, it will drag down the performance of the entire battery pack, significantly reducing both the actual capacity and service life of the battery pack.

2. Connection Operation Specifications

(1) Preparations before connection

1. ** Clean battery terminals ** : Before connecting batteries in series, it is necessary to carefully clean the positive and negative terminals of each battery with a clean cloth or brush. During long-term storage or use, battery terminals may be contaminated with dust, oil stains, electrolyte and other impurities, which will increase the contact resistance between the terminals and the connecting wires. When current passes through, contact resistance will cause electrical energy loss and generate heat. This not only reduces the charging and discharging efficiency of the battery pack but also may damage the terminals and connecting wires due to overheating, and even cause safety accidents such as fires.

2. ** Check the connection wires ** : The connection wires are the key components for achieving battery series connection, and their quality and condition directly affect the performance and safety of the battery pack. It is necessary to check whether the material of the connecting wire is a suitable conductive material, such as copper or aluminum, and the wire diameter should be thick enough to meet the large current transmission requirements during the charging and discharging of the battery pack. At the same time, carefully check whether the connecting wires have any damage, breakage, corrosion or other conditions. If the connecting wires are damaged, leakage may occur under high voltage and large current conditions, posing an electric shock hazard to the operators. If the connecting wire breaks, it will cause the battery pack to open circuit and fail to work properly. If the connecting wires are corroded, their resistance will increase, affecting the efficiency of current transmission and possibly further damaging the battery terminals.

3. **Prepare appropriate tools**: Get the necessary connection tools ready, such as wrenches, screwdrivers, etc., and the specifications of the tools should match the connection bolts of the battery terminals and connection wires. Using mismatched tools may prevent the connecting bolts from being tightened correctly, causing the connection to loosen. Loose connections will increase the contact resistance, generate a large amount of heat during charging and discharging, damage the connection points, and even cause battery pack failures.

(2) Connection Process

1. ** Determine the connection sequence ** : According to the designed battery pack series connection scheme, determine the connection sequence of the batteries. Generally, the positive terminal of the first battery is connected to the negative terminal of the second battery, then the positive terminal of the second battery is connected to the negative terminal of the third battery, and so on. All batteries are connected in sequence. During the connection process, proper markings should be made to ensure the correct connection sequence. Incorrect connection sequence can lead to polarity confusion of the battery pack, preventing the forklift from providing the correct voltage and current, and may even damage the forklift's electrical system.

2. ** Tighten the connecting bolts ** : Firmly connect one end of the connecting wire to the battery terminal, and use a wrench or screwdriver to tighten the connecting bolts according to the specified torque value. The torque value is generally provided by the battery manufacturer. It is essential to operate strictly in accordance with the requirements. If the bolts are tightened too loosely, it will cause the connection to become loose, increase the contact resistance, and lead to problems such as heating and sparking. If it is tightened too much, it may damage the battery terminals or the threads of the connecting wires, which will also affect the reliability of the connection.

3. ** Ensure correct polarity ** : When connecting each battery, carefully confirm whether the positive and negative terminals are connected correctly. The positive and negative pole markings clearly marked on the battery casing can be used for judgment. At the same time, after the connection is completed, check again whether the polarity of the entire battery pack meets the requirements of the forklift's electrical system. Once the polarity is reversed, when the power is connected, a powerful reverse current will instantly pass through the battery pack and the electrical components of the forklift, which may burn out the battery, damage the forklift's controller, motor and other key components, causing serious economic losses.

(3) Inspection after connection

1. ** Visual Inspection ** : After the connection is completed, conduct an visual inspection of the entire battery pack. Check whether the connection between the connecting wire and the battery terminal is firm, and whether there are any abnormal conditions such as looseness or gaps. Check whether the connecting wires are twisted or overly stretched. Check whether the battery casing has any problems such as damage, deformation or leakage. If any abnormality is found in the appearance, it should be dealt with in time. If leakage is found in the battery casing, the battery should be replaced in time to prevent the leakage of electrolyte from causing corrosion and damage to the surrounding environment and equipment.

2. ** Voltage Measurement ** : Use professional measuring tools such as a multimeter to measure the total voltage of the battery pack. The measured value should be close to the theoretical total voltage calculated based on the number of series batteries and the nominal voltage of individual batteries. If the measured voltage deviates too much from the theoretical value, there may be problems such as poor connection, battery damage or reversed polarity.

