NiMH battery packs

How to extend the service life of NiMH battery packs? The secret to extend the service life of NiMH battery packs

Recently, Fujita Electric Vehicles and the well-known domestic battery brand BAK held an exchange meeting on lithium battery electric vehicle technology on site.

"Why are NiMH battery packs better than lead-acid batteries?" "How to extend the service life of NiMH battery packs?" Dr. Mao Huanyu, general manager of BAK Battery, explained the working principle and usage skills of lithium-ion batteries on site, and answered questions from the audience.

The use of NiMH battery packs is completely different from that of lead-acid batteries

Dr. Mao introduced that lithium-ion batteries have been widely used in daily life. For example, mobile phone batteries and laptop batteries are basically NiMH battery packs, and they have also begun to develop in the field of power batteries. He pointed out that NiMH battery packs and lead-acid batteries are very different in terms of internal reactions and usage.

The characteristics of NiMH battery packs are small size, light weight, high energy storage per unit mass, and the movement of internal electrons is a physical change, so the battery has a long service life. The characteristics of lead-acid batteries are low requirements for the use environment and can still be used in some low temperature environments, but because the internal charging and discharging process is a chemical change, the service life is short.

"Although NiMH battery packs have so many benefits, to extend their life and allow electric bicycles to ride longer, we still need to pay attention to some issues." Dr. Mao pointed out that the current use of power batteries is to group batteries and connect single batteries in series and parallel. Due to its characteristics, lithium battery packs have high requirements for the consistency of single batteries. "In a group of batteries, as long as one battery is different from the other batteries, the effect of the whole group will decrease." Therefore, consumers of lithium-ion electric bicycles must pay attention to the care of batteries when using them. At present, most of the batteries used in communication power supplies are advanced valve-regulated sealed lead-acid batteries. The voltage of each single cell of this battery is generally 2V, which is connected in series to form a 48V or 24V system. It plays two major functions of protecting communication equipment and facilities and ensuring the smooth operation of the network. In terms of protecting communication power equipment and facilities, batteries, UPS, and switching power supply systems play a role in preventing voltage surges, waves, spikes (drops), transients, and undervoltage (overvoltage) in the city power grid, effectively protecting communication equipment and preventing station downtime accidents. In terms of ensuring the smooth operation of the network, the battery, UPS and switching power supply system play the role of maintaining the normal operation of the system when the mains power is interrupted, and also play the role of filtering out noise voltage and maintaining the communication quality.

Once these batteries are installed and put into operation in the communication base station, they will not be replaced within a few years. Therefore, it is of great significance to strengthen the maintenance of the battery, improve its use condition, and effectively extend the service life of the battery. The online detection of the battery currently plays an indispensable role in the maintenance of the communication power supply.

2. The technical status of the battery in the communication system

The battery pack is an important component of the base station to achieve DC uninterrupted power supply, and its investment amount is basically equivalent to that of the switching power supply equipment. At present, most of the mobile base stations use valve-controlled sealed lead-acid batteries (referred to as VRLA batteries) developed at the end of the 20th century. Due to the use of valve-controlled sealing structure, there is no need to add acid or water for maintenance, no acid liquid or acid mist leaks, and it can be installed in the same machine room as the equipment. Due to its small size, light weight, small self-discharge, less maintenance, long life, easy use, safety and reliability, it is very popular among users. However, we must see that, on the one hand, the basic electrochemical principle of this battery remains unchanged, so its inherent electrical characteristics requirements have not only not changed, but have become more stringent; on the other hand, in the early stage of the promotion of this battery, the manufacturer's manual sometimes more or less referred to this battery as a "maintenance-free" battery, which led some maintenance personnel to believe that this battery does not require maintenance, so that battery maintenance and testing have not received due attention. This misleading still has a profound impact.

From the current use of base station batteries, there is a general problem of rapid battery capacity decline, short service life, and frequent station drop accidents. From the current quality of batteries produced by several large domestic valve-controlled sealed battery manufacturers, they should be able to meet the requirements of various operators, although the quality and performance of batteries produced by various manufacturers are different. The quality factor of the battery should not be the main reason for the rapid decline in capacity and shortened service life of the base station batteries of various operators. From the comprehensive factors such as the product structure, product performance, and on-site survey of the use of base station batteries of valve-regulated sealed batteries, combined with the use of exchange stations, the capacity of valve-regulated sealed batteries should not decrease so quickly after 1 to 4 years of use under normal circumstances. The main reasons for the rapid decrease in capacity and shortened service life of base station batteries should be related to the use environment and maintenance of the base station.

