VRLA Batteries – What are the special precautions when charging?

What are the requirements and precautions for VRLA battery charging

Because VRLA battery has the advantages of low price, stable voltage and no pollution, it has been widely used in communication, electric power, transportation, photovoltaic power generation and other fields in recent years. However, recently, many users have reported that VRLA batteries that should have worked for 10 ~ 15 years are mostly damaged within 3 ~ 5 years, and some even fail after less than 1 year of use, resulting in great economic losses. Through the statistical analysis of the damaged VRLA battery, it is known that the proportion of VRLA battery life termination caused by unreasonable charge and discharge control is high, such as early capacity loss of VRLA battery, irreversible sulfation, heat out of control, electrolyte drying up and so on. Reasonable charge and discharge control of VRLA battery is the basis to make VRLA battery reach its design life.

  1. Technical requirements for charging VRLA battery

The technical index for ensuring the service life of the battery provided by the VRLA battery manufacturer is given when the ambient temperature is 25 ℃. Because the voltage of the single VRLA battery has the characteristic that the temperature decreases by about 4MV every 1 ℃ rise, the floating charge voltage of a 12V battery pack composed of six single VRLA batteries in series at 25 ℃ is 13.5V; When the ambient temperature drops to 0 ℃, the floating charge voltage shall be 14.1v; When the ambient temperature rises to 40 ℃, the floating charge voltage should be 13.14v. At the same time, VRLA battery has another characteristic. When the ambient temperature is constant and the charging voltage is 100mV higher than the required voltage, the charging current will increase several times. Therefore, it will lead to thermal runaway and overcharge damage of VRLA battery. When the charging voltage is 100mV lower than the required voltage, the VRLA battery will be undercharged and damaged. In addition, the capacity of VRLA battery is also related to the temperature. The capacity will decrease by 1% when the temperature decreases by 1 ℃, so it is required that in the use of VRLA battery, the VRLA battery should be charged in time after releasing 50% of the rated capacity in summer and 25% in winter.

Obviously, the VRLA battery in daily use cannot be in the environment of 25 ℃ for a long time. At present, the commonly used thyristor rectifier, transformer step-down rectifier and switching regulated power supply chargers can not meet the technical conditions required for VRLA battery charging by constant voltage or constant current. Looking at the methods used to charge VRLA batteries in the past and the VRLA battery chargers developed according to these methods, it is not difficult to see that their technology is not perfect. Charging VRLA batteries with these products is bound to directly affect the service life of VRLA batteries. At the same time, these chargers still have problems such as narrow working voltage adaptation range, large volume, low efficiency and poor reliability.

  1. Natural balance charger of VRLA battery
Natural balance charger of VRLA battery
Intelligent balance charger

The natural balance charging principle of VRLA battery has two power sources EA and EB. When the power source EA and EB are at the same ambient temperature, the positive electrode and the positive electrode are connected, and the negative electrode and the negative electrode are connected. In the closed circuit formed by them, there is the following relationship. If EA is higher than EB, the voltage difference between EA and EB is ea-eb= Δ E. Press Δ The size of E has- Δ I current flows from power supply EA to power supply EB, when EB absorbs the power provided by EA Δ I current, when EB rises to completely equal to EA (in VRLA battery, it shows the rise of VRLA battery terminal voltage and the increase of charge storage), the power supply EA will stop supplying current to the power supply EB, that is, EA = EB, Δ E=0, Δ i=0。

In the above description, the EB is replaced by the charged VRLA battery, and the EA is carefully designed as a power supply that can automatically adjust the output voltage and current according to the needs of VRLA battery charging balance under different ambient temperatures. In the case of complete idealization, the power EA can charge the VRLA battery according to the current that the VRLA battery can accept at any ambient temperature. After the VRLA battery is fully charged, Δ E=0, Δ If I = 0, EA power supply will no longer consume power. Thereafter, EA only provides tracking balance compensation for the charged VRLA battery with the change of ambient temperature. Because the whole charging process of VRLA battery is completely automatic, it is called natural balance method.

The VRLA battery is charged by the natural balance method. After the VRLA battery is fully charged, the voltage difference between EA and the charged VRLA battery EB Δ E = 0, natural Δ I = 0, because EA has no power to supply VRLA battery (EB), the electrolyte of VRLA battery cannot boil, nor can it decompose the water in the electrolyte of VRLA battery, let alone increase the pressure and temperature in VRLA battery, resulting in potential safety hazards. Therefore, using this method to charge VRLA battery will neither overcharge VRLA battery nor overcharge VRLA battery. It is a more convenient, safer and more reliable charging method.

