Withenergy shortages and environmental pollution issues becoming increasinglyprominent, electric vehicles have received widespread attention for theirenergy saving and environmental protection advantages. When the capacity of thepower battery used in electric vehicles falls to the extent that it does notmeet the requirements for the range of electric vehicles, it is necessary todecommission the power batteries. As the electric vehicle market becomes moreprosperous, the problem of the "outlet" of retired power batteries isbecoming increasingly apparent. The capacity of the electric vehicle's powerbattery has declined to 80%, and it has been retired due to insufficientbattery life. However, it can still be used for base station power backup aftera step utilization process. The vehicle power battery pack uses the 48V backuppower supply for communication as the basic module. The electric vehicle powerbattery is connected in series and connected through multiple sets of 48Vmodules to form a vehicle power battery module for powering the car. Can bedirectly applied in the field of communication.
BASICCHARACTERISTICS OF LADDER LIFEPO4 BATTERY
1. Batterycapacity rate characteristics
Asthe discharge current increases, the discharge capacity of the battery willdecrease. When the discharge rate is less than 0.33C10, the discharge capacityof the lithium-ion battery is little affected by the discharge rate, and thedifference in discharge capacity is not large. The capacity can be released100%.
2. Temperaturecharacteristics of battery capacity
Whenthe ambient temperature is above 0 ° C, the battery capacity decays slowly, andwhen the ambient temperature is below 0 ° C, the battery capacity decaysfaster, and the internal resistance of the battery increases sharply as thetemperature decreases.
3. Comparisonof Ladder LiFePO4 Battery with Traditional Lead Acid
1.High temperature resistance: The stable operating temperature range oflead-acid battery is 25 ～ 28 ℃. The temperature rise will damage the battery and reduce thebattery life.
2.High energy density: The weight specific energy of LiFePO4 battery products canexceed 130Wh / kg (0.2C, 25 ° C), the volume specific energy is 210Wh / L; theweight specific energy of lead-acid battery products is 32 ～ 37Wh / kg (0.2 C, 25℃), volume specific energy is 70Wh / L.
3.High-current charge and discharge performance: The LiFePO4 battery can becharged and discharged quickly at a large current of 2C, and the startingcurrent can reach more than 5C. Lead-acid batteries do not have thisperformance now. So the LiFePO4 battery has a short charging time.
4. Environmentalprotection: LiFePO4 battery does not contain any heavy metals and rare metals(nickel metal hydride batteries need rare metals) and is non-toxic (SGScertification passed); a large amount of lead exists in lead-acid batteries.Improper will still cause enough pollution to the environment.
Thecomparison between lead-acid batteries and stepped LiFePO4 batteries is shownin Table 1.
|
Battery performance index |
Lead-Acid |
Ladder LiFePO4 battery |
|
Cycle life (times) |
200 |
1200~2000 |
|
Mass specific energy (Wh/kg) |
30~45 |
60~110 |
|
Volume specific energy (Wh/L) |
70 |
125~250 |
|
Mass specific power (W/kg) |
200~300 |
4000 |
|
Volume specific power (W/L) |
|
10000 |
|
Self-discharge rate |
High |
Low |
|
range of working temperature (℃) |
25~28 |
-20~+55 |
|
Operating voltage range (V) |
1.75~2.35 |
2.5~3.65 |
|
Nominal cell voltage (V) |
2 |
3.2 |
|
Memory effect |
Yes |
No |
5.The use of cascade power lithium batteries has a long service life and a largenumber of cycles. After cascade use, theoretically, it can still have 6 yearsof actual life and 400~2,000 actual cycles, which is 3~6 years compared withthe traditional lead-acid battery The actual number of cycles of 200 times hasbeen greatly improved.
6. Hightemperature resistance, lithium battery meets the use of extreme workingconditions below 45 ° C, and the current upper limit of lead-acid batteriescommonly used in communication base stations is only 35 ° C.
7. Gooddischarge characteristics, high capacity utilization when high currentdischarge.
8.High charge-discharge conversion efficiency, the energy conversion efficiencyof the cascade battery is 10% -15% higher than that of the lead-acid battery.
