Failure discussion and online monitoring of valve-regulated lead-acid batteries

At present, most battery monitoring modules are voltage patrol meters, which monitor the floating charge voltage of the battery online and give an alarm when it exceeds the set value. Compared with the previous whole group voltage monitoring method, single cell voltage monitoring is a big step forward, but for the monitoring of capacity decay and even failure during the long-term operation of the battery, the voltage can reflect very limited problems: the difference in floating charge voltage between a 100Ah battery and a battery that decays to 10Ah is difficult to distinguish. Therefore, it is necessary to explore the failure mode of the battery to solve the battery monitoring problem.

2. Failure mode of valve-regulated lead-acid battery

For valve-regulated lead-acid batteries, the common performance deterioration mechanisms are as follows:

1. Heat accumulation

When an open lead-acid battery is charged, in addition to the regeneration of active substances, the water in the sulfuric acid electrolyte is gradually electrolyzed to generate hydrogen and oxygen. When the gas is released into the atmosphere from the vent hole on the battery cover, 11.7 kcal of heat is generated for every 18 grams of water decomposed.

For valve-regulated lead-acid batteries, the oxygen generated internally during charging flows to the negative electrode, where it oxidizes the active material, sponge-like lead, and effectively replenishes the water lost by electrolysis. The oxygen cycle inhibits the precipitation of hydrogen, and the oxygen participates in the reaction to generate water. Although this eliminates the problem of discharging explosive gas mixtures, this sealed type reduces an important path for heat diffusion, and the only way to release heat is through heat conduction through the battery shell wall.

Valve-regulated lead-acid batteries rely on heat conduction from the battery shell wall to dissipate heat. Good ventilation and low room temperature are important conditions when installing the battery. In order to further reduce the risk of thermal runaway, the float charge voltage is usually determined by different manufacturers and different room temperatures. Manufacturers generally provide the float charge voltage and temperature compensation coefficient of the battery.

2. Sulfation

The problem that valve-regulated batteries are more prone to than open-cell batteries is sulfation of the negative plates. This is due to:

1) The lower potential of the negative plate caused by the circulation of oxygen;

2) Acid stratification at the bottom of the battery where the strong acid electrolyte collects, which is difficult to avoid in this non-flowing, non-circulating electrolyte system.

Both of these may produce a certain amount of residual sulfate under floating charge conditions, which then transforms into a permanent sulfate form. Therefore, as the plate deactivation accelerates, the available discharge ampere-hour capacity will decrease. This situation will be exacerbated as the temperature of the negative plate increases. Due to the occurrence of oxygen cycle reactions, the surface of the negative plate is oxidized and a considerable amount of heat is released.

3. Corrosion and shedding of positive plate group

In valve-regulated lead-acid batteries, this form of performance deterioration is inherently more serious. Due to the oxygen cycle reaction, the negative active material is continuously oxidized to form lead sulfate, which effectively maintains the discharge state, thereby reducing the potential of the negative plate. For a given floating charge voltage, the potential of the positive plate group is correspondingly higher. As a result, the oxidizing atmosphere is intensified, causing more oxygen to precipitate, which intensifies the corrosion and shedding of the active material.

4. Battery drying up

During use, the gas recombination mechanism is not 100% efficient. Although the rate at which water is electrolyzed to produce hydrogen and oxygen is less than 2% of the electrolysis rate of a flooded cell of the same size, water is still gradually lost.

When dehydration is the primary cause of failure, the specific gravity of the electrolyte will increase, and when the specific gravity increases from the initial 1.30 to 1.36, it means that the dehydration degree has reached about 25%. At the dehydration degree of 25%, the high concentration of acid accelerates sulfation, and the specific gravity of the electrolyte begins to decrease again. The battery voltage is directly proportional to the specific gravity of the electrolyte, so the battery voltage is not a reliable indicator of the health of the battery.

5. Corrosion of lead on the negative electrode

The corrosive nature of the positive grid and pack is inherent in all designs of lead-acid batteries. In sharp contrast, the negative plates are located in a highly reducing atmosphere and in open cells the pack busbars are usually submerged below the electrolyte level, thus protecting them from corrosion due to oxygen escaping from the positive plate pack. However, many designs of valve-regulated batteries do not protect the plate lugs, pack and busbars, especially the welded joints between them. They are therefore exposed to a continuous stream of oxygen escaping from the oxygen cycle and above the battery pack. Depending on the consistency of the lead alloy selected for the grid (lugs) and pack and the production quality (which requires complete melting of the grid welds and low porosity of the busbars), rapid oxidation may occur.