From the above analysis, it is not difficult to see that this method is especially suitable for the daily maintenance and charging of VRLA battery used for intermittent discharge, which is conducive to improving the reliability of VRLA battery in daily use and the service life of VRLA battery.

  1. Charging mode of VRLA battery

1) Initial charge

The initial charging time is related to the charging rate. When the charging rate is greater than C / 5, the overcharge reaction starts before the capacity of VRLA battery recovers to 80% of the discharged capacity. Only when the charging rate is less than C / 100, the battery capacity can be restored to 100% and then the overcharge reaction can be started. When a large charging rate is adopted, in order to restore the capacity of VRLA battery to 100%, a certain overcharge must be allowed. After the overcharge reaction occurs, the voltage of single cell VRLA battery rises rapidly, reaches a certain value, the rising speed decreases, and then the voltage of VRLA battery begins to decline slowly. Therefore, after the VRLA battery is fully charged, the best way to maintain the capacity of the VRLA battery is to add a constant voltage at both ends of the VRLA battery pack. In other words, after the VRLA battery is fully charged, the charger should output a constant floating charge voltage.

2) Floating charge

(1) Floating charge voltage.

The floating charge voltage of lead-acid battery can be calculated according to the following formula:

Floating charge voltage = open circuit voltage + polarization voltage = (specific gravity of electrolyte + 0.85) + (0.10 ~ 0.18) v = (1.30 + 0.85) + (0.10 ~ 0.18) v = 2.15v + 0.10v = 2.25V (single 2V battery)

(2) Floating charge current.

The floating charge current of ordinary lead-acid battery has two functions: one is to supplement the loss of self discharge of lead-acid battery; The second is to provide current to daily loads. The floating charge current of VRLA battery has three functions: one is to supplement the loss of self discharge of VRLA battery; The second is to provide current to the daily load; Third, maintain the internal oxygen circulation of VRLA battery.

In the floating charge state, the current charged into the VRLA battery shall be able to supplement the power lost by the VRLA battery due to self discharge and maintain the oxygen circulation in the VRLA battery. The floating charge voltage should not be too high to avoid shortening the service life of VRLA battery due to serious overcharge. If the floating charge voltage is properly selected, the floating charge life of VRLA battery can reach more than 10 years. Practice has proved that when the difference between the actual floating charge voltage and the specified floating charge voltage is 5%, the service life of VRLA battery will be shortened by half.

(3) Recombination of gases.

Under normal floating charge voltage, when the current is below 0.02c, the oxygen precipitated from the positive electrode diffuses to the surface of the negative electrode, 100% is reduced at the negative electrode, there is no surplus oxygen around the negative electrode, and the hydrogen precipitated from the negative electrode is trace. If the floating charge voltage is raised or the ambient temperature is raised, the charging current rises sharply, and the gas recombination efficiency decreases with the increase of charging current. At 0.05c, the recombination rate is greater than 90%. When the current is at 0.5c, the gas recombination efficiency is approximately zero, as shown in Figure 2-5. At this time, there are a lot of oxygen accumulated on the negative electrode and hydrogen precipitated on the negative electrode surface, the internal pressure of VRLA battery rises sharply, and the safety valve is opened, resulting in serious water shortage of VRLA battery.

When the VRLA battery is charged, its internal gas recombination itself is an exothermic reaction, which increases the temperature of the VRLA battery, the floating charge current and the gas evolution, which makes the temperature of the VRLA battery rise higher. The VRLA battery itself is a “lean liquid” design, with tight assembly and difficult internal heat dissipation. If the heat is not removed in time, it will cause the heat out of control. At the end of floating charge, if the voltage is too high and the ambient temperature around VRLA battery rises, the thermal runaway of VRLA battery will be aggravated.

The relationship between voltage and temperature of VRLA battery is that when the temperature increases by 1 ℃, the voltage of single cell VRLA battery will decrease by about 3MV / monomer. In other words, the voltage of VRLA battery has a negative temperature coefficient, which is – 3MV / ℃. It can be seen from this that the ideal charger works when the ambient temperature is 25 ℃. When the ambient temperature drops to 0 ℃, the VRLA battery will not be fully charged. When the ambient temperature rises to 50 ℃, the VRLA battery will be overcharged, and the service life of VRLA battery will be shortened due to serious overcharge. When the temperature is lower than – 40 ℃, VRLA battery can work normally, but the capacity of VRLA battery will be reduced. Therefore, in order to ensure that the VRLA battery can be fully charged in a wide temperature range, the output voltage of the charger must change with the voltage temperature coefficient of the VRLA battery.