9. Smallfootprint, light weight, low transportation costs, the weight and volume of thestep battery is 1/2 or 2/3 of the lead-acid battery of the same capacity.
Technicalsolution for the application of ladder LiFePO4 battery (Table 2)
|
Technical solutions |
specific contents |
Supply side |
Processing side |
Processing cost |
|
Cell-level reorganization plan |
Disassembling the cascade battery into the cell level, sorting, reorganizing, and processing into a battery product |
Battery companies |
Battery companies |
High |
|
Module-level reorganization plan |
Disassembling cascaded batteries into module levels, sorting, reorganizing, and processing into battery products |
Battery and automotive companies |
Foundry companies |
Medium |
|
PACK application scheme |
Measure and divide the entire retired battery pack and apply it to the base station |
Automotive companies |
Foundry companies |
Low |
1. Centralizeddisassembly of retired power batteries, centralized screening of cells, andreassembly into standard modules are conducive to centralized screening andmaintenance of retired cells to ensure quality; the source of retired cells isnot limited to the guaranteed number of specific electric vehicle projects; Thefinal battery module can be standardized to ensure compatibility.
2. The direct transformation on the basisof retired power batteries is conducive to the simple modularization of thebattery pack's step utilization, which has advantages in capacity, simple andeasy production methods, low labor costs, but high land requirements.
3.Ladder battery process: Screen battery cells, test voltage, battery cellassembly, internal connection lines, BMS, chassis or rack.
BASICSTRUCTURE OF LADDER LIFEPO4 BATTERY
The LiFePO4battery is composed of a positive electrode plate and a negative electrodeplate (the positive electrode active material is LiFePO4 and the negativeelectrode active material is graphite), a separator, an electrolyte, a tab, andan aluminum plastic film shell. The positive and negative plates are the areasfor electrochemical reactions. The separator and electrolyte provide a Li +transmission channel. After the process of chemical conversion and otherprocesses, a dense SEI film (also called a solid electrolyte interface film)will be formed on the surface of the battery plate. To the role of the pilotcurrent. The positive electrode active material is LiFePO4, which has anolivine structure.
LiFePO4is mixed with a conductive agent and a binder in a certain ratio, and is coatedon an aluminum foil to form a positive electrode. The negative electrode activematerial is usually a graphite material and is attached to a copper foilthrough a binder. Positive and negative electrodes are separated by a polyethyleneseparator (or a polypropylene and polyethylene composite separator) to preventbattery short-circuiting. The separator is a thin film with a porous structure.During the charge and discharge process, Li + can pass through its pores, whileelectrons e- cannot pass through. The electrolyte of the battery is an organicsolvent of lithium hexafluorophosphate.
WORKINGPRINCIPLE OF LADDER LIFEPO4 BATTERY
Whenthe battery is charged, Li + migrates from the LiFePO4 material to the crystalsurface, and is released from the positive plate material. Under the action ofthe electric field force, it enters the electrolyte, passes through theseparator, and then migrates to the surface of the negative graphite crystalthrough the electrolyte. Then it is embedded in the anode layered graphitematerial. At the same time, electrons flow through the aluminum foil of thepositive electrode, through the tab, battery pole, load, negative pole, andnegative electrode to the copper foil electrode of the negative electrode, and thenflow to the graphite negative electrode through the conductor to balance thecharge.
Whenthe battery is discharged, Li + is de-embedded from the layered graphitecrystal, enters the electrolyte, passes through the separator, migrates to thesurface of the LiFePO4 crystal through the electrolyte, and then re-embedded inthe LiFePO4 material. At the same time, electrons flow through the conductor tothe copper foil electrode of the negative electrode, flow through the tab, thebattery negative pole, the load, the positive pole, and the positive poleelectrode to the aluminum foil electrode of the battery, and then flow throughthe conductor to the LiFePO4 positive electrode. Equilibrium the charge.
MANAGEMENTSYSTEM OF LADDER LIFEPO4 BATTERY
Thebattery management system is mainly used to manage the charging and dischargingprocesses of the battery, to improve the battery life, and to provide userswith the general information of the circuit system.
