LiFePO4 battery is the safest high specificenergy battery in the field of lithium ion batteries. The discharge voltage of LiFePO4battery is very stable, generally 3.2V. The voltage after the discharge (mainlyrefers to the remaining 10% capacity) changes rapidly, and the cut-off voltageis generally 2.5V. Ambient temperature, especially low temperature, will affectthe 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 is80% of normal temperature, 0 °C It is 90% of normal temperature, and thedischarge capacity change of 0 °C to 20 °C is very small. The low temperatureperformance of LiFePO4 battery is better than that of lead acid battery.
INSTALLATION PRECAUTIONS FOR LIFEPO4
Although the LiFePO4 battery is charged anddischarged at the time of shipment from the factory, if the installation dateof the LiFePO4 battery is far from the factory date, there will be a loss afterlong-term self-discharge capacity. In addition, the LiFePO4 battery isgenerally 60% charged at the factory, and the battery pack should be rechargedat the initial installation. Due to the difference in self-discharge of theindividual cells, there may be a phenomenon in which the voltage at eachbattery terminal is unbalanced. The open circuit voltage must be measuredbefore the LiFePO4 battery pack is installed. The open circuit voltagedifference cannot be greater than 50mV. The battery test should be done andrecorded. With the dummy load, the battery pack can be tested for capacity by0.1C10 and 0.2C5. This test does not need to be connected to the BMS. It onlyneeds to connect the battery packs in series, but the battery cell voltage mustbe strictly checked during the discharge process. The total voltage, dischargecurrent, and cell voltage are measured and recorded. The battery detects thebattery with low voltage of the discharge battery every 10 minutes in the latestage of discharge. If one of the battery terminals stops discharging at 2.5V,it is calculated whether the actual battery discharge capacity is consistentwith the rated capacity of the battery. If it is basically the same, it provesthat the battery is discharged. The test is passed and the battery is charged.If the voltage is discharged to the termination voltage, the difference betweenthe capacity released by the battery pack and the rated capacity is greaterthan 15%, indicating that there may be a problem with the factory capacity ofthe battery pack. Contact the manufacturer for disposal.
FACTORS AFFECTING LIFEPO4
(1) Effect of temperature on LiFePO4
The effect of temperature on LiFePO4 batteriesis much smaller than that of lead-acid batteries, especially in the case ofdischarge. For the temperature characteristics test of two types of batteriesof the same specification, the LiFePO4 battery is 20% higher than the sealedvalve controlled lead acid battery. Due to the electrochemical properties ofthe LiFePO4 material itself, the low temperature charging performance of thebattery is slightly poor, and the charging temperature is required to be higherthan 0 ° C, otherwise the irreversible capacity of the battery will increase asthe temperature decreases. The LiFePO4 battery can still be circulated andcharged well under the environment of about 55 °C.
(2) Effect of intermittent float charging on LiFePO4battery
Since the electrolyte of the LiFePO4 battery isan organic liquid, and the lithium metal is very active, the battery must besealed. The battery generates almost no gas under normal charging anddischarging conditions, and even if the battery is in an intermittent floatingstate for a long time, the battery does not generate a high pressure inside. Inaddition, the backup power supply of the communication base station is in afloating state for a long time, and for the intermittent floating charge of thebattery, the voltage is long at about 3.65V. This voltage is relatively stablefor both the battery plate and the electrolyte, so the LiFePO4 battery is verysuitable for communication base stations.
BMS
Charge, extrusion, acupuncture and other abuseconditions are still safe, but will have a great impact on the cycle life ofthe battery. The production process of LiFePO4 battery is relativelycomplicated, and the consistency difference of single battery is larger thanthat of sealed valve-regulated lead-acid battery, which causes the voltage ofindividual battery cells to rise rapidly in the late stage of charging, and thecommunication base station is in the long-term. The state of duty is not easyto find in time, resulting in a shortened or damaged life of the LiFePO4battery pack. In order to avoid the above phenomenon, it is necessary to useBMS to ensure the safety and reliability of the battery. BMS is a core componentof battery protection and management, not only to ensure the battery is safeand reliable, but also to fully utilize the performance of the battery andextend its service life. As a backup power for communication, the BMS acts as abridge between the switching power supply and the battery. The BMS securitymanagement mode is critical to the security of the battery, including the dataacquisition unit, the calculation and control unit, the equalization unit, thecontrol execution unit and the communication unit.