VRLA battery has high requirements for temperature. Therefore, temperature compensation measures should be considered when designing charger, but the selection of temperature sampling point is very important, which is directly related to the effect of compensation. There are three temperature sampling points, namely, the air temperature near the VRLA battery, the surface temperature of the VRLA battery shell and the electrolyte temperature inside the VRLA battery. The first is the easiest. At present, this method is basically adopted, but this method is very inaccurate, because the temperature of VRLA battery rises for some reason, but the rise of VRLA battery temperature is difficult to cause the rise of air temperature near VRLA battery, so this compensation measure is basically useless; The third part can best reflect the actual situation of VRLA battery, but it is difficult to realize; The second part is the most practical and easy to implement. At present, enterprises have designed temperature compensation units according to the sampling of the second part.

3) Equalizing charge

The so-called balanced charging is to connect each single VRLA battery in parallel and charge it with a unified charging voltage. If the VRLA battery pack has a lagged VRLA battery in the float charging process (the monomer voltage is less than 2.20V, relative to the 2V battery), or after floating charge for three months, the VRLA battery should be charged evenly at once. During the equalization charging process, the VRLA VRLA battery voltage will be controlled at 2.35V, charging 6~8h (note that the time of one equalization charge should not be too long), and then return to the floating charging voltage. Then observe the voltage change of the backward VRLA battery. If the voltage is not in place, charge it evenly after two weeks. Generally, the voltage of the new VRLA battery pack will tend to be the same after 6 months of floating charge and even charge. The equalizing charging current is generally 0.3C or slightly less than 0.3C. For VRLA batteries with rated voltage of 12V, the equalizing charging voltage is generally 14.5V.

When equalizing the VRLA battery according to the regulations, in addition to the charging voltage, the setting of equalizing charging time is also very important. In order to prolong the service life of VRLA battery, the equalizing charging time must be accurately set according to the equalizing charging voltage and current. In other words, in the process of equalizing charging, when the charging current remains unchanged for three consecutive hours, it must be immediately transferred to the floating charging state, otherwise, it will be seriously overcharged and affect the service life of VRLA battery.

4) Cyclic charging

In the field of cycle application, VRLA batteries adopt thin plate design to improve specific energy and high current performance. For VRLA battery with thin plate design, the best charging methods are pulse mode and current decreasing mode. The pulse mode can increase the charging current in a short time, fill quickly, and has little overcharge; The current decreasing charging method has the same advantages. The key to high current fast charging is the composite process. Provide sufficient current and control this process. When VRLA battery is aging, the composite efficiency becomes more and more intense, but the electrode plate is thin, the surface area is large, the electrode plate spacing is small and the charging efficiency is high.

5) Charging current limiting

After the VRLA battery is discharged, the initial charging current is too large, and the generated heat may fuse the grid vertical bars, bus bars, terminals, etc.: it will relax, soften and fall off the combination between the active material PbO2 particles of the positive plate. In serious cases, it will lead to out of control heating, deformation, cracking and failure of the VRLA battery. Therefore, the charging current value needs to be limited. Charging current limiting setting methods include:

(1) Turn off current limiting and turn off several chargers when current limiting is required

(2) There is a level setting to limit the output current of the charger, which can be selected in gear 1 / 3 or gear 2 / 3 of the rated current.

(3) Local stepless setting, current limiting point can be selected at 50% ~ 100% of the rated current of the charger.

(4) Stepless setting, the current limiting point can be selected in the 0 ~ 100% section of the rated current of the charger. The advanced technology of several current limiting methods is: 4.

6) Charging operation

After the VRLA battery pack is discharged, it should be charged immediately and start to control the charging current, which should not be greater than 0.2C (for example, for 200ah VRLA battery, the charging current should not be greater than 0.2 x 200 = 40a). When the current becomes small, the charging voltage of VRLA battery pack can be slowly increased to reach the average charging voltage value, charged for another 6h, and then adjusted back to the floating charging voltage value. The initial charging current of VRLA battery is generally set according to the value specified in the manual or 1 / 10 of the rated capacity.

The ideal charging current is the phased constant current charging mode, that is, a larger current is used at the initial stage of charging, which is changed to a smaller current after charging for a certain time, and a smaller current is used at the end of charging. The setting of charging current is generally 0.1C. When the charging current exceeds 0.3C, it can be considered as overcurrent charging. Avoid charging with ordinary fast charger, otherwise the VRLA battery will be in the state of “instantaneous overcurrent charging” and “instantaneous overvoltage charging”, resulting in the decrease of available power of VRLA battery and even damage to VRLA battery. Over current charging will lead to the bending of VRLA battery plate and the falling off of active substances, which will reduce the power supply capacity of VRLA battery and damage VRLA battery in serious cases.

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