Maintenance and monitoring of valve-regulated lead-acid batteries VRLA

Abstract: This paper analyzes the many factors that affect the performance of VRLA batteries during operation, compares the technologies of manual battery detection and online monitoring, and proposes a solution for battery maintenance based on online monitoring of internal resistance.

Preface: With the rapid development of my country's communications, power, UPS and other industries, the use of maintenance-free batteries is also increasing rapidly. The quality of their performance is particularly important to ensure the normal operation of backup DC power supplies, but at the same time, various problems are gradually emerging:

• The service life is shorter than expected;
• The failure of individual batteries leads to the failure of the entire battery group;
• It is difficult to ensure timely detection of sudden battery failures;
• The risk of battery discharge testing is very high;
• Due to the limitations of on-site conditions, it is difficult to carry out manual inspections, and the analysis of test data requires the operation and maintenance personnel to have a high level of professionalism;
• The daily inspection costs of unmanned stations are very high;
• There is a lack of scientific and effective monitoring and management methods, and accurate judgments cannot be made in a timely manner on the reasonable use of batteries;
• Power equipment with "battery management function" does not really play the role of battery managers.

Relevant data show that after 3-4 years of use, most batteries are difficult to pass capacity tests, and only a few can exceed 6 years. In actual use, only a few users regularly check batteries and conduct regular capacity tests on batteries. In many cases, it is discovered that the battery discharge capacity does not meet the design requirements after a power outage. Some battery packs even continue to "work" when the capacity does not reach 50% of the rated capacity.

This shows that battery users urgently need to be able to monitor battery performance online in real time, and battery online monitoring equipment is of great significance to battery management.

I. Factors affecting battery performance
1. Technical issues affecting battery quality
1) Battery composition
VRLA battery consists of positive plate, negative plate, AGM diaphragm, positive and negative bus bars, electrolyte, safety valve, cover and shell. Among them, the thickness of the positive plate grid, alloy composition, uniformity of AGM diaphragm thickness, bus bar alloy, electrolyte volume, safety valve opening and closing pressure, shell cover material, battery production process, etc. have an important impact on battery life and capacity uniformity.

2) Grid alloy
VRLA battery negative grid alloy is generally Pb-Ca series alloy, positive grid alloys include Pb-Ca series, Pb-Sb (low) series and pure Pb, among which Pb-Ca and Pb-Sb (low) alloy positive grid batteries have similar floating charge life, but the cycle life varies greatly. For areas with frequent power outages, low antimony alloy batteries are used for good reliability.

3) Grid thickness
The thickness of the positive grid of the plate determines the design life of the battery.

4) Safety valve
The safety valve is a key component of the battery, with acid filtering, explosion-proof and one-way opening functions. YD/T7991 996 stipulates that the safe opening and closing pressure range is 1-49kPa. However, for long-life batteries, one-way sealing must be considered to prevent air from entering the battery and prevent internal water vapor from escaping at higher temperatures.

5) AGM diaphragm
The porosity and thickness uniformity of the diaphragm directly affect the acid absorption saturation and assembly compression ratio of the diaphragm, thereby affecting the battery life and capacity uniformity.

6) Shell cover materials
VRLA battery shell cover materials include PP, ABS and PVC, and PP material is relatively better.

7) Acid quantity and formation process
It is divided into battery formation and tank formation. Battery formation can quantitatively inject acid and record the data of the whole formation process of each battery cell, which can accurately judge the comprehensive production quality of each battery shipped, but the formation time is longer. Tank formation is the formation of the plate, the formation time is short, and the plate formation is more complete, but the quality of battery assembly cannot be judged by the formation process data record.

8) Plate coating process
The plate coating process must ensure the uniformity of the plate thickness and the active material of each plate.

9) Sealing technology
VRLA battery sealing technology includes pole sealing, shell cover material permeability, shell cover sealing and safety valve sealing.

10) Oxygen recombination efficiency
AGM batteries have good oxygen recombination efficiency. Under lean liquid conditions, the oxygen recombination efficiency is generally greater than 98% as tested according to relevant standards. Therefore, they have good maintenance-free performance.

2. Environmental factors that affect battery life
1) Ambient temperature
The normal operating temperature of the battery is 20-40°C, and the optimal operating temperature is 25°C. When the temperature rises by 5°C, the battery life decreases by 10%, and thermal runaway is more likely to occur.