LIFEPO4 BATTERY SHOULD BE USED WITH SWITCHINGPOWER SUPPLY
In view of the characteristics of the LiFePO4battery described above, it is only necessary to adjust the float voltage orthe average charging voltage to the charging voltage required for the LiFePO4battery when selecting or setting the power source device. What needs to bespecially stated here is that, compared with the sealed valve-regulatedlead-acid battery, the charging process of the LiFePO4 battery is not thecharging mode of the floating charge plus the equalization, as long as thecharging voltage required for charging the LiFePO4 battery is satisfied. can. LiFePO4battery does not change its battery performance even if it is in a state ofconstant charge for a long time. The LiFePO4 battery has obvious advantages inthis respect. In addition, the real-time monitoring of the switching powersupply, voltage limiting current limit, automatic floating charge automaticconversion, battery temperature compensation, intelligent management andsecondary power-off settings can directly refer to the setting of lead-acidbatteries, just increase the charging voltage The charging voltage required forthe LiFePO4 battery. For example, a mobile base station uses two groups of48-300 Ah LiFePO4 batteries. In order to ensure power supply safety whenreplacing batteries, two sets of LiFePO4 batteries are tested offline. Aftertesting the total voltage of each group of batteries, adjust the DC powersupply voltage of the switching power supply to correspond to it, andrespectively connect two sets of iron-lithium batteries into the system tosupplement the electricity. At this time, the switching power supply chargingvoltage 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 and31A respectively. As the battery pack voltage increases, the charging currentwill decrease accordingly. When the battery pack voltage and the chargingvoltage of the switching power supply are very close, the charging current ofthe battery pack is gradually reduced. The small is 0. At this time, the BMS ofthe battery pack plays a protective role. The charging system is disconnectedfrom the battery through the BMS, and the battery is charged. After fullcharge, the two groups of batteries were static for 8 h for discharge test. Inorder to test the high current discharge capability of the two sets of batterypacks, a 0.2 C5 current discharge was used, that is, 120 A. The total dischargetime is 5h, the current displayed in the intelligent load cabinet display isbasically 120A, and the capacity released by the battery pack is 600Ah. The LiFePO4battery emits 100% of the rated capacity, so the capacity discharged by thebattery pack fully meets the rated capacity requirement, and the intelligentdummy 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 LiFePO4battery, the measurement of the battery terminal voltage cannot be performedonly in the intermittent floating state, and should also be detected in thedischarge state. The battery terminal voltage is an important parameter thatreflects the battery's operating conditions. Due to the presence of an appliedvoltage, the battery terminal voltage is measured in an intermittent floatingstate, and the voltage thereof is liable to cause an artifact. Therefore, thebattery should be periodically tested with a load-on-charge test every year torelease 30% to 40% of the actual capacity of the battery; and the batterymonitoring system can be used for real-time detection and printing andarchiving; and the contact condition of the battery connection bar is checked.The battery connecting strip is loosely fastened to ensure safe and stableoperation of the battery.
(2) Battery capacity discharge test
In the current communication power supplysystem, the switching power supply and the LiFePO4 battery are connected inparallel intermittent charging, so the battery pack cannot be separated fromthe power supply system, and the capacity test of the battery cannot beperformed separately. Due to the low environmental temperature requirements andlong battery life of the LiFePO4 battery, the capacity test of the battery packcan be carried out every 3 years. After the battery is used for 6 years, thecapacity test is carried out once a year, and more than 80% of the requireddischarge capacity is qualified. . In general, this test is carried out underconditions of actual load (or actual load plus dummy load). In order to ensurethat the DC power supply system is safely and reliably supplied under suchconditions, the diesel generator set should be inspected to ensure that thediesel generator set is powered normally. At the same time, for the loadcondition of the DC power supply system, the discharge rate of the battery packis determined, which is in accordance with the 3-hour rate. 5 hour rate or 10hour rate discharge, 3 hour rate discharge current is 0.25C3, 5 hour ratedischarge 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. Sincethe LiFePO4 battery has BMS protection, when the battery capacity test isperformed, 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 DCdischarge meter is used to detect the battery discharge current to ensure thepower supply safety of the base station.
MAINTENANCE OF LIFEPO4 BATTERY
(1) Environmental requirements
According to the environmental requirements ofthe battery, the room temperature should not exceed 55 °C. It is recommendedthat the ambient temperature should be kept at 0-55 °C; avoid direct sunlightto the battery, and the sun-facing windows should be shaded; ensure sufficientmaintenance space between the battery packs.
(2) Notes
LiFePO4 batteries of different specificationsand different types are forbidden to be used in the same DC power supplysystem. The batteries with different old and new levels should not be mixed inthe same DC power supply system. If there is a centralized monitoring system forpower and environment, the total voltage, current, cell voltage and temperatureof the battery pack should be monitored in real time through the centralizedmonitoring system and the BMS. At the same time, through the battery monitoringdevice 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 andconnecting 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 theconnection rises? Abnormal; according to the technical parameters provided bythe manufacturer and the on-site environmental conditions, check whether thebattery pack and the cell voltage meet the requirements, and check whether thecharging current of the battery pack intermittently floats within the requiredrange; detect the charging current limit value setting of the battery packWhether the low voltage alarm and high voltage alarm settings are correct. Ifthere is a secondary power-off device in the DC power supply system, it shouldbe checked whether the secondary power-off voltage setting is correct.
(4) Voltage requirements
The intermittent charging voltage of the LiFePO4battery is generally set to 3.60 to 3.76V for the single battery and 56.4V forthe battery. The battery pack voltage and the terminal voltage of the singlecell are measured monthly.