LiFePO4 battery is the safest high specific energy battery in the field of lithium ion batteries. The discharge voltage of LiFePO4 battery is very stable, generally 3.2V. The voltage after the discharge (mainly refers to the remaining 10% capacity) changes rapidly, and the cut-off voltage is generally 2.5V. Ambient temperature, especially low temperature, will affect the discharge capacity of LiFePO4 battery: discharge capacity at -20 °C is 45% of normal temperature capacity, -10 °C is 65% of normal temperature, -5 °C is 80% of normal temperature, 0 °C It is 90% of normal temperature, and the discharge capacity change of 0 °C to 20 °C is very small. The low temperature performance of LiFePO4 battery is better than that of lead acid battery.
INSTALLATION PRECAUTIONS FOR LIFEPO4
Although the LiFePO4 battery is charged and discharged at the time of shipment from the factory, if the installation date of the LiFePO4 battery is far from the factory date, there will be a loss after long-term self-discharge capacity. In addition, the LiFePO4 battery is generally 60% charged at the factory, and the battery pack should be recharged at the initial installation. Due to the difference in self-discharge of the individual cells, there may be a phenomenon in which the voltage at each battery terminal is unbalanced. The open circuit voltage must be measured before the LiFePO4 battery pack is installed. The open circuit voltage difference cannot be greater than 50mV. The battery test should be done and recorded. With the dummy load, the battery pack can be tested for capacity by 0.1C10 and 0.2C5. This test does not need to be connected to the BMS. It only needs to connect the battery packs in series, but the battery cell voltage must be strictly checked during the discharge process. The total voltage, discharge current, and cell voltage are measured and recorded. The battery detects the battery with low voltage of the discharge battery every 10 minutes in the late stage of discharge. If one of the battery terminals stops discharging at 2.5V, it is calculated whether the actual battery discharge capacity is consistent with the rated capacity of the battery. If it is basically the same, it proves that the battery is discharged. The test is passed and the battery is charged. If the voltage is discharged to the termination voltage, the difference between the capacity released 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. Contact the manufacturer for disposal.
FACTORS AFFECTING LIFEPO4
(1) Effect of temperature on LiFePO4
The effect of temperature on LiFePO4 batteries is much smaller than that of lead-acid batteries, especially in the case of discharge. For the temperature characteristics test of two types of batteries of the same specification, the LiFePO4 battery is 20% higher than the sealed valve controlled lead acid battery. Due to the electrochemical properties of the LiFePO4 material itself, the low temperature charging performance of the battery is slightly poor, and the charging temperature is required to be higher than 0 ° C, otherwise the irreversible capacity of the battery will increase as the temperature decreases. The LiFePO4 battery can still be circulated and charged well under the environment of about 55 °C.
(2) Effect of intermittent float charging on LiFePO4 battery
Since the electrolyte of the LiFePO4 battery is an organic liquid, and the lithium metal is very active, the battery must be sealed. The battery generates almost no gas under normal charging and discharging conditions, and even if the battery is in an intermittent floating state for a long time, the battery does not generate a high pressure inside. In addition, the backup power supply of the communication base station is in a floating state for a long time, and for the intermittent floating charge of the battery, the voltage is long at about 3.65V. This voltage is relatively stable for both the battery plate and the electrolyte, so the LiFePO4 battery is very suitable for communication base stations.
Charge, extrusion, acupuncture 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 relatively complicated, and the consistency difference of single battery is larger than that of sealed valve-regulated lead-acid battery, which causes the voltage of individual battery cells to rise rapidly in the late stage of charging, and the communication base station is in the long-term. The state of duty is not easy to find in time, resulting in a shortened or damaged life of the LiFePO4 battery pack. In order to avoid the above phenomenon, it is necessary to use BMS to ensure the safety and reliability of the battery. BMS is a core component of battery protection and management, not only to ensure the battery is safe and reliable, but also to fully utilize the performance of the battery and extend its service life. As a backup power for communication, the BMS acts as a bridge between the switching power supply and the battery. The BMS security management mode is critical to the security of the battery, including the data acquisition unit, the calculation and control unit, the equalization unit, the control execution unit and the communication unit.