2) Ambient humidity
The operating humidity of the battery should be between 5% and 95% (without condensation). If the ambient humidity is too high, condensation will form on the surface of the battery, which may cause a short circuit. If the ambient humidity is too low, static electricity may be generated.

3) Dust
: Too much dust can easily cause the battery to short-circuit and the safety valve to become blocked and ineffective.

3. Battery failure mode
1) Battery water loss
There are conditions for valve-regulated lead-acid batteries not to release gas, namely: no gas should be released during storage; no gas should be released when the charging voltage is below 2.35V/cell (25°C); and no gas should be released during discharge. However, when the charging voltage exceeds 2.35V/cell, gas may be released. At this time, a large amount of gas is generated in the battery in a short period of time and cannot be absorbed by the negative electrode in time. When the pressure exceeds a certain value, it begins to exhaust through the one-way exhaust valve. Although the exhausted gas is filtered out of acid mist by the filter pad, the battery loses gas (that is, water loss) after all, so valve-regulated sealed lead-acid batteries cannot be overcharged.

2) Sulfation
of negative plates When the valve-regulated sealed lead-acid battery is undercharged, PbSO4 will be present on the positive and negative grid plates of the battery. This phenomenon is called sulfation of active substances. Sulfation reduces the active substances of the battery, reduces the effective capacity of the battery, and also affects the gas absorption capacity of the battery, which will cause the battery to fail over time.

3) Positive plate corrosion
Due to water loss in the battery, the specific gravity of the electrolyte increases, and the excessive acidity of the electrolyte aggravates the corrosion of the positive plate.

4) Thermal runaway
Thermal runaway refers to the cumulative enhancement of charging current and battery temperature when the battery is charged at a constant voltage, which gradually damages the battery. According to the current battery usage survey, thermal runaway is one of the main causes of battery failure. The direct consequence of thermal runaway is that the battery shell bulges and leaks, the battery capacity decreases, and in severe cases, it can also cause the plate to deform and eventually fail. The floating charge voltage is the charging voltage used for a long time by the battery, and it is a crucial factor affecting the battery life. In general, it is more appropriate to set the floating charge voltage to 2.23 ~ 2.25V/cell (25℃).

4. Problems with the battery in the backup power supply operation
1) The battery life cannot meet the design requirements
In practice, the battery will be seriously degraded after three years, and there are few batteries that have been used for more than 5 years. The reason is that the battery is not managed and maintained effectively and reasonably during use, causing the battery to degrade at an early stage, and the outdated battery is not discovered in time, resulting in the accumulation and aggravation of degradation, leading to the premature scrapping of the battery.

2) Unclear about the operation and performance of the battery
If there are lagging batteries in the battery pack, the lagging difference can be reduced to a certain extent through a certain depth of discharge and charging cycle. However, due to the lack of good management methods, the internal performance parameters of the battery, such as the internal resistance of the battery and the current remaining capacity, cannot be clearly understood, so the corresponding measures cannot be implemented.

3) For single cells, the reliability of the charging mechanism needs to be improved
Since the charging mechanism of the current domestic DC system is not very perfect, there is voltage drift in practice. The battery is in a floating state for a long time. If the floating voltage deviates from the normal range, it will cause the battery to be overcharged or undercharged. Long-term overcharge or undercharge has a great impact on the performance of the battery.

4) Imbalance between single cells
At present, the battery pack is composed of a large number of single cells. In actual operation, there are large differences in charging voltage, internal resistance, etc. between single cells. Especially under floating charge, this imbalance phenomenon is very serious. If the lagging battery is not fully charged, this lagging will be aggravated if it is not discovered and handled in time. In this way, this imbalance will be aggravated, causing the lagging battery to fail, thereby causing the capacity of the entire battery pack to be lost prematurely.

5) The maintenance work of unattended sites lacks good management and monitoring methods
For many unattended sites, due to the lack of network management and monitoring methods, the maintenance of batteries is even weaker, especially the operation and performance of batteries cannot be clearly understood. A large amount of maintenance and management work is done manually, and the collation and analysis of data requires maintenance personnel to have strong professional knowledge.

6) The end of battery life cannot be judged in advance and there is a lack of scientific basis for battery replacement
We hope to make a judgment on the end of battery life in advance to buy time for battery replacement. However, there is no reliable means to determine the end of battery life at present, and it is only based on years of experience. Therefore, in practice, the end of battery life is often found during discharge only after the battery discharge capacity is lower than the minimum requirement.

