Significance of battery internal resistance monitoring

1 Introduction

Because VRLA's operational requirements are relatively strict, running under deviation from the correct conditions of use will have serious consequences. Therefore, it is very important to monitor the operating parameters of VRLA.

Locations where spare batteries are used are all very important departments, and failed battery packs do not provide power backup. Once the main power supply fails, the system will be shut down, resulting in huge economic and social losses. It is also very important to detect and deal with battery failures in a timely manner.

As we all know, the VRLA's terminal voltage does not reflect the battery's capacity characteristics, and the battery with severely degraded capacity has almost no difference in the float charge voltage of the entire group of float-charged batteries. Once the battery pack is discharged, these batteries will drop quickly due to the small amount of charge, and they will impede the discharge performance of the battery pack. At this time, it is easy to find them from the terminal voltage of the battery, but it is already too late, and the battery pack has no backup effect when it needs backup power.

2 Importance of internal resistance monitoring in lead-acid batteries

Measuring the internal resistance of a battery using the AC impedance method, the conductance method, or the direct current method has been recognized as a rapid and convenient method for diagnosing the health of a battery. More and more literatures believe that there is a certain relationship between the internal resistance of the aging battery and the discharge capacity.

(1) Basic resistance model

The impedance of the new VRLA product is basically the same as the sum of the ohmic resistance of each component. For example, for a 12V/100A VRLA, the ohmic resistance is composed of the following components:

Omits 40% of grid ohmic resistance with lead paste

32% of ohmic resistors in connection bars, terminals and solder joints

Terminal ohmic resistors account for 12%

Solder ohmic resistors account for 7%

Electrolyte and separator ohmic resistors account for 16%

This number varies with the battery manufacturer, battery model, and capacity.

For simplicity, capacitance and inductance are negligible. It is worth noting that the internal resistance of the battery rapidly increases with the temperature drop. This is mainly due to changes in the electrolyte resistance. Therefore, when considering the influence of time on internal resistance, temperature is an important factor.

Another item of resistance that depends on the chemical reaction kinetics of the plate is called the "charge transfer resistor," which can be measured based on the voltage drop and the current during discharge. Therefore, the total battery resistance is the sum of the ohmic resistance and the "charge transfer resistance." The charge transfer resistance depends on the discharge current, the temperature, the specific area of ​​the paste, and the sulfuric acid composition. At a discharge rate of 15 minutes, the ohmic resistors accounted for 40% of the total resistance, while the charge transfer resistors accounted for 60%; at a discharge rate of 8 hours, the charge transfer resistance accounted for only 5%.

[next] (2) The significance of real-time monitoring of internal resistance of lead-acid batteries

In the discharge process, the initial voltage drop follows Ohm's law V = IR, I is the discharge current, and R is the total internal resistance of the battery. The larger the initial voltage drop, the closer the voltage is to the final termination voltage, thus reducing the battery life. As the discharge process progresses, the three active substances (sulfuric acid, positive and negative pastes) begin to undergo electrochemical transformation. A decrease in paste utilization and electrolyte leakage will inhibit the discharge reaction, which will cause the battery voltage to drop faster.

VRLA is a poorly designed acid. Compared to pasting pastes, the electrolyte has a smaller ampere-hour capacity and the discharge process is often constrained by the electrolyte. If the resistance value is proportional to the utilization of the active material or the available electrolyte, the relationship with the discharge capacity can be improved.

For any new battery, R does not usually have a linear relationship with the discharge capacity. The saturation of the electrolyte, the degree of complete formation (especially on the surface of the plate), the separator--contact area of ​​the plate interface, and the slight pressure variations all have only a slight effect on the resistance, but may have a large effect on the discharge process. .

A slight increase in the initial electrolyte volume will only cause a slight decrease in the total battery resistance R. However, due to the lack of acid, a small increase in the volume of the electrolyte will lead to an increase in the discharge time, and there will be a difference between the batteries in the 12V battery pack. Measurements of resistance and open circuit voltage can be used to find out those that are not qualified: their voltage drops too quickly beyond the normal range. The main drawbacks of these defective products are generally poor tip connection, too little electrolyte volume, air leakage or short circuit. These undesigned defects can easily be measured by resistance and open-circuit voltage methods during battery use. Many battery manufacturers use the open circuit voltage method and discharge load method to make final quality checks on battery products. Users can also use this method to inspect battery products during their reception, installation, and use.

All VRLAs have a certain lifetime, which is due to the corrosion of the positive grid, especially during float charging. Increasing the mass of the positive grid or reducing its corrosion rate can extend the life of the battery. The positive grid is the conductive and supporting skeleton of the positive paste. Corrosion not only increases the resistance of the positive grid, but also makes the grid thicker, thus losing electrical contact with the paste. The negative grid will not be corroded. Other design parameters, such as the volume of the electrolyte, the degree of compression of the separator, and the composition of the components, the air permeability of the battery case, the vent design, the physicochemical parameters of the paste, and the manufacturing parameters can all affect the lifetime.