The main reasons for the rapid decrease in capacity and shortened service life of base station batteries are:

First, the base station has frequent power outages, long power outages, and irregular power outages, which cause the battery to charge and discharge frequently. According to the current dissection of scrapped batteries in base stations by battery manufacturers, the reason for the end of battery life is the sulfation of the negative plate of the battery, which is a typical phenomenon of early capacity failure (PCL) of the battery.

The reasons for the sulfation of the negative plate of the battery are as follows: The frequency of power outages in the base station is too high, with power outages several times a day, or even continuous power outages for several days, causing the base station battery to discharge again when it is not fully charged after discharge, resulting in undercharge of the battery. If undercharging occurs repeatedly, it will cause cumulative loss of battery capacity, and the battery capacity of the base station will decrease in a short time, and its service life will end quickly. Generally speaking, the rate of battery capacity decrease is proportional to the number of times the base station battery is undercharged continuously. The internal reason for the decrease in battery capacity is that the battery is discharged again without being fully charged after discharge, and the lead sulfate generated by the positive and negative electrodes after discharge cannot be completely restored to lead dioxide and metallic lead respectively. The positive and negative plates are discharged again, causing the battery to be undercharged. Continuous undercharging causes the negative plate to gradually sulfate, producing irreversible crystalline lead sulfate. Especially when the battery is in a deep over-discharge, the sulfation of the negative plate of the battery will be more serious, and the sulfation rate will be faster, causing the surface of the negative plate to be shielded, and its function will gradually decrease until it fails, resulting in a decrease in the battery service life until it ends.

Second, the parameters of the switch power supply are unreasonable, the voltage of the base station battery under-voltage protection is set too low, and the reset voltage is set too low, causing the battery to be over-discharged or even deeply over-discharged, which on the other hand aggravates the sulfation of the negative plate of the battery.

At present, the combined switching power supply of base stations is equipped with low voltage isolation protection function or secondary power-off function. When the battery is discharged to a certain set voltage value, the switching power supply system will automatically cut off the power supply to some heavy loads or all loads to protect the battery from over-discharge and ensure the battery life.

If the minimum undervoltage protection value of the battery is set too low, the battery will be over-discharged. Multiple over-discharges and failure to replenish power in time after over-discharge or insufficient charging will seriously affect the battery life; in addition, if the reset voltage of the switching power supply is set too low, the battery will be repeatedly discharged during the discharge process; the specific minimum undervoltage protection value of the battery should be set according to the load current size, and the current minimum undervoltage protection value of the base station battery is generally set at about 1.8V per single cell voltage, and some are even set to 1.75V per cell. According to the discharge performance of the valve-controlled sealed battery combined with the actual load current of the base station (the actual load current of the base station is mostly less than 0.1C10A), the minimum undervoltage protection value of the base station battery should be set at more than 1.8V per battery cell voltage.

Therefore, the reference voltage of the current base station battery undervoltage protection setting is too low. If the base station is out of power for a long time, the battery will be over-discharged, or even deeply over-discharged with a small current. It takes a long time to fully charge the over-discharged battery to restore its capacity. It is generally difficult for the deeply over-discharged battery to fully restore its rated capacity under the only constant voltage charging condition of the base station. Therefore, the unreasonable setting of the switching power supply parameters will aggravate the sulfation of the negative plate of the battery, thereby causing the battery capacity to decrease and the service life to be shortened.

Third, the base station is used in a harsh environment. After the base station is out of power, the ambient temperature of the base station gradually rises due to the lack of air conditioning. Or due to air conditioning failure, the indoor temperature of the base station is high, thereby reducing the service life of the battery.

Indoor base stations are equipped with air conditioners, which are general cabinet or split air conditioners. Long-term uninterrupted use causes some base station air conditioners to fail and shut down. After the air conditioner is damaged, it is sometimes not repaired in time. The indoor base station is a closed machine room. After the air conditioner is shut down, the indoor temperature of the base station rises sharply. The indoor temperature of the color steel plate machine room can even reach above 70℃.