3. Development of battery monitoring system

In order to provide a good operating environment for batteries and monitor the working condition of batteries online, the Battery Management System (BMS) came into being and became a key part of the high-reliability power supply system.

1. Measurement of internal resistance of battery cells

The internal resistance R is inversely proportional to the cross-sectional area A for transmitting current. The shedding of active materials, sulfation and corrosion of the plate grid and busbar, and drying can reduce the effective cross-sectional area A, so the failure of the battery can be detected by measuring the internal resistance.

The degree of correlation between internal resistance and battery condition is highly variable. Reported correlations range from 0% to 100%. The UK Electronics Association (ERA) conducted a large battery survey using both laboratory and commercial designs of impedance monitoring and found the accuracy of both to be above 50%. A fundamental difficulty is the accuracy of measuring small changes in value. The resistance of a normal 300 ampere-hour standby current is only on the order of 0.25 x 10-3 ohms. Therefore, small and significant changes in resistance may not be observed. The problem is exacerbated under the following operating conditions.

1) The interference caused by transformer "noise" and floating charge voltage fluctuation during online measurement.

2) The effect of corrosion cracks on internal resistance is highly directional, and the internal resistance value is relatively insensitive to cracks parallel to the direction of current flow.

3) Changes in electrolyte concentration and subsequent changes in the battery make the results difficult to interpret.

Although it is difficult to accurately measure the capacity of the battery by internal resistance measurement, and the corresponding relationship between internal resistance and capacity is difficult to reproduce, for BMS, internal resistance testing is only used for comparison between battery cells, and the computer can record and process the changes in internal resistance to predict battery capacity attenuation and failure. Therefore, internal resistance testing is one of the key technologies for BMS.

For measuring internal resistance offline or with the battery open circuit, the excitation current loop and the voltage measurement loop can be conveniently connected to the single cells in the battery pack in a 4-terminal manner. However, for online measurement, it is difficult to solve the excitation and measurement problems.

Currently, most methods use a discharger connected in parallel at both ends of the battery pack. Because the charger and the battery pack are connected in parallel, the charger needs to be stopped, and the current and voltage changes of the battery need to be measured synchronously in real time, which makes it difficult to deal with sampling interference.

Compared with the currently used internal resistance test device that applies excitation to the positive and negative ends of the battery pack, the excitation device with a midpoint tap has a midpoint tap. The current of the excitation device passes through the midpoint tap and then reaches the positive and negative electrodes of the battery pack through the upper battery pack and the lower battery pack, eliminating the parallel influence of the external charger and power load of the battery pack, generating a stable current excitation on the battery, and can accurately test the internal resistance of the battery.

2. System structure

The configuration of valve-regulated lead-acid batteries (VRLAB) in general systems is generally:

500kV substation DC system: 2 sets of full-capacity batteries, 3 chargers.

220kV substation DC system: 1 set of full-capacity batteries, 2 chargers.

110kV substation DC system: 1 set of full-capacity batteries, 2 chargers.

The main components are 108 2V batteries, 18 or 19 12V batteries. The placement of the batteries varies greatly, and the distance between the batteries and the operating room is uncertain.

BMS consists of a control unit, a measurement module, related software and auxiliary components. One control unit can be connected to multiple measurement modules to complete the monitoring and management of multiple groups of batteries with different numbers and voltages. The control unit is used for data transmission, data processing and human-machine interface control. It has RS-232 connection interface and RS-485 remote (centralized) management interface, measurement module control interface, operation keyboard, display panel, sound and light alarm and alarm output control contacts. The control unit displays battery data in real time, analyzes data intelligently, and issues timely alarms for abnormal battery operation.

The measurement module is used for inspection of battery data. The built-in CPU works independently at high speed. In addition to conventional voltage, current, temperature and other measurements, it can accurately test the internal resistance of the battery online after connecting with the internal resistance test module. The measurement module is installed near the battery and communicates with the control module to facilitate on-site wiring installation.

