Comprehensive solution for batteries in harsh application environments

Communication DC power supply is a basic telecommunication equipment, providing power supply guarantee for communication main equipment. Battery pack is an important component of communication DC power supply system, equivalent to backup power supply, and is the last line of defense of communication DC power supply system.

In the early development of telecommunication business, operators paid little attention to batteries, and it was enough for batteries to provide power when AC power outage occurred. In recent years, competition in the field of telecommunication operation has intensified and intensified. Operators are very concerned about the service life, maintenance workload, and TCO of batteries, and their requirements are getting higher and higher.

With the development of communication networks and technological advances, in order to save construction costs and speed up the construction cycle, operators often do not build computer rooms or mobile cabins in urban-rural fringe areas, small towns and rural areas, but use outdoor cabinet solutions to place communication main equipment and DC power supply systems. In recent years, the proportion of outdoor base stations in the newly built base stations of mainstream operators around the world has increased year by year. For low-latitude and desertified countries or regions (such as South Asia, Africa, etc.), high temperatures have a great impact on outdoor base stations. Outdoor base stations are generally located in remote areas with poor power supply, especially in developing countries. Outdoor base stations often face harsh working environments with high temperatures and frequent power outages. Outdoor applications of communication DC power supply systems are becoming more mainstream, and batteries are often in harsh application environments with high temperatures and frequent power outages.

Problems faced by batteries in harsh application environments

With the increasing use of outdoor base stations, battery failures in harsh application environments have gradually become prominent, such as in South Asia, such as Pakistan and India, causing economic losses to operators and damaging their customer satisfaction. In response to the large number of battery damages in harsh application environments, ZTE conducted extensive research, gained an in-depth understanding of battery application scenarios, and investigated and analyzed the causes of battery failures. The key to the problem is not the battery itself, but the fact that the outdoor battery cabinet does not consider high temperature protection for the battery. In order to fundamentally solve the problem, a comprehensive solution for the application of batteries in harsh outdoor environments must be provided.

Comparative analysis of active cooling technologies for outdoor battery cabinets

There are many options for heat dissipation in outdoor cabinets. Which heat dissipation method is suitable for outdoor battery cabinets? This starts with the product characteristics of batteries. For lead-acid batteries in communication DC power supply systems, users are most concerned about service life. The main factors affecting the service life of lead-acid batteries are ambient temperature and grid conditions.

The service life of lead-acid batteries is closely related to the ambient temperature. The higher the ambient temperature, the shorter the battery life. When the ambient temperature is higher than the battery design life requirement temperature (25 ºC), the service life is shortened by half for every 10 ºC increase in temperature.

The number of times a battery is discharged and the depth of discharge directly affect the battery life. The more times a battery is discharged and the deeper the discharge depth, the shorter the battery life. In other words, frequent power outages in the power grid will reduce the battery life.

For outdoor base stations, operators are usually unable to improve grid conditions or the cost of improving grid conditions is too high and unaffordable, so we start by lowering the working environment temperature of the battery to increase the battery life.

The traditional heat dissipation method of outdoor cabinets is direct ventilation by fans or heat exchangers, but neither of these two methods can make the temperature inside the cabinet lower than the ambient temperature outside the cabinet. For application scenarios in high-temperature areas (generally above 40°C), active heat dissipation is required to make the temperature inside the outdoor battery cabinet lower than the ambient temperature outside the cabinet. ZTE breaks through conventions, combines innovations, and introduces refrigeration components into outdoor battery cabinets.

The main communication equipment (such as GSM, transmission, etc.) and the power changing part (rectifier) ​​of the DC power supply will generate heat during the operation of the equipment, but the battery is different. According to the electrochemical mechanism of battery charging and discharging, the battery does not generate heat when discharging. It basically does not generate heat during normal charging (not overcharging). That is, the heat generated by the battery during normal use can be ignored. Therefore, there is no heat source in the outdoor battery cabinet, and the required cooling capacity is small. According to calculations, under normal circumstances, the outdoor battery cabinet only needs a cooling capacity of 200-400W. Thermoelectric Cooler (TEC) air conditioners use emerging semiconductor refrigeration technology. For the application scenarios of outdoor battery cabinets, TEC air conditioners have many advantages over traditional compressor air conditioners:

1) Simple structure and high reliability. The entire refrigerator is connected by a thermoelectric cooling module and wires. It does not require a compressor and has no mechanical rotating parts, so there is no vibration, friction or noise. It has high reliability and long life (life is greater than 100,000 hours at 32°C).

2) Refrigeration is not affected by AC power outages. With a DC 48V power supply, the battery will power the TEC air conditioner when the AC power is off, and refrigeration can still be achieved in the outdoor battery cabinet.