The battery management system BMS iscomposed of monitoring, protection circuits, electrical, communicationinterfaces, and thermal management devices. It is a core component of batteryprotection and management. It must not only ensure the safe and reliable use ofthe battery, but also give full play to the performance and extension of thebattery. Service life, as a backup energy source for communication, themanagement system plays an important bridge role between the switching powersupply and the battery. The requirements for the battery management system mustconform to the requirements of the communication power supply system, so thesafety management mode of the battery management system is very important tothe safety of the battery. The battery management system mainly includes a dataacquisition unit, a calculation and control unit, an equalization unit, acontrol execution unit, and a communication unit.
PRACTICALAPPLICATION OF LIFEPO4 BATTERY PACKS IN TOWER BASE STATIONS
Accordingto the characteristics of the lithium battery pack, when the base station DCswitching power supply application is set, it is only necessary to adjust thefloating charging voltage and the average charging voltage to the chargingvoltage required by the lithium battery pack. (At the same time, it must bewithin the DC power supply voltage range of the communication equipment. )Because the lithium battery pack is in a long-term charging state, the batteryperformance will not change due to its own BMS protection function.
Forexample, a base station backup battery pack uses a 48V 300Ah LiFePO4 battery pack.Each battery pack consists of 16 pieces 3.2V 100Ah single cells connected inseries, of which 300Ah batteries are composed of 3 pieces 100Ah battery packsin parallel. Yes, each battery pack has a BMS control system.
Aftertesting with a smart battery pack discharge meter, it is incorporated into theDC power supply system online. At this time, the charging voltage of theswitching power supply is set to 56.8V, and the charging current is limited to30A per piece.
LADDERLIFEPO4 BATTERY CONFIGURATION REQUIREMENTS
1.Cascade battery modules can be divided into 15, 25, 30, 50, 100, 130, 150,200Ah and other capacity series according to their nominal capacity. Thenominal capacity should be the capacity of the decommissioned lithium batteryafter grouping;
2. The cascade battery specificationsseries can be divided into three types according to the installation method:embedded, floor-standing and box-type.
3. Capacity requirements: The cascadebattery should meet the capacity requirements shown in Table 3 under differentoperating temperature conditions:
|
Ambient temperature |
Discharge current |
Battery capacity requirements |
|
-10℃ |
1.0I3 |
The measured capacity should not be less than 70% of the nominal capacity |
|
0℃ |
1.0I3 |
The measured capacity should not be less than 80% of the nominal capacity |
|
25℃ |
1.0I3 |
The measured capacity should be between 100% -110% of the nominal capacity |
|
40℃ |
1.0I3 |
The measured capacity should not be less than 98% of the nominal capacity |
|
55℃ |
1.0I3 |
The measured capacity should not be less than 97% of the nominal capacity |
4. Requirementsfor cascade battery cells: The capacity of the single cell used by the cascadebattery must reach 70% of the initial nominal capacity of the battery cell.
5. Outputvoltage range: The cascade battery should use 16 series, the rated voltage ofthe battery pack is 51.2V, and the working voltage range is 41.6V ~ 60.0V.
6.Environmental requirements: The cascade battery pack should work normally in anon-corrosive, explosive, and insulating gas and conductive dust environment.Operating temperature range: -5 to 45 ° C; Note: Heating and insulationmeasures should be taken below -5 ° C. Relative humidity range: ≤95% (45 ℃ ± 2℃), atmospheric pressure range: 70kPa ～ 106kPa;
7.Service life: At an ambient temperature of 25 ° C ± 2 ° C, the cycle life ofthe battery pack 80% DOD0.33C3 should be not less than 2000times.
At anambient temperature of 25 ° C ± 2 ° C, the life of the LiFePO4 battery packunder electrical conditions should not be less than 6 years.
