Analysis of the application of LiFePO4 batteries in base stations

LiFePO4 batteries are the safest high-ratio energy batteries in the lithium-ion battery family. The discharge voltage of LiFePO4 battery is very stable, generally 3.2V, the voltage changes rapidly in the later stage of discharge (mainly refers to the remaining 10% capacity), and the cut-off voltage is generally 2.5V. Ambient temperature, especially low temperature, will affect the discharge capacity of LiFePO4 battery: the discharge capacity of -20 ℃ is 45% of the normal temperature capacity, -10 ℃ is 65% of the normal temperature, -5 ℃ is 80% of the normal temperature, 0 ℃ is 90% of the room temperature, 0 ℃ to 20 ℃ of the discharge capacity change is very small. The low temperature performance of LiFePO4 battery is better than that of lead-acid battery.

Installation considerations.

Although the lithium phosphate battery at the factory positive and negative plates have been charged and discharged, but if the installation date of LiFePO4 battery from the factory date is far away, after a long period of self-discharge capacity will inevitably be lost. In addition, LiFePO4 battery at the factory charge is generally 60%, the initial installation should be recharge the battery pack. Due to the difference in self-discharge of the unit battery, the voltage imbalance at each end of the battery may occur. The open circuit voltage must be measured before the LiFePO4 battery pack is installed, the open circuit voltage difference should not be greater than 50mV, and the battery test should be done and recorded. With false load, the battery pack can be pressed 0.1C10 and 0.2C5 capacity test, this test does not need to be connected to the battery management system, only the battery pack series, but the discharge process must strictly detect the battery unit voltage, the total voltage of the battery per hour, discharge current, battery unit voltage measurement and recording. Battery in the discharge after every 10min detection discharge battery unit voltage low battery, if there is a battery end voltage to 2.5V immediately stop discharge, calculate the actual battery discharge capacity and battery rated capacity is consistent, if basically consistent to prove that the battery discharge test qualified, and then charge the battery. If discharge to termination voltage, the difference between the capacity discharged by the battery pack and the rated capacity is greater than 15%, indicating that there may be a problem with the factory capacity of the battery pack, the manufacturer should be promptly contacted to deal with.

Influencer factors.

1.    Effect of temperature on LiFePO4 batteries.

The effect of temperature on LiFePO4 batteries is much smaller than that of lead-acid batteries, especially in the case of discharge. For temperature characteristics testing of two batteries of the same size, LiFePO4 batteries have a 20% higher capacity than sealed valve-controlled lead-acid batteries. Due to the electrochemical properties of the lithium phosphate material, the low-temperature charging performance of the battery is slightly worse, the charging temperature is higher than 0 ℃, otherwise the irreversible capacity of the battery will increase with the decrease of temperature. LiFePO4 batteries can still be recycled and discharged in an environment of about 55 ℃.

2.     Effect of intermittent floating charging on LiFePO4 batteries.

Since the electrolyte of the lithium phosphate battery is an organic liquid, and lithium metal is very active, the battery must be sealed. The battery produces almost no gas under normal charge and discharge conditions, and even if the battery is intermittently charged for a long time, there will be no higher pressure inside the battery. In addition, the backup power supply of the communication base station is in the floating charge state for a long time, and the voltage of the battery is about 3.65V for the long time. This voltage is stable for both the battery plate and the electrolyte, so LiFePO4 batteries are ideal for communication base stations.

BMS

Charge and discharge, extrusion, needles and other abuse conditions are still safe, but will have a great impact on the cycle life of the battery. The production process of LiFePO4 battery is more complex, the consistency difference of single battery will be longer than the sealing valve-controlled lead-acid battery, which leads to the rapid rise of the voltage of individual unit batteries in the later charging period of the battery pack, coupled with the communication base station long-term unattended state, not easy to find in time, resulting in a short life of lithium phosphate battery pack or damage phenomenon. In order to avoid the above phenomenon, BMS is required to ensure the safety and reliability of the battery. BMS is a core component of battery protection and management, not only to ensure the safety and reliability of the battery, but also to give full play to the performance of the battery and extend its life. As a backup power for communication, BMS acts as a bridge between switching power supplies and batteries. BMS's safety management mode is critical to battery safety and includes data acquisition units, computing and control units, balancing units, control execution units and communication units.

LiFePO4 batteries should be used in support of switching power supplies.