Figure 1 System structure diagram

3. System parameter settings

As a complete monitoring system, the BMS system should first be applicable to DC 220V system, DC 110V system, DC 48V system, and DC 24V system. Its versatility was taken into consideration during the design. The main monitoring module and internal resistance detection module are universal. For different systems, only different numbers of acquisition modules need to be added, and the number of battery samples for each acquisition module needs to be set. Therefore, the system needs to set the following system parameters and alarm parameters:

1) Number of acquisition modules
2) Number of batteries collected by the acquisition module with the least number of batteries
3) Background communication address setting
4) Background communication baud rate setting
5) Upper and lower limits of battery pack float charge voltage
6) Upper and lower limits of single battery float charge voltage
7) Internal resistance threshold
8) Capacity alarm
9) Overcurrent alarm
10) Abnormal temperature

The first four items are system settings, and the last six items are alarm settings.

4. Voltage and current inspection and data analysis

The original battery monitoring device only detects the terminal voltage, current and temperature of the battery pack, and compares the detection data with the set upper and lower limits to give an alarm prompt. The battery inspection instrument can measure the voltage of each battery cell and alarm when the floating charge voltage exceeds the limit.

Most battery manufacturers' technicians put voltage measurement first. For batteries in the floating charge state, the slight difference in the floating charge voltage can reflect the battery's charge state and determine the serious failure of the battery. Because the floating charge current is very small, the performance difference between batteries (mainly capacity difference) is difficult to show. BMS monitors the complete working process of the battery, measures the voltage and current in different states of charging, floating charge and discharging in real time, and uses different data processing methods to improve the accuracy of data analysis.

The relationship between float charge voltage and temperature can be compensated according to the slope provided by the manufacturer.

VF = V0+k (T-T0)

Generally, k is 3~5mV.

5. Calculation of remaining capacity

Trying to measure the actual remaining capacity of the battery online through some method has always been the most urgent hope of battery users, but so far, there is no such method or algorithm. Some materials or product advertisements introduce the use of battery internal resistance to calculate the remaining capacity, but in actual use, the corresponding relationship of the data is not strict, and the internal resistance can only be used to distinguish the large changes in battery capacity. In particular, the relative change of battery internal resistance can accurately predict the battery lag.

When the battery is in discharge operation, it is necessary to know the remaining capacity and power supply time of the battery in many occasions. According to the rated capacity of the battery and the monitoring of the discharge current, it is not difficult to calculate the remaining capacity in real time. Assuming that the load is relatively stable, the power supply time can be converted. Generally, battery manufacturers give the battery capacity at different discharge multiples.

Using the least squares method, based on the discharge capacity at different rates provided by the battery manufacturer, we can simplify the relationship between current and capacity using a quadratic curve to obtain a, b, and c respectively:

6. Battery operation event record

Another important function of BMS is to record operating data so that it can be tracked when a battery fails to determine whether it is due to battery quality or abnormal use. For long-term continuous operation, recording all data not only requires high hardware requirements, but also has no practical significance. BMS is designed with an event generator, which stores the normal operation of the battery in the form of events according to the event generation rules, greatly reducing the amount of data and facilitating query management. It mainly includes:

1) Float charge voltage is too high or too low
2) Charging current is too large
3) Discharging current is too large
4) Operating temperature is too high or too low
5) Internal resistance changes
6) Deep discharge

The event records the data and duration at that time. For the battery operation characteristics of the power system, the event generation rules are required to have strong robustness to shield the closing shock and measurement interference.

If there are individual lagging cells in the battery pack, the discharge capacity is determined by the worst cell.

7. Remote management

With the promotion of unmanned substations, online monitoring of batteries is more necessary. Battery monitoring equipment can be connected to the centralized monitoring system. Through the remote management software, you can view the current operating status of the battery and the recorded historical operating events, and promptly learn the alarm information issued during the monitoring process to decide whether to send maintenance personnel. You can also conduct further testing through remote control.

8. Measured data analysis

Six batteries with different capacities and voltage levels were tested and compared, with the standard internal resistance tested by a single battery internal resistance tester imported from Japan, and the standard voltage tested by a 0.1-level standard digital multimeter. The online measurement was conducted by a BMS battery inspection instrument, and the specific data are as follows (internal resistance is in milliohms, voltage is in volts):

Through testing and analysis, it is found that the BMS battery inspection instrument has accurate testing and high precision, and is fully capable of online monitoring of the battery system.

IV. Summary

The battery is the core part of the power supply system. Adding corresponding effective monitoring equipment can, on the one hand, ensure that the battery works under reasonable conditions and extend the floating charge service life of the battery; more importantly, measures can be taken before the battery fails completely to avoid discovering battery problems after a power outage.