LIFEPO4 BATTERY SHOULD BE USED WITH SWITCHING POWER SUPPLY
In view of the characteristics of the LiFePO4 battery described above, it is only necessary to adjust the float voltage or the average charging voltage to the charging voltage required for the LiFePO4 battery when selecting or setting the power source device. What needs to be specially stated here is that, compared with the sealed valve-regulated lead-acid battery, the charging process of the LiFePO4 battery is not the charging mode of the floating charge plus the equalization, as long as the charging voltage required for charging the LiFePO4 battery is satisfied. can. LiFePO4 battery does not change its battery performance even if it is in a state of constant charge for a long time. The LiFePO4 battery has obvious advantages in this respect. In addition, the real-time monitoring of the switching power supply, voltage limiting current limit, automatic floating charge automatic conversion, battery temperature compensation, intelligent management and secondary power-off settings can directly refer to the setting of lead-acid batteries, just increase the charging voltage The charging voltage required for the LiFePO4 battery. For example, a mobile base station uses two groups of 48-300 Ah LiFePO4 batteries. In order to ensure power supply safety when replacing batteries, two sets of LiFePO4 batteries are tested offline. After testing the total voltage of each group of batteries, adjust the DC power supply voltage of the switching power supply to correspond to it, and respectively connect two sets of iron-lithium batteries into the system to supplement the electricity. At this time, the switching power supply charging voltage is set to 56.4V, and the charging current is limited to 30A per group. The initial charging currents of the two groups of battery packs are 29A and 31A respectively. As the battery pack voltage increases, the charging current will decrease accordingly. When the battery pack voltage and the charging voltage of the switching power supply are very close, the charging current of the battery pack is gradually reduced. The small is 0. At this time, the BMS of the battery pack plays a protective role. The charging system is disconnected from the battery through the BMS, and the battery is charged. After full charge, the two groups of batteries were static for 8 h for discharge test. In order to test the high current discharge capability of the two sets of battery packs, a 0.2 C5 current discharge was used, that is, 120 A. The total discharge time is 5h, the current displayed in the intelligent load cabinet display is basically 120A, and the capacity released by the battery pack is 600Ah. The LiFePO4 battery emits 100% of the rated capacity, so the capacity discharged by the battery pack fully meets the rated capacity requirement, and the intelligent dummy load is very stable during the entire discharge process.
TEST OF LIFEPO4 BATTERY
(1) Loaded nuclear discharge test
According to the working characteristics of the LiFePO4 battery, the measurement of the battery terminal voltage cannot be performed only in the intermittent floating state, and should also be detected in the discharge state. The battery terminal voltage is an important parameter that reflects the battery's operating conditions. Due to the presence of an applied voltage, the battery terminal voltage is measured in an intermittent floating state, and the voltage thereof is liable to cause an artifact. Therefore, the battery should be periodically tested with a load-on-charge test every year to release 30% to 40% of the actual capacity of the battery; and the battery monitoring system can be used for real-time detection and printing and archiving; and the contact condition of the battery connection bar is checked. The battery connecting strip is loosely fastened to ensure safe and stable operation of the battery.
(2) Battery capacity discharge test
In the current communication power supply system, the switching power supply and the LiFePO4 battery are connected in parallel intermittent charging, so the battery pack cannot be separated from the power supply system, and the capacity test of the battery cannot be performed separately. Due to the low environmental temperature requirements and long battery life of the LiFePO4 battery, the capacity test of the battery pack can be carried out every 3 years. After the battery is used for 6 years, the capacity test is carried out once a year, and more than 80% of the required discharge capacity is qualified. . In general, this test is carried out under conditions of actual load (or actual load plus dummy load). In order to ensure that the DC power supply system is safely and reliably supplied under such conditions, the diesel generator set should be inspected to ensure that the diesel generator set is powered normally. At the same time, for the load condition of the DC power supply system, the discharge rate of the battery pack is determined, which is in accordance with the 3-hour rate. 5 hour rate or 10 hour rate discharge, 3 hour rate discharge current is 0.25C3, 5 hour rate discharge current is 0.168C5, 10 hour rate discharge current is 0.10C10, preferably at ambient temperature of 20 °C at 10 hour rate Capacity test. Since the LiFePO4 battery has BMS protection, when the battery capacity test is performed, the floating voltage of the switching power supply is set to 45V, and then the AC input of the switching power supply is turned off, and the DC discharge meter is used to detect the battery discharge current to ensure the power supply safety of the base station.
MAINTENANCE OF LIFEPO4 BATTERY
(1) Environmental requirements
According to the environmental requirements of the battery, the room temperature should not exceed 55 °C. It is recommended that the ambient temperature should be kept at 0-55 °C; avoid direct sunlight to the battery, and the sun-facing windows should be shaded; ensure sufficient maintenance space between the battery packs.
LiFePO4 batteries of different specifications and different types are forbidden to be used in the same DC power supply system. The batteries with different old and new levels should not be mixed in the same DC power supply system. If there is a centralized monitoring system for power and environment, the total voltage, current, cell voltage and temperature of the battery pack should be monitored in real time through the centralized monitoring system and the BMS. At the same time, through the battery monitoring device to understand the battery charge and discharge curve and performance, regular measurements, timely detection of faults and processing.
(3) Inspection items
Frequently check whether the poles and connecting strips of LiFePO4 battery are loose, damaged, deformed or corroded; whether there is looseness at the joint, whether the battery case is damaged, leaked or deformed, and whether the temperature of the battery and the connection rises? Abnormal; according to the technical parameters provided by the manufacturer and the on-site environmental conditions, check whether the battery pack and the cell voltage meet the requirements, and check whether the charging current of the battery pack intermittently floats within the required range; detect the charging current limit value setting of the battery pack Whether the low voltage alarm and high voltage alarm settings are correct. If there is a secondary power-off device in the DC power supply system, it should be checked whether the secondary power-off voltage setting is correct.
(4) Voltage requirements
The intermittent charging voltage of the LiFePO4 battery is generally set to 3.60 to 3.76V for the single battery and 56.4V for the battery. The battery pack voltage and the terminal voltage of the single cell are measured monthly.