2. Comparison of manual inspection and online monitoring technologies for batteries
1. Manual inspection
Currently, most of the methods are manual inspection to achieve battery maintenance. In addition to the discharge test, the manual measurement of this method mainly measures the battery pack voltage, single cell voltage, temperature and single cell internal resistance.
Battery pack voltage measurement can find out whether the charger parameter settings are correct. Since the battery is operated in series, the voltage of the entire battery group is determined by the output of the charger.
Single cell voltage monitoring can find out whether the floating charge voltage of a single cell is incorrect, whether the single cell is overcharged or over-discharged, etc.
Temperature measurement can find out whether the working environment of the battery is poorly ventilated or the temperature is too high.
Battery internal resistance can reflect the capacity decline and battery aging of the battery. The accuracy and anti-interference ability of internal resistance testers from different manufacturers vary greatly; due to the different working frequencies used, the reading values ​​will also be different; especially the measurement fixture is difficult to directly contact the battery terminal, and the measurement value often includes the connection resistance.

There are many shortcomings in manual measurement:
a. The accuracy of manual measurement will be affected by many factors;
b. Since manual testing is mostly conducted on a regular basis, outdated and failed batteries cannot be discovered in time;
c. Discharge testing will cause irreversible damage to the battery;
d. A large number of manual measurements are time-consuming and labor-intensive, with poor safety and long cycles.

2. Online monitoring of batteries
Online monitoring management of batteries is designed to measure the operating conditions of batteries and detect the condition of the batteries themselves. Its development has roughly gone through three stages: ① whole group voltage monitoring, ② single cell voltage monitoring, ③ single cell internal resistance inspection

1) Whole group voltage monitoring
The whole group battery monitoring function is generally designed in the rectifier power supply to measure the voltage, current and temperature of the battery group, and perform charging and discharging management, especially adjusting the floating charge voltage of the battery according to the change of ambient temperature. When the battery is discharged, the battery group voltage drops to a certain lower limit and an alarm is sounded. The current UPS still uses this method.

However, there are major deficiencies in the monitoring of the entire group. For example, when the battery pack is discharging, the discharge cut-off voltage is N×1.8V/battery (N is the number of batteries). However, since the consistency of the batteries in the battery pack cannot be strictly guaranteed, during discharge, when individual batteries have reached the discharge cut-off voltage, but the battery pack has not reached N×1.8V/battery, individual batteries will be over-discharged.

2) Single cell voltage monitoring
Fully electronic monitoring can provide comprehensive monitoring and management of the battery operation, such as single cell voltage, battery pack voltage, charge and discharge current, battery ambient temperature, etc. By monitoring the battery operating parameters, the battery can be guaranteed to operate and work under normal conditions. However, when the battery operating conditions cannot be guaranteed, the monitoring of the battery operating parameters cannot reflect its performance parameters.

3) Single cell internal resistance monitoring
The total internal resistance of a battery is the sum of the charge transfer resistance and the ohmic resistance of each component. Experiments have shown that ohmic impedance is the biggest hidden danger of early battery failure.

The following are the most common factors that affect the change of internal resistance:
Corrosion As the grid and busbar corrode, the metal conductive circuit changes, increasing the internal resistance.
Grid corrosion and long-term use will cause the active material to fall off the grid, increasing the internal resistance.
Sulfidation As part of the active material sulfides, the resistance of the paste also increases.
Battery dryness Since VRLA batteries cannot be watered, water loss may make the battery scrapped.
Manufacturing defects, such as cast lead and paste, can lead to high metal resistance and capacity problems.
The charging state from floating charge to 20% capacity discharge has almost no effect on the internal resistance. Experiments show that the effect of 20% discharge on the internal resistance is less than 3%.
High temperatures within 39°C have little effect on the internal resistance of the battery, and low temperatures have some effect, but it needs to be below 18°C.

Experiments have shown that batteries with internal resistance 50% higher than the baseline value cannot pass the standard capacity test, and VRLA batteries fail one after another. It is also common for battery packs that have been used for 3 to 4 years to have internal resistance values ​​that are higher than 0 to 100% of the baseline value. The use time at high discharge rates seems to be more sensitive to these factors. Generally, the battery life is reached when the internal resistance increases by 20 to 25%. At low discharge rates, the battery life ends after the internal resistance generally increases by 20 to 35%.