In view of the characteristics of the above-mentioned LiFePO4 battery, in the selection or setting of power supply equipment, only the floating charging pressure or the average charging voltage is adjusted to the charging voltage required for the LiFePO4 battery. It should be noted here that, as opposed to sealed valve-controlled lead-acid batteries, the charging process of LiFePO4 batteries is not a float-charged charging mode, as long as the charging voltage required to charge the lithium phosphate battery can be met. Iron phosphate lithium battery even if the long-term average charging state, due to its own protection function, battery performance will not change, LiFePO4 battery in this regard is very obvious advantages. In addition, the real-time monitoring of switching power supply, regulatory limit flow, automatic conversion of floating charge, battery temperature compensation, intelligent management and secondary power down and other functions of the setting can directly refer to the lead-acid battery settings, only need to increase the charging voltage to the LiFePO4 battery required charging voltage. For example, a mobile base station uses 48 to 300Ah LiFePO4 battery two sets, in order to replace the battery in order to ensure the safety of power supply, two sets of LiFePO4 battery offline testing. After testing the total voltage of each set of batteries, adjust the switching power supply DC supply voltage corresponding to each other, respectively, two sets of LiFePO4 battery access system to supplement the power. At this point, the switching power charging voltage is set to 56.4V and the charging current is limited to 30A per set. The initial charging current of the two battery packs is 29A and 31A, respectively, as the battery pack voltage increases, the charging current will decrease accordingly, when the battery pack voltage and switching power supply charging voltage is very close, the charging current of the battery pack is gradually reduced to 0, at this time the battery pack BMS played a protective role, the charging system through the BMS and battery disconnect, battery charging is complete. When fully charged, the two sets of batteries are stationary 8h for discharge testing. In order to test the high current discharge capacity of the two battery packs, 0.2C5 current discharge is used, i.e. 120A. The total discharge time is 5h, the current shown in the smart load cabinet display is basically 120A, the battery pack released capacity of 600Ah. LiFePO4 battery released 100% of the rated capacity, so the battery pack released the capacity to fully meet the rated capacity requirements, the entire discharge process intelligent false load is very stable.

Test of LiFePO4 battery.

(1)   The test of the discharge of sex.

According to the operating characteristics of LiFePO4 battery, the measurement of battery end voltage should not only be in the intermittent floating state, but also in the discharge state of detection. The voltage at the end of the battery is an important parameter to reflect the working condition of the battery. Due to the presence of the added voltage, the voltage of the battery end voltage is measured in the intermittent floating charge state, and its voltage can easily create the illusion. Therefore, the battery should be regularly carried out on a regular basis with a checked discharge test, released 30% to 40% of the actual capacity of the battery, and the battery monitoring system for real-time testing and printing archive of the battery pack, while checking the battery connection strip contact, the battery connection strip is loose to fasten, to ensure the safe and stable operation of the battery.

(2)   Quantity discharge test.

In the current communication power supply system, switching power supply and LiFePO4 battery for the side-by-side intermittent floating charge power supply, so the battery pack can not be separated from the power supply system, can not be separated to the battery capacity test. Due to the low environmental temperature requirements of lithium phosphate battery, long battery life and so on, the battery pack can be tested every 3 years, the battery after 6 years of annual capacity test, requiring more than 80% of the released capacity is qualified. In general, such tests are carried out under conditions with actual load (or actual load plus false load). In order to ensure the safe and reliable supply of dc power in this case, the diesel generator set should be inspected to ensure that the diesel generator set is properly powered, and the discharge multiple of the battery pack is determined to meet the discharge rate of 3 hours, 5 hours or 10 hours for the load of the DC power supply system. The 3-hour rate discharge current is 0.25C3, the 5-hour rate discharge current is 0.168C5, and the 10-hour rate discharge current is 0.10C10, preferably at an ambient temperature of about 20 ℃ at a capacity test of 10 hours. Because the lithium phosphate battery has BMS protection, the floating charging voltage of the switching power supply is set to 45V when the battery capacity test is carried out, and then the AC input of the switching power supply is disconnected, and the discharge current of the battery is detected with the DC clamp type meter to ensure the safety of the base station power supply.

Maintenance of LiFePO4 batteries.

(1)   Environmental requirements.

According to the environmental requirements of the battery, room temperature should not exceed 55 ℃, it is recommended that the ambient temperature should be kept at 0 to 55 ℃;

(2)   Precautions.

LiFePO4 batteries of different sizes and models are prohibited from using in the same DC power supply system, and batteries of different degrees, old and new, should not be mixed in the same DC power supply system. If there is a dynamic and environmental centralized monitoring system, the total voltage, current, unit voltage and temperature of the battery pack should be monitored in real time through the dynamic environment centralized monitoring system and BMS. At the same time, through the battery monitoring device to understand the battery charge and discharge curve and performance, regular measurement, timely detection of faults and treatment.

(3)   Check the item.

The pole column and connecting strip of LiFePO4 battery should be checked frequently for looseness, damage, deformation or corrosion, looseness at the connection, damage, leakage and deformation of the battery housing, abnormal temperature rise at the battery and connection, according to the manufacturer. Provide technical parameters and on-site environmental conditions, check whether the battery pack and unit voltage meet the requirements, detect whether the charging current when the battery pack is intermittently charged is within the required range, and detect whether the battery pack charging limit setting, low voltage alarm, high voltage alarm setting is correct. If there is a secondary power-down device in the DC power supply system, the secondary down-voltage setting should be checked correctly.

(4)   Voltage requirements.

The intermittent charging voltage of LiFePO4 battery is generally set to 3.60 to 3.76V for single battery and 56.4V for battery pack. Measure the battery pack voltage and the end voltage of the unit battery on a monthly level.