Maintenance of Valve Regulated Lead Acid Battery
VRLAB is provided with the following advantages:
① It can be placed horizontally, piled and placed with communication equipment to save space;
② The design of barren liquor omits the specific gravity measurement in the maintenance and is applicable for large-current discharging;
③ In the normal case, hydrogen and oxygen will be compounded well and no hydrogen will be generated;
④ No acidic gas will be produced during charging to pollute the environment, etc.
When VRLAB was first used at the beginning of 1990, it was called as “maintenance-free battery”, which in fact is misleading. Moreover, as the quality of early VRLAB was not good, different faults would always happen during its use. Certainly, as the computer technology develops, the LSI (large scale integration) elements emerges continuously, and the technologies of switching power supply and UPS etc are improved continuously, much progress has been made for the power system equipment, the power supply safety and reliability etc has been promoted greatly, and the workload for power system equipment maintenance has been reduced greatly as well. Nevertheless, the VRLAB supporting the power system equipment is still haunted with some problems at times. Thus, maintenance of VRLAB is vital either in the DC power supply system or the AC/DC UPS system.
So, how can VRLAB be well maintained under the present conditions? I’d like to express my opinion about the problem here.
Firstly, the quality of VRLAB operation should be analyzed. It depends on three aspects, namely product quality, installation quality and maintenance quality, which are all quite important. Especially, the product quality is critical to maintain the good operation quality of VRLAB. It is related to all procedures in the battery production course, i.e. every working procedure form lead powder production to assembly. Thus, all aspects and procedures should be controlled strictly, including thickness & weight of grid, prescription of lead paste, solidifying effect, air permeability of grid, technical design of safety valve, filling manner of electrolyte, control of electrolyte feeding amount, formation manner, shell material, shell cover, pole terminal, and sealing between the shell cover and the shell, etc.
Installation quality as well as storage, installation and capacity test, etc, will all directly affect the operation and maintenance of VRLAB. Thus, ensure that no collision occurs during the transport and storage and the intercell connector fits the pole terminal during installation. Have the uneven pole terminal leveled. When the pole terminal is fastened, the force exerted cannot be too large or too small. If it is too large, the cooper sheathings inside the pole terminal will be loosened; if it is too small, the intercell connector will not contact well with the pole terminal. Thus, at installation, it is better that the wrench capable of over-force tripping provided by the manufacturer or the wrench consistent with the reference force provided by the manufacturer is used. In addition, attention should also be paid to the following aspects during installation: 1. try to make the length of anodes or cathodes for all batteries between the storage battery and the DC cabinet be the same to ensure the operation balance between batteries during the large-current discharging; 2. Ensure the positive manifold and the negative manifold of battery are connected firmly and reliably; 3. remember to charge the battery additionally after installation.
Maintenance quality is also an important aspect to ensure normal operation of VRLAB. The high maintenance quality enables VRLAB to exert the maximum efficiency and extend the service life. Thus, the electric maintenance personnel should not conduct the maintenance work until they fully understand all requirements in the instruction manual of VRLAB. In addition, the following relationship and issues should be figured out during the maintenance work:
(1) Relationship between Temperature and Capacity
At 25℃, capacity of VRLAB is 100%; the relationship between temperature and capacity is showed in charts of the specification manual.
It can be known easily from Table 1 that capacity of VRLAB changes with the temperature. The maintenance personnel should adjust the discharging current of battery properly according to the actual temperature change, and control the battery temperature between 22℃～25℃.
(2) Relationship between Charging & Discharging and Life & Capacity
① Relationship between charging and life
An exact charging system should be established and implemented for maintenance of VRLAB to ensure the battery reaches the optimized performance and the longest life. According to a large number of researches both at home and abroad, the charging manner determines the service life of battery. Some batteries are damaged by inappropriate charging rather than use. Regarding this, many battery manufacturers and scientific research institutes or colleges in China have conducted the similar experiment. For example, an agency has prepared two groups of batteries for experiment. For one group, the full-capacity life test with the ordinary constant-voltage limiting -current manner was conducted; for the other group, the charging capacity was controlled with the phased constant-current charging manner and the capacity & cycle life test was conducted with the short-time medium current impulse manner after charging. As a result, greatly different cycle lives were got due to different charging manners. The battery tested with the phased constant-current charging manner had the longer cycle life. It can be seen that the constant-voltage limiting-current charging manner widely applied has the large defect in the later phase of charging. As the rectifying equipment currently used, especially the switching power supply, has not the characteristic of constant current, there is still certain difficulty with the second charging manner. Further research should be made on the problem. In addition, some research agencies have been studying the pulse charging manner for VRLAB charging recently. Mainly, the pulse charging is divided into one or several phases and each phase has several pulse periods. The electrolyte is cooled down, etc. It is said that it is an ideal way to eliminate vitriolization.