With the corrosion of the positive grid and the depletion of the electrolyte in the separator, the battery resistance increases and the battery capacity decreases. Both of these conditions can cause a drop in the initial voltage and a decrease in available active material. Periodic R measurements can track and monitor these changes and find defective products.

The relationship between battery capacity and lifetime is similar to the curve of voltage vs. discharge time. At first, the curve is relatively flat, but then it rapidly decreases with time.

In an uninterruptible power supply, battery capacity is likely to fall below 80% of rated capacity during both tests due to fewer battery inspections and discharges. If internal resistance testing is used, these problems can be easily found and system reliability can be improved.

Experiments have shown that as the porosity of the paste increases, the volume of electrolyte contained in the plate increases, resulting in a decrease in electrolyte in the separator. In the early stage of use, the porosity increases as the positive and negative plates respectively change to lead dioxide and spongy lead, during which the sulfuric acid redistributes. This has little effect on the resistance, but increases the paste utilization and battery capacity. With the aging of the battery, the positive electrode grid continues to erode and the effective porosity of the positive electrode plate also increases, and the total volume of the electrolyte in the battery gradually decreases. However, due to redistribution of the electrolyte into the positive plate, the loss of electrolyte in the separator is much faster. The separators and electrolyte resistances show a (2-3) power e with decreasing electrolyte saturation, but their resistance only accounts for 5%-10% of the total new battery resistance at a discharge rate of 15 minutes (16%) The ohmic resistance is multiplied by the ohmic resistance ratio of 40% of the total resistance.

The [next] test results show that at the 15 minute discharge rate, the initial discharge voltage of the new battery is slightly lower than the battery with the end of the life cycle by 20mV-50mV. As the water is electrolyzed during floatation use or consumed by grid corrosion, the remaining electrolyte becomes more concentrated. Therefore, the open circuit voltage also increases. Although the battery internal resistance R and the total voltage drop may increase, this can be partially offset by the increase in the open circuit voltage.

As the battery ages, their voltage-time curves show a similar initial voltage value, but the slope of the curve increases with the discharge time. The constant decline in the voltage-time curve does not coincide with the reduction of electrolytes and the use of active substances. The large amount of electrolyte loss can seriously affect the battery capacity, and is in good agreement with the increased internal resistance R of the acid-lean valve-regulated lead-acid battery products.

The reading of internal resistance R is not sensitive to the possible reaction at the initial stage due to the growth of the paste due to grid growth, or the reaction to the deterioration of the degree of bonding between the particles in the paste due to the utilization or recycling of the balance of the active material. Not sensitive. The paste may initially have sufficient electrical contact with the grid, but as the discharge process proceeds, the degree of bonding may deteriorate, thereby reducing the utilization of the paste.

The sensitivity of the internal resistance R to the pasting performance may also be related to the internal resistance R and the inconsistency of the battery capacity.

Some theories point out that the failure of some battery components may be related to the AC frequency. However, most of the internal resistances R are relatively flat in the frequency range of 8 Hz to 1000 Hz.

At present, we have not yet found a clear difference between impedance or admittance and battery life.

The decrease in the conductance of the positive grid due to corrosion can be reflected by the change in internal resistance R. The contribution of the gate resistance and its variation during use largely depend on the discharge rate, grid design, formation, and manufacturing methods. It is not surprising that the gate resistance increases by 5%-30% at the end of its useful life. The high-speed discharge is more sensitive to the increase of the internal resistance of the grid, and the change of the internal resistance of the top lead pole has little effect on the products with good performance.

The total internal resistance of the battery is the sum of the charge transfer resistance and the ohmic resistance of the component. Due to the inconsistency of the performance of some components, the value of the initial internal resistance R shows a certain distribution between ±20%.

As the resistor ages, the change in the internal resistance of a component may be obscured by changes in other components. When the change in internal resistance is large enough and is related to the reduction of electrolytes and the use of active substances, the corresponding relationship between the internal resistance and the battery capacity is obvious. Increased internal resistance of battery components due to plate corrosion and growth, electrolyte loss, or redistribution is accompanied by a similarly flat exponential curve.

The loss of battery capacity is similar to this. The increase in internal resistance is related to the decrease in battery capacity, especially when the battery life does not reach 80%. The use time at high discharge rate seems to be more sensitive to these factors, and generally the battery life is reached when the internal resistance increases by 20%-25%. At low discharge rates, the battery's internal resistance generally increases by 20% to 35% before its end of life.

3 Conclusion

Some articles think that the remaining capacity of VRLA can't be reflected by the internal resistance of the battery. They think that the internal resistance of the battery corresponding to a 20% drop in battery capacity is not obvious. However, one thing is generally acknowledged, that is, the increase in internal resistance of the battery corresponds to a decrease in the battery capacity. When the internal resistance of the battery can be clearly confirmed, the battery should retain 60% or more of the capacity. Such a battery cannot pass the battery. Floating-voltage measurement found. So the real-time monitoring of VRLA internal resistance is revolutionary than the terminal voltage.