3) Refrigeration efficiency and cooling capacity. In the case of large capacity, the efficiency of thermoelectric refrigeration is not as good as that of vapor compression refrigeration. However, the efficiency of vapor compression refrigerators decreases as the capacity decreases, and the compressor cannot be made too small. The efficiency of thermoelectric refrigeration has nothing to do with the capacity size, and it has advantages in applications with small cooling loads. For outdoor battery cabinet applications (small cooling load), the use of TEC air conditioners is an ideal choice.

4) Small size. Especially suitable for outdoor cabinet installation.

5) Cost-effectiveness. Based on the above analysis, the outdoor battery cabinet uses TEC air conditioners, which are cost-effective.

6) Easy maintenance. TEC air conditioners do not require refrigerant circulation and compressor rotation. You only need to regularly check that the insect screen is not blocked. The maintenance workload is very small.

7) Environmentally friendly. It does not use Freon refrigerants, does not damage the atmospheric ozone layer, and is environmentally friendly.

Based on the above analysis, we selected TEC air conditioners with comprehensive performance superior to traditional compressor air conditioners as the refrigeration components of the outdoor battery cabinet.

Series TEC outdoor battery cabinet with good high temperature protection capability

TEC outdoor battery cabinet consists of TEC air conditioner, cabinet, thermal insulation layer, etc. In order to meet the needs of installing batteries of different capacities and types, ZTE has designed a series of TEC battery cabinets. Two typical cabinets are shown in Figure 1. The cabinet on the left can install up to 2 groups of narrow 12V 150Ah AGM or gel batteries. The cabinet on the right can install up to 6 groups of narrow 12V 150Ah AGM or gel batteries (3 groups of wide 12V 150Ah or 200Ah gel batteries can also be installed). In order to achieve the ideal cooling effect, the cabinet is required to have good thermal insulation effect. We have professionally designed the thermal insulation cotton, trying to use the whole raw material to reduce openings and cutting, and the thermal insulation cotton is 40mm thick. Like the TEC module, it adopts IP55 protection level design.

TEC battery cabinet fully considers the cooling effect and product reliability when designing the product, and uses advanced cooling chips, high-reliability fans, and thermal insulation cotton with good thermal insulation. TEC battery cabinet has good high temperature protection capabilities.

Figure 1 Appearance of two typical TEC battery cabinets

Comprehensive solution to extend battery life and reduce TCO

For outdoor applications of communication DC power supply systems in harsh application environments, the key point of the comprehensive solution is to greatly improve the high temperature protection capability of the outdoor battery cabinet by using TEC air conditioners, and significantly improve the working environment of the battery. The experimental test results of the outdoor battery cabinet using TEC air conditioners show that when the ambient temperature outside the cabinet is above 40°C, the temperature inside the cabinet is 15°C lower than the temperature outside the cabinet, and the battery works within the optimal operating temperature range. Compared with the ordinary outdoor battery cabinets of the operator's existing network, the service life of the battery using the TEC outdoor battery cabinet has increased by 1.5 times, which is a very large increase.

The comprehensive solution uses TEC outdoor battery cabinets. By adding some cost investment to the battery cabinets, valuable high-temperature protection capabilities are obtained, so that the batteries can work within the optimal operating temperature range, avoiding the batch replacement of batteries due to low service life, and avoiding the battery procurement costs, related transportation costs and replacement labor costs required for batch replacement of batteries on site, thereby reducing the operator's operation and maintenance costs. The significant increase in battery service life effectively guarantees the safe and reliable operation of the communication network and significantly improves the operator's customer satisfaction. Therefore, the comprehensive solution reduces TCO (Total Cost of Ownership) for operators while increasing the battery service life.

Application Examples and User Value

The comprehensive solution has been put into practical use in Pakistan. In mid-May 2010, the TEC battery cabinet arrived at an outdoor base station of a certain operator in Pakistan, and the DC power supply system of the station was transformed on-site using the comprehensive solution. After the TEC battery cabinet was put into use, its cooling capacity was tested. The temperature trend curve generated based on the on-site test data is shown in Figure 2. T2 is the internal temperature of the TEC battery cabinet, Tc is the outdoor ambient temperature (measured in the shade under the awning), and ΔT is the temperature difference between the ambient temperature and the temperature inside the cabinet.

Figure 2 TEC battery cabinet temperature trend curve

From the test results, it can be seen that during the period between 12:00 noon and 5:00 pm, when the outdoor ambient temperature is high and reaches up to 45°C, the TEC battery cabinet can achieve a cooling effect with a temperature difference of 15°C between the inside and outside. The internal temperature of the battery cabinet can be controlled between 25°C and 30°C, meeting the requirements for the optimal operating temperature range of the battery.

Through the on-site transformation of the DC power supply system of an outdoor station of an operator in Pakistan, the user highly recognized ZTE's comprehensive solution and believed that the adoption of the comprehensive solution can greatly improve the survivability of batteries in harsh application environments, significantly reduce the maintenance workload of batteries, and effectively reduce the comprehensive TCO of the outdoor station DC power supply system.