② Relationship between discharging and capacity
As we all know, discharging currents with the different multiplying power make the battery have different capacities as showed in the specification table.
Capacities of batteries installed in the DC power supply system of communication power are different as well. The capacity of battery with the actual discharging current should be calculated exactly. It should be noted that it is quite difficult for redox of lead sulfate formed under the condition of small-current discharging, as the size of lead sulfate granules formed under such condition are far larger than that formed under the condition of large-current discharging, i.e. the crystal formation speed under the condition of large-current discharging is lower than that under the condition of small-current discharging, and the crystal has been oxidized and reduced before it grows, so the granules are smaller. Under the condition of small-current discharging, the larger lead sulfate crystal cannot be oxidized and reduced easily. If the lead sulfate crystal is not cleared for a long time, capacity and life of battery will surely be affected.
(3) Influence of Unbalance on VRLAB
According to relevant research results, alloy components and structures at different parts of grid are distributed differently, so the electrochemistry unbalance of grid will be caused and produce difference of voltages at the state of floating discharge and charging/discharging. Moreover, as charging and discharging circulate, the difference will become larger to form the so-called “failed battery (failure of battery)”. According to the existing standard in China, the maximum floating charge voltage difference for a group of batteries shall not be over 50mV. However, it cannot be over 20mV according to the standard of developed countries.
(4) Phenomenon of Thermal Runaway
Due to the barren liquor design for VRLAB, all electrolyte filled in the battery are absorbed on the glass fiber baffle. When the charging current increases, the safety valve should be used to release gas, so desiccation, internal resistance increase and capacity attenuation will be caused to the battery and large amount of heat will be produced during charging and discharging. The heat fails to radiate timely and makes the temperature rise sharply, so thermal runaway is caused.
Causes to thermal runaway also include the following: the floating voltage is not reduced timely; the safety valve is not tight; the valve-opening pressure is too low, etc. In the case of thermal runaway, discharging may make the transient voltage of battery drop suddenly and the battery shell temperature rise to 70℃~80℃, which will lead to shell distortion. Much attention should be paid to thermal runaway.
Through the above analysis, we have a certain understanding of VRLAB maintenance. The following requirements should be met for maintenance of VRLAB:
1. As conditions permit, the storage battery room should be provided with air conditioning equipment, and the temperature should be controlled between 22℃~25℃, which will not only extends the battery life but also enables the battery have the best capacity.
2. In any case, the floating charge voltage of battery should not be over the value specified by the manufacturer. In addition, the floating charge voltage should be adjusted with the voltage adjustment factor ±3mV/℃ according to the environment temperature change.
3. In view of influence of unbalance on the VRLAB, the lower limit value of floating charge voltage should be used for floating charge.
4. When the battery unbalance is large or the battery is discharged greatly, and when the battery has operated for a quarter, it should be charged additionally with the balanced manner. During the balanced charging, pay attention to variation of environment temperature, and reduce the value of balanced voltage as the environment temperature rises. For example, if the environment temperature rises by 1℃, the voltage value should be reduced by 3mV.
5. Try charging the “failed battery” with the manner of pulse charging to recover the battery.
6. The battery should be maintained carefully. Data of voltage and internal resistance for each single battery should be kept down and filed as the original data before VRLAB is put into operation. After ever half year of operation, compare the operation data with the original data. Settle the abnormal cases found timely.
7. When the VRLAB operation time reaches 1/2 of its life, the testing frequency should be increased appropriately. Especially testing frequency of the 12V single battery should be increased. If the internal resistance of battery increases suddenly or the voltage values (especially the two digits after the decimal point) are found unstable and vary all the time when measured, the battery should be treated as the “failed battery”.
8. When conditions permit, it is suggested that the 2V single VRLAB for UPS above 40Kvac be used.
9. Inspect the safety valve of VRLAB regularly and observe carefully whether there is injection stain around the safety valve to ensure the safety valve is fastened or damaged.
10. During the type selection and purchase of battery, the production crafts, manufacture flow, and quality control means of manufacturer as well as technical characteristics etc should be well understood. When necessary, it can be required that the first capacity combination be carried out in the factory to select batteries with great difference.
In short, during the daily maintenance, emphasis should be laid on maintenance of VRLAB. Different maintenance requirements should be proposed based on different features and characteristics of batteries. Moreover, the maintenance operation level should be promoted through experience and knowledge accumulation to make VRLAB fully exert its function and reach the expected purpose.