1 Overview
With the rapid development of precision technologies such as information processing technology and microelectronics, the requirements for the quality and reliability of power supply systems are also increasing. Therefore, there is an urgent need for an AC uninterruptible power supply with stable voltage, synchronous tracking of grid frequency and high reliability, and UPS power supply has emerged. The UPS power supply is mainly an AC-DC-AC conversion system. When the AC power is normal, after rectifying the AC to DC, the battery is charged on the one hand, and the DC is reconverted to AC to supply power to the load through the inverter. When the AC power is interrupted, the DC power of the battery is immediately converted into AC to the load through the inverter to ensure the continuity of the power supply. The battery in the UPS system is the most important, and its selection and maintenance becomes very important.
2 UPS battery selection
2.1 battery selection steps
Select the rated voltage of the battery according to the battery charging circuit voltage in the UPS power supply.
If the battery charging circuit voltage in the UPS power supply is 110V, the value is the floating charging voltage of the 96V battery pack, and the battery with rated voltage of 12V can be selected for 8 knots. The battery discharge termination voltage Ei is 85V.
Calculate the maximum discharge current Imax of the battery pack.
Where: P is the rated apparent power of the UPS power supply; Cosφ is the power factor of the load; η is the efficiency of the inverter. The discharge rate XC is determined from the horizontal axis delay time requirement of the discharge characteristic curve and the vertical axis discharge termination voltage.
Calculate the safety time C1 of the battery pack.
C1=Imax/XC(2)
From the horizontal axis minimum temperature requirement and discharge rate of the temperature characteristic curve, the available battery capacity percentage Y is found on the vertical axis.
Calculate the number of hours C2 of the final battery pack,
CC=C1/Y(3)
2.2 Calculation example
(1) Known conditions
The rated apparent power P of the UPS power supply is 1kVA;
The power factor COSφ of the load is 0.8;
The efficiency η of the inverter is 0.8;
The battery with rated voltage of 12V is selected for 8 knots, and the discharge end voltage Ei of the battery pack is 85V.
(2) Requirements
In the operating temperature range of the UPS power supply (-10 °C - 40 °C), the rated power of the battery is selected under the condition that the UPS power supply is powered by the battery for not less than 1 h.
(3) Battery rated ampere-hour calculation
1 The above known conditions are brought into the formula (1) to obtain Imax ≤ 11.76A;
2 from the discharge characteristic curve of the horizontal axis delay time requirement 1h and the vertical axis discharge end voltage of 85V, find the discharge rate XC = 0.5C;
3 Bring the Imax and XC values ​​into the formula (2), and calculate the safety time of the battery pack C1=23.52Ah;
4 from the temperature characteristic curve of the horizontal axis minimum temperature requirement -10 ° C and discharge rate 0.5 C on the vertical axis to find the available battery capacity percentage Y = 50%;
5 taking the battery pack's safety time C1 and the available battery capacity percentage Y value into equation (3), calculating the final battery pack's safety time C2 = 47.04Ah;
6 In the above known conditions and usage requirements, the rated number of hours of the battery pack should be chosen to be 50 Ah.
Since the characteristics of the battery vary from manufacturer to manufacturer, the specific selection must be made in accordance with the instructions provided by the manufacturer.
3 correct use of the battery
3.1 connection
Different capacities, different performances, different old and new, different manufacturers of batteries should not be used together.
Insulating tools should be used when connecting to prevent accidental short circuit between positive and negative.
When the battery is connected to the charger or load, the circuit switch should be in the off position, the positive pole of the battery should be connected to the positive pole of the charger or load, and the negative pole of the battery should be connected to the negative pole of the charger or load.
The nuts, bolts, washers and connecting wires for coupling should be loose and moderate, and the screws should be loose and too tight.
3.2 Charging
Charging is divided into initial charging, normal charging, and balanced charging.
Initial charging. The first charge of the new battery is called initial charge, and the purpose is to reduce the active material of the plate that is oxidized during the assembly process, increase the active material content, and improve the discharge performance of the battery.
Normal charging. Charging a battery that has been discharged is called normal charging.
Floating charge. The battery pack is connected in parallel with the power supply to the load. When the AC power supply is normal, it rectifies the AC power to DC power, and then charges the battery while re-converting the DC power to AC power to supply the load. When the AC power supply is interrupted, the DC power of the battery is immediately converted into AC power by the inverter to supply power to the load to ensure the continuity of the power supply. This type of battery charging is called floating charging.
Balanced charging. In the process of using the battery, imbalances such as specific gravity, capacity, and voltage are often generated. The output voltage of the battery pack is too low and the output power is too small. To this end, the battery pack is overcharged so that each of the battery cells in the battery pack is fully charged. This charging process is called equalization charging.
When the battery pack float voltage is low or the battery needs to be recharged after being discharged, or the battery pack capacity is insufficient, the battery pack needs to be balancedly charged (referred to as equal charge). The appropriate average charge voltage and equal charge frequency are to ensure the long battery life. Foundation. It is not recommended to charge the VRLA battery at all times, and the charging voltage is related to the ambient temperature. After the battery is discharged, especially after deep discharge, whether it is using floating charge voltage or using uniform charging pressure, attention should be paid to current limiting to prevent the charging current from being too large and causing accidents.