3. ** Current Test ** : When conditions permit, a small current test can be conducted on the battery pack to simulate the current situation during normal operation of the forklift and observe the performance of the battery pack when it is powered on. Check whether there are any abnormal phenomena such as overheating or sparking at the connection points, and at the same time monitor whether the output current of the battery pack is stable. If during the test, it is found that the connection part is severely heated or there is a sparking phenomenon, it indicates that there is a problem with the connection and it needs to be rechecked and dealt with. If the output current is unstable, it may be due to an internal fault in the battery pack, and the cause needs to be further investigated.

3. Precautions during Charging Process

(1) Charger matching

1. ** Voltage and current matching ** : It is crucial to select a charger that matches the voltage and capacity of the series battery pack. The output voltage of the charger should be able to meet the voltage requirements for charging the battery pack, and the output current should be moderate. If the output voltage of the charger is too high, it will cause overcharging of the battery, raise the internal temperature of the battery, accelerate water loss and plate aging of the battery, shorten the battery life, and even may cause battery swelling, explosion and other dangers. If the output voltage is too low, the battery cannot be fully charged, which will affect the working time of the forklift. At the same time, the output current of the charger should be selected based on the capacity of the battery pack. Generally speaking, the charging current should not be too large to avoid damaging the battery. Generally, the charging current can be controlled at around 10% to 20% of the battery pack's capacity. For instance, for a 200Ah battery pack, the charging current can be selected as 20A to 40A.

2. ** Charging Characteristic matching ** : Different types of batteries have different charging characteristics. For instance, the charging curves of lead-acid batteries and lithium batteries vary significantly. Therefore, the charging characteristics of the charger should match the type of battery pack. Lead-acid batteries typically adopt a constant current - constant voltage charging method, that is, they are charged at a constant current first. When the battery voltage rises to a certain value, they switch to constant voltage charging until the battery is fully charged. Lithium batteries, on the other hand, require dedicated lithium battery chargers. Precise charging control is carried out based on the characteristics of lithium batteries, including multiple stages such as pre-charging, constant current charging, and constant voltage charging, to ensure the safety and lifespan of lithium batteries. If a mismatched charger is used to charge the battery pack, it may not be possible to charge the battery in the best way possible, resulting in problems such as undercharging, overcharging or low charging efficiency.

(2) Charging environment

1. **Appropriate temperature**: The charging efficiency and lifespan of the battery are significantly affected by the ambient temperature. Generally speaking, the optimal charging temperature range for lead-acid batteries is between 20℃ and 40℃. When the ambient temperature is too low, the viscosity of the electrolyte inside the battery increases, the ion diffusion speed slows down, resulting in an increase in the battery's internal resistance, a decrease in its charging acceptance capacity, an extension of charging time, and it may not be able to fully charge the battery. At the same time, low-temperature charging may also cause irreversible chemical reactions inside the battery, damaging it. When the ambient temperature is too high, the heat generated by the battery during charging is not easily dissipated, which will further increase the battery temperature, accelerate the battery's water loss and aging, and may even lead to dangerous situations such as thermal runaway. For instance, in a high-temperature summer environment, if a battery pack is placed in direct sunlight for charging, the battery temperature may rise rapidly, exceeding the safe range and causing serious damage to the battery. Therefore, the battery pack should be placed in an environment with a suitable temperature for charging as much as possible. If the ambient temperature cannot meet the requirements, it is advisable to consider using heating or cooling equipment to adjust the charging environment. ​

2. **Good ventilation**: During the charging process, the battery undergoes a series of electrochemical reactions, which may produce flammable and explosive gases such as hydrogen. If the charging environment is poorly ventilated, these gases will accumulate locally. When they reach a certain concentration and encounter fire sources such as open flames or static electricity, they may cause an explosion accident. Therefore, the charging site must maintain good ventilation conditions to ensure that the generated gas can be discharged in a timely manner. For instance, when charging the battery pack of a forklift indoors, it is necessary to ensure that the charging room has sufficient ventilation openings or is equipped with ventilation devices such as exhaust fans to guarantee air circulation. At the same time, smoking and open flames are strictly prohibited in charging areas, and the use of equipment or materials that may generate static electricity, such as synthetic fiber clothing, should be avoided to reduce safety risks.