FUNCTIONALREQUIREMENTS OF STEPPED LIFEPO4 BATTERIES
Sleepfunction
Thecascade battery should have the hibernation function, and the battery pack BMSshould be completely disconnected in the state of transportation, storage oroffline; when the battery pack is switched from the online state (that is, thestate of the battery pack output terminal and negative pole, and thecommunication interface is connected to the outside world) When it is offline(that is, the state where the positive and negative terminals of the batterypack output terminal and the communication interface are disconnected from theoutside world), the BMS should have a discrimination function and automaticallygo to sleep according to the power and battery pack conditions. When thebattery pack is switched from offline state (i.e., the state of the batterypack output terminal, the communication interface is disconnected from theoutside world) to online state (i.e., the state of the battery pack outputterminal, the communication interface is connected to the outside world), theBMS should be able to identify and Automatic activation, and adjust the workingstatus according to power and battery pack conditions.
Electricheating function
Whenthe cascade battery is used in the scene of -5 ℃ or below, a DC electricheating device should be configured (the temperature should be controlled andadjusted according to the actual situation), and the battery pack should have aspecial heat dissipation design to ensure uniform heating and normal operationof the equipment.
Chargingcurrent limit management function
Thecascade battery should have an autonomous current-limiting charging function toensure that the battery pack can be charged normally when the voltage is inputwithin the working range. The charging current limit value should be setbetween 0.1C3 (A) to 0.2C3 (A). The default value is 0.2C3 (A).
Overchargeprotection
Thecascade battery should have the function of protecting the total charging voltagefrom being too high. It will alarm when charging reaches the total voltagewarning point, and protect when reaching the protection point.
Lowtotal discharge voltage protection
Thecascade battery should have the protection function of low total dischargevoltage. When the discharge reaches the low alarm point of the total voltage,the discharge circuit should be cut off and alarmed, and the battery packshould enter the sleep mode after a period of time.
Lowdischarge cell voltage protection
Thecascade battery should have the protection function of the low voltage of thesingle cell when discharged. It will alarm when the single cell voltage isdischarged to the point of protection, and it will protect when it reaches theprotection point.
Dischargeovercurrent management
Thecascade battery should have an output overcurrent protection function setaccording to the needs of the user. The circuit should be cut off and an alarmshould be provided during the protection.
Batteryhigh temperature protection
Thecascade battery itself should have a high-temperature battery protectionfunction. When the battery temperature reaches the alarm point, it will warn;when it reaches the protection point, it will protect and act on the cutoff;the temperature will automatically recover when the temperature drops to acertain value.
Batterylow temperature protection function
Thecascade battery itself should have a low-temperature battery protectionfunction. When the battery temperature reaches the alarm point, it will warn;when it reaches the protection point, it will protect and act on the cutoff;the temperature will automatically recover after the temperature rises to acertain value.
Batterypack state of charge (SOC) calculation
Thecascade battery should have a dynamic charge capacity calculation function, andthe error between the calculated value and the actual battery capacity shouldnot be greater than 5%.
Outputshort circuit protection
Whenthere is a direct short circuit between the positive and negative terminals ofthe output of the cascade battery, the circuit should be automatically cut offand alarmed immediately. The BMS and battery should not be damaged (includingno fire, deformation, leakage, smoke, fire or explosion); Can resume workmanually or automatically.
Technicalrequirements for cascade battery monitoring
Telemetry
Canperform battery pack / battery voltage, state of charge (SOC), battery packcharge / discharge current, number of cycles (discharge once 80% of the nominalcapacity is a cycle), ambient temperature / battery pack temperature, batterypack internal resistance (Optional) Telemetry monitoring and historical dataquery, fault log query and other functions.
Telemetry
Cancollect the charging / discharging status of the cascade battery, battery packovercharge / overcurrent alarm, battery pack undervoltage / overcurrent alarm,single charge overvoltage alarm (optional), single discharge undervoltage alarm(optional) , Battery pack polarity reverse alarm, environment / battery pack /PCBA board high temperature alert (optional), low ambient temperature alert,low battery pack alert, battery pack temperature / voltage / current sensorfailure alert, cell failure alert ( (Optional), remote signal indicator such asbattery pack failure alarm (optional).
Remotecontrol
Remoteoperation such as alarm sound on / off, intelligent intermittent charging,current limiting charging, charging on / off, discharge start / stop, etc.