On the one hand, even if the air conditioner is normal, the base station will not be able to cool due to the power outage without AC power, especially in summer, which will cause the indoor temperature of the base station to rise sharply, thus affecting the normal operation of the battery. This will increase the water loss inside the valve-controlled sealed battery, reduce the electrolyte saturation (reduced electrolyte in the glass fiber cotton diaphragm), reduce the battery capacity and shorten the battery life.

On the other hand, due to the high indoor temperature, the thermal runaway effect of the battery will be aggravated, resulting in the corrosion rate of the positive plate of the battery, deformation and expansion of the plate, bulging and even cracking of the battery shell, etc., which will eventually lead to a rapid decrease in battery capacity and shortened battery life. According to the accelerated life test, if the ambient temperature rises by 10 degrees and the charging voltage is not adjusted, the battery life will be shortened by half.

Fourth, after the base station is powered off, the battery is discharged to the termination voltage, and the battery is not replenished in time, which will also lead to a decrease in battery capacity and a shortened service life.

Since some base stations are located in suburbs or remote mountain villages, the city power supply is poor, the number of city power outages is high and the outage time is long. Once the city power outage occurs, the battery is discharged to the termination voltage, and the city power has not been restored. This may cause the battery to be over-discharged on the one hand, and the battery cannot be replenished in time after discharge on the other hand. According to relevant data, if the battery cannot be replenished in time after discharge, the battery capacity will gradually decrease. After several cycles, the battery life will be significantly shortened.

In addition, the floating charge voltage value of the switching power supply output of some base stations is more than 1V lower than the set value and the display value, causing the battery to be in an undercharged state for a long time.

Despite these problems with batteries used in communication stations, the maintenance system and maintenance personnel of our current major communication operators cannot guarantee timely detection of battery problems, timely maintenance, and timely replacement of outdated batteries to avoid station drop accidents. The battery online monitoring system came into being in such an environment.

3. Necessity of battery monitoring

From the above analysis, communication power supply is the foundation of communication network, and backup battery is the last barrier of the entire communication power supply. Once an accident caused by battery, especially the battery in the communication room, occurs, it will cause huge losses! Therefore, in order to ensure the smooth operation of communication network, various communication operators have strengthened the maintenance and testing of communication room batteries, and introduced strict maintenance procedures of IEEE1189-2005. The current maintenance procedures mainly rely on regular and mandatory verification discharge to monitor the health status and charging status of batteries. However, due to the limitation of the establishment of maintenance technicians, the remote and scattered maintenance locations, and the long time of detection operations, the completion rate of battery maintenance work in the entire communication industry is very low. Now the only feasible maintenance technical means without test blind spots is to use a battery monitoring system. Battery monitoring has the following advantages over traditional regular monitoring;

Reduce the labor time required for manual measurement and inspection of batteries on site.

The data provided by the monitoring system can be used to understand the battery health status in a planned manner without the need to go to the site for inspection according to the calendar, thereby saving costs.

Improve power supply reliability and reduce system downtime through online continuous battery system monitoring.

Replace failing batteries before they affect other batteries in the same group, avoiding mutual impact and extending the life of the overall battery system.

Know which batteries are due for replacement, so you have more initiative in battery procurement and reduce the number of batteries replaced in batches.

Increase customer satisfaction and uptime by reducing the chance of power outages.

Avoid annual revenue loss due to power outages.

The monitoring system's ability to identify batteries with signs of damage enhances system reliability and increases production capacity.

Using our patented impedance measurement technology, reliable trend data will correctly display battery problems at the beginning of battery failure.

No need to rely on voltage measurement to determine battery status. Voltage measurement values cannot accurately indicate the quality and availability of batteries.

The battery monitoring management system has remote monitoring, data collection and trend analysis functions, which can remotely monitor multiple battery systems and improve management efficiency.

It can predict when and where the battery system will be unable to provide DC power supply due to power outages from the power company.

Accurately track and record the real-time performance of the battery during power outages.

Accurately monitor all items that have a direct impact on battery performance:

Although installing an online battery system will increase the cost of operation and maintenance at one time, it is completely worth it in terms of the return of increasing the battery life by 30%, reducing maintenance workload by 75%, reducing maintenance costs by 80%, reducing station drop accidents, and improving communication customer satisfaction.

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