Field test data show that when the internal resistance of individual batteries deviates from the average value by 25%, a discharge capacity test should be performed. Placing a temperature sensor on the surface of the battery can detect battery overheating, thereby promptly discovering abnormalities in the battery operation process.

4) Internal resistance test method
In recent years, battery monitoring equipment manufacturers have launched products that monitor the internal resistance of single batteries, which has brought about a qualitative change in battery monitoring technology, that is, from passive voltage monitoring to active testing of the internal state of the battery. On the one hand, the internal resistance inspection can monitor the operating parameters of the battery such as voltage, current, and temperature, and on the other hand, it can timely detect the health of the battery through the monitoring of internal resistance.

Online internal resistance testing technology is difficult, and each manufacturer has its own specific implementation technology, and their internal resistance accuracy and anti-interference capabilities vary greatly. The methods for real-time online monitoring of internal resistance can be divided into two categories: DC discharge method and AC method.

DC discharge method
The DC method measures the battery voltage drop during instantaneous high current discharge (70A), thereby obtaining the internal resistance of the battery, and analyzing the battery's backwardness or failure trend through the change of the battery's internal resistance. At the same time, it is supplemented by the monitoring of operating parameters such as voltage and current. It is currently a leading monitoring technology.

The shortcomings of the DC method:
a) The use of large current discharge will cause certain damage to the battery performance; if the measurement frequency is high, this damage will accumulate;
b) The DC method can only measure the ohmic impedance of the battery internal resistance, but cannot measure the polarization impedance. It is not sufficient to judge the failure and backwardness of the battery;
c) The connection wire with the battery needs to be more than 10 square millimeters, and the connection method has high requirements. The reliability of the discharger and the connection wire must be high.

AC method
In recent years, with the development of digital signal processing technology, it has become possible to effectively eliminate interference from other electromagnetic signals, and a breakthrough has been made to solve the difficulties in the practical application of the AC method, so that this method can be applied in actual work. The
AC method is to inject an AC signal of a certain frequency into the battery. Due to the impedance inside the battery, the current signal fed back is measured, the signal is processed, and the difference between the injected signal and the feedback signal is compared to measure the internal resistance of the battery.

Features of the AC method:
a) Since there is no need to discharge, the damage to the battery performance caused by high current discharge is avoided.
b) Since there is no need to take the battery offline or static, the hidden dangers of system safety are avoided, and real-time online measurement is truly achieved.
c) The AC method measures the ohmic impedance and polarization impedance of the battery at the same time, making the analysis of the battery health more real and reliable.
d) Since there is no load, its cost is greatly reduced.

3. Battery Online Monitoring Solution
In response to the above factors, LEM has developed an effective battery online monitoring solution. In an intelligent and networked form, it monitors the battery's internal resistance, voltage, temperature, and charge and discharge current to truly and effectively reflect the battery's health status, thereby promptly discovering outdated batteries and preventing battery failures in advance. It provides a scientific and accurate basis for the safe operation and maintenance of batteries and relieves users' worries.

1. Core Component Sentinel sensor module
Sentinel is the core component of the battery online monitoring system. It can simultaneously measure three important parameters related to battery performance of the connected single battery: voltage, temperature and impedance, and convert them into digital quantities. It communicates with the controller through the bus to realize the transmission of data and commands.

Temperature measurement, as specified in the international standard IEEE 1188, temperature is one of the parameters that must be detected during regular maintenance of stationary batteries.

Temperature is an environmental factor that affects the life of batteries. Batteries are generally designed based on a standard ambient temperature of 25°C, and their ideal operating range is 21-27°C. When working at a lower temperature, the battery discharge capacity will not reach the rated capacity and the standby discharge time will be reduced; when working at a higher temperature, the battery life will be shortened.

Sentinel has a built-in temperature measuring element, which is directly attached to the battery body to monitor the battery temperature at any time and detect battery overheating in time.
 Voltage measurement is the main operating parameter of the battery. Battery voltage monitoring can detect whether the battery floating charge voltage is correct and whether the battery is overcharged or over-discharged.

Sentinel is connected in parallel to the positive and negative poles of the single battery, and measures the terminal voltage of the battery in different states in real time, with a measurement range of 0.90 to 16V DC. Since Sentinel is directly powered by the single battery pack, the current consumption is very low and no external working power supply is required. 

Internal resistance measurement is the most effective parameter for detecting battery failure modes. The change trend of battery internal resistance can reflect whether the battery capacity is decreasing and whether the battery is aging.