Due to the use of floating charge and unattended, the charger that requires VRLA battery has the following functions: automatic steady current, constant voltage current limit, high temperature alarm, ripple factor not more than 5%, fault alarm, floating charge / equal charge automatic conversion . It is worth noting that the peak value of the floating charge voltage under different ripple coefficients, when the charging voltage of the battery at 25 ° C exceeds 2.40 V / only, the water of the battery will be decomposed, and the floating charging voltage does not match the ripple coefficient of the charger. This leads to accelerated corrosion of the battery and an increase in water loss, which causes the battery to fail prematurely.
3.3 floating charge operation
In the power system, the battery is always on-line, so that the battery is basically in a long-term floating state, and the selection of the float voltage plays a crucial role in the long-term reliable operation of the battery. As seen earlier, the high float voltage will cause the battery to lose water slowly and cause thermal runaway to cause the battery to fail. The low float voltage will cause the battery to be fully charged for a long time, causing the battery to sulfate. This causes the battery to fail. The correct float voltage should generally be selected from 2.23V-2.25V/cell, and should be adjusted accordingly with the battery operating temperature. Due to the different battery manufacturers, there will be some differences in this parameter, which should be selected in strict accordance with the parameters provided by the manufacturer. Figure 1 is a plot of the battery float voltage versus temperature for a manufacturer.
Figure 1 relationship between float charge voltage and temperature
The choice of VRLA battery float voltage is a question worth exploring. The float voltage directly affects the service life and reliability of the battery. The float voltage is set during battery installation. Many users do not adjust according to temperature changes during use. Therefore, it is especially important to choose a suitable float voltage. The float voltages set by different VRLA battery manufacturers range from 2.23V to 2.35V/, and what is the right value? The following discussion is based on both theoretical and practical experience.
The float voltage is a charge voltage set to supplement the self-discharge of the battery. The selection principle is to make the anode gate alloy anodization potential at the potential region where the corrosion current is the smallest. In the relationship between the anodic oxidation potential and the oxidation current density of lead, different positive grid alloys have different anodic oxidation currents, and the floating charge voltages are different. For the flooded battery, the positive grid is generally made of Pb-Sb alloy, and the battery float voltage is higher than the open circuit voltage by l00Mv. For example, the acid-proof battery open circuit voltage is 2.05V-2.07V, and the float voltage is 2.15V-2.17V. For VRLA batteries, the float voltage selection value is different due to different alloys, and the Pb-Sb alloy series battery floats. The pressure is 2.23V-2.27V/only, and the floating charge voltage of the Pb-Ca alloy series battery is 2.23V-2.35V/only. The initial VRLA battery float voltage is relatively high, both users and manufacturers believe that higher float voltages cause battery corrosion and water loss, causing early battery capacity failure. Therefore, after years of use, the low float voltage of VRLA batteries is considered to be one of the ways to prevent early failure of VRLA batteries. Relevant experts and production plant technicians believe that the VRLA battery float value is lower, preferring the battery to undercharge, but also to prevent overcharge.
In a battery pack, the batteries are always charged in series. Since there are always individual differences in the batteries, the terminal voltage of each battery is not strictly consistent. In order to ensure the long-term safe operation of each battery in the battery pack, it must be ensured that the floating terminal voltage of each battery in the battery pack is in the correct range. Balanced charging is a commonly used method to ensure the battery pack by appropriate over-charging. The battery is fully charged. This method should limit the use of the battery pack because it should be overcharged. Instead of equalizing the charging, a single battery should be used. If the battery pack must be balanced, the equalized charging voltage must be strictly controlled. The voltage of the balanced charging should be selected in strict accordance with the regulations of the battery manufacturer.
3.4 discharge
The discharge current should not be too large, and it is necessary to avoid short-circuit discharge.
When discharging, the battery terminal voltage should not be lower than the termination voltage, in case the battery is over-discharged, resulting in a decrease in battery performance and a shortened life.
After discharge, it should be charged in time. The battery is not allowed to stand for a long time in the discharge state.
3.5 storage
The actual capacity that the battery can release is directly related to the temperature. The lower the temperature, the smaller the actual capacity that the battery can release, that is, the lower the efficiency of the battery. For temperature characteristics, please refer to the battery temperature characteristics provided by the battery manufacturer.
The longer the storage time of the battery, the smaller the capacity available for practical use. The higher the storage temperature, the faster the capacity is reduced. To this end, batteries that have not been used for a long time should be periodically charged to facilitate battery use and extend life. For storage characteristics, please refer to the battery storage characteristics provided by the battery manufacturer.
The battery should be stored in a clean, well-ventilated warehouse with suitable ambient temperature; keep away from heat and avoid sunlight. The battery should be charged regularly. When the UPS power supply is not used for a long time, it should be charged and charged periodically.
4 routine maintenance
(1) Check the float voltage, battery case or pole temperature of each battery cell every quarter. If the float voltage is raised or the temperature is too high, it should be handled according to the instructions or submitted to the manufacturer.
(2) Capacity discharge shall be carried out once a year or every two years. If the capacity is insufficient, it shall be promptly submitted to the manufacturer and disposed of.
(3) It is not recommended to charge evenly. After the battery is discharged or after an accident, the management personnel should go to the battery room in time to balance the battery and check the charging current of the charger to prevent the charging current from being too large.
(4) Check the pole connection bolts loosely every six months or frequently, clean the dust on the battery, especially the dust on the pole and connecting rod, prevent the battery from leaking or grounding, and observe whether the appearance of the battery is abnormal or not. Handle in time.
5 summary
The use of UPS will become more and more extensive. The correct selection and maintenance of UPS battery is directly related to the stability of UPS system operation, so we must pay great attention to it.
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