Comparison between manual inspection and online monitoring of batteries

1. Manual inspection

Currently, most of the battery maintenance is done by manual inspection. In addition to the discharge test, the manual measurement mainly measures the battery pack voltage, single cell voltage, temperature and single cell internal resistance.

The battery pack voltage measurement can find out whether the charger parameter settings are correct. Since the batteries are connected in series, the voltage of the entire battery pack is determined by the charger output.

Single cell voltage monitoring can detect whether the single cell floating charge voltage is incorrect, whether the single cell is overcharged or over-discharged, etc.

Temperature measurement can reveal whether the battery's working environment is poorly ventilated or the temperature is too high.

The internal resistance of a battery can reflect the decrease in battery capacity and battery aging. The accuracy and anti-interference ability of internal resistance testers from different manufacturers vary greatly; due to the different operating frequencies used, the readings will also be different; in particular, it is difficult for the measuring fixture to directly contact the battery terminals, and the measured value often includes the connection resistance.

Manual measurement has many shortcomings:

a. The accuracy of manual measurement will be affected by many factors;

b. Since manual testing is mostly performed periodically, it is impossible to detect outdated and failed batteries in time;

c. Discharge testing may cause irreversible damage to the battery;

d. A large amount of manual measurement is time-consuming and labor-intensive, with poor safety and a long cycle.

2. Online monitoring of batteries

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

1) Whole group voltage monitoring

The whole battery monitoring function is generally designed in the rectifier power supply to measure the voltage, current and temperature of the battery pack, and manage charging and discharging, especially adjusting the floating charge voltage of the battery according to changes in ambient temperature. When the battery is discharging, an alarm will be issued when the battery pack voltage drops to a certain lower limit. 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 a more comprehensive monitoring and management of the battery's operating conditions, such as single cell voltage, battery pack voltage, charge and discharge current, battery ambient temperature, etc. By monitoring the battery's operating parameters, the battery can be guaranteed to operate and work under normal conditions. However, when the battery's operating conditions cannot be guaranteed, the monitoring of the battery's operating parameters cannot reflect its performance parameters. [page]

3) Single cell internal resistance monitoring

The total internal resistance of the 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 can cause active materials to fall off the grid, increasing internal resistance.

Sulphurization As part of the active material is sulphated, the resistance of the paste also increases.

Battery drying up Since VRLA batteries cannot be watered, water loss may render the battery useless.

Manufacturing imperfections, such as lead casting and pasting, can lead to high metal resistance and capacitance problems.

The charge state changes from floating charge to 20% capacity discharge, which 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 below 39°C have little effect on the internal resistance of the battery, while low temperatures have some effect, but the temperature must 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. 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.

a. 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. The battery's backwardness or failure trend is analyzed through the change in 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 relatively leading monitoring technology.

Disadvantages of DC method:

a) Using high 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's 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 to 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.

b. Communication 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 has made a breakthrough in solving the difficulties in the practical application of the AC method, thus enabling this method to 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, signal processing is performed, and the difference between the injected signal and the feedback signal is compared to measure the internal resistance of the battery.

Characteristics of communication method:

a) Since there is no need to discharge, the damage to battery performance caused by large current discharge is avoided.

b) Since there is no need to take the battery offline or static, hidden dangers to 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 realistic and reliable.

d) Since there is no load, its cost is greatly reduced.