In actual applications, the correct operation of the charger does not mean that each single battery is in normal working condition, and the terminal voltage of the battery cannot truly reflect the capacity characteristics of the battery. For batteries with severely reduced capacity, the difference in floating charge voltage is not enough to determine whether the battery has failed due to reduced capacity. In fact, only by discharging the battery pack regularly can we understand the capacity status of the battery, but at this time, the outdated battery found has already entered the late stage of its life, and can only provide users with delayed information, and the battery pack can no longer play a backup role.

A large amount of experimental data shows that there is a certain relationship between the internal resistance and discharge capacity of aging batteries. The sharp increase in internal resistance is related to the reduction in battery capacity (Figure 2), especially when the battery life is less than 80%. Using internal resistance testing to assess the condition of batteries is a fairly reliable method.

Figure 3: LEM resistance test method
By comparing the test results with the laboratory equipment of AACHEN University in the United States, the two test equipments obtained very close test data (Figure 4). From the test data, it can be seen that the LEM Sentinel battery internal resistance measurement results are accurate and reliable, and can truly reflect the health of the battery.

Figure 4: Test data of LEM Sentinel and AACHEN experimental equipment

2. LEM Sentinel battery online monitoring system

Figure 1 Block diagram of LEM Sentinel battery online monitoring system
Due to the increase in unmanned sites, the intelligent and networked online monitoring of batteries has become a trend. LEM Sentinel battery online monitoring system measures battery parameters online, uses RS 232 or network (IP) to achieve communication between PC and measurement module, and the monitoring software calculates and analyzes the collected data, and displays the operating status of the battery in the form of graphs and status tables. LEM Sentinel battery online monitoring system functions:

Measurement function: Continuously monitor the current, voltage, and temperature operating parameters of the battery during the entire process of charging and discharging, regularly monitor the internal resistance of the battery, and convert it into digital quantities.

Communication function: The communication between Sentinel and controller is realized through serial bus. Each controller can connect up to 254 Sentinels, each Sentinel has a unique address (ID), and the communication method is flexible. The controller can be directly connected to the PC, or communicate through Ethernet or LAN. The Webserver function allows users to understand the status of the battery anytime and anywhere.

Analysis function: The monitoring software records and analyzes the collected battery parameters, and reflects the different battery health conditions in the form of graphics and status tables in different working modes (normal, discharge, charge) (Figure 5, Figure 6, Figure 7).

Alarm function: For abnormal conditions such as battery voltage, temperature, overcharge, over discharge, etc., there will be an alarm display in the monitoring software and a corresponding alarm node output on the controller, which is convenient for users to access the alarm system.

3. Advantages of LEM Sentinel battery online monitoring system
1) Realize real-time and online monitoring of battery parameters during operation, make early judgments, prevent problems early and deal with them in time for possible problems.
2) The measurement data has good repeatability, avoiding the randomness of manual test data, realizing continuous measurement of battery parameters, forming a historical data record curve, and users can find and prevent battery defects according to the changing trend of operating parameters.
3) One-time distributed installation, one-to-one correspondence between modules and batteries, simple on-site wiring and low engineering costs, reducing the planned maintenance costs during the battery life cycle, reducing the large amount of repeated investment in labor and equipment, and eliminating the adverse factors brought by manual measurement to the safety of personnel and power systems, especially reducing the interruption of external power during battery discharge testing.
4) The supporting software automatically analyzes the various parameters of the monitored batteries, reducing the stringent requirements of battery maintenance on the professional quality of personnel.
5) Independent modules make monitoring very flexible. Compared with centralized monitoring products, they are more advantageous in places with a small number of batteries, and meet the current needs of informatization and networked equipment management in the fields of power and communications.
6) Sentinel uses a LEM customized system-on-chip SoC; the high degree of integration ensures higher reliability and anti-interference performance of the module, and the module warranty period is 5 years. 7) The module
uses a pulse current discharge method to measure internal resistance. The discharge current is small, which is not only safe but also not easy to cause damage to the battery. The module can detect the temperature of a single battery, which is rare in other similar products on the market.

Figure 5: Bar graph showing the voltage, temperature, and internal resistance of each battery in the network

Figure 6: Continuously recording battery voltage, temperature, internal resistance, and current

Figure 7: Webserver function 4. LEM Sentinel battery online monitoring field application case