Remoteadjustment
Thefunctional status and parameter setting range of various BMS test items forcascade batteries.
LADDERLIFEPO4 BATTERY INSTALLATION AND MAINTENANCE REQUIREMENTS
1. Thesurface of the battery pack should be clean, without obvious deformation, nomechanical damage, and no rust on the interface contacts; the surface of thebattery pack should have the necessary product identification, and theidentification should be clear; the positive and negative terminals andpolarity of the battery pack should be clearly marked The wiring method shouldbe the front outlet method for easy connection; the power interface andcommunication (or alarm) interface of the battery pack should be clearlymarked;
2. The19-inch standard mechanical and electrical unit of the stepped lithium batterypack should be made of metal, and the structure should be easy to handle;
3. Installationof step batteries in order to facilitate commissioning and subsequentmaintenance, the lithium-ion battery panel should be facing outwards, and thestep batteries should be reliably fixed to the battery rack or integratedcabinet;
4.Lay the battery in the cascade. Connect the battery cable to the upper terminalof the safety copper bar in the power cabinet or the battery management circuitbreaker, and label the cables.
5. Laythe battery monitoring cable and connect the LiFePO4 battery pack to theFSU-RS485 communication terminal;
6.LiFePO4 battery cascade battery access system. After the connection of variouscables is completed, use a multimeter to test the output voltage of the battery,record the detected data, and adjust the output voltage of the switching powersupply to the current voltage value of the step battery.
7. Adjustthe parameters of the switching power supply. After the various types of cablesare connected, use a multimeter to test the output voltage of the battery andrecord the detected data;
8. Requirementsfor operating environment of Ladder LiFePO4 battery: According to theenvironmental requirements of the battery, the room temperature should notexceed 55 ° C, avoid direct sunlight to the battery, and the sun-shadingwindows should be shaded to ensure that sufficient maintenance space isreserved between the battery packs;
9.Precautions for using LiFePO4 battery in a step-by-step manner Monitor thetotal voltage, current, cell voltage SOC, SOH, and temperature of the batterypack in real time through the moving ring centralized monitoring system andBMS. At the same time, understand the battery charge and discharge curve andperformance through the battery monitoring device, and perform regularmeasurements to find faults and deal with them in time;
10.Items that are frequently checked for ladder-type LiFePO4 batteries: You shouldalways check whether the pole connecting wires (bars) of the ladder-type LiFePO4battery module are loose, and whether they are damaged, deformed, or corroded.Check whether there is any damage, leakage and deformation of the battery,whether the temperature rise of the battery and the connection is abnormal; thecontact condition of the BMS data line; and conduct warning experiments on theoutput fuse temperature check and signal fuse of the battery pack. According tothe technical parameters and on-site environmental conditions provided by themanufacturer, check whether the total voltage of the battery pack and the cellvoltage meet the requirements through the BMS system, and check whether thecharging current during the intermittent charging of the battery pack is withinthe required range. Check whether the charging voltage and current limit of theswitching power supply, battery pack are set correctly. Check whether the lowvoltage alarm, high voltage alarm, and high temperature alarm of the batterypack are set correctly.
TECHNICALAND ECONOMIC JUSTIFICATION FOR LADDER LIFEPO4 BATTERY PACKS
Comparedwith lead-acid batteries currently used, electric vehicle retired batterieshave high energy density, high power density, (small size, light weight), goodtemperature characteristics, long cycle life, and low self-discharge rate.These excellent characteristics make them more suitable As a backup powersource for tower base stations, the current ladder battery has a cycle life ofmore than 800 times. Strong manufacturers have longer battery cycle life; withthe development of electric vehicles, by the end of the 13th Five-Year Plan and2020 In the future, the cycle life of retired batteries will generally bebetter than 1,000 times, and good quality is expected to reach 2,000 times.
Atpresent, according to the current market conditions, batteries with a low cyclelife (as long as more than 400 times can be achieved at present) are used inthe first, second, third, and fourth categories of municipal electricalconditions and high-temperature conditions. The battery is used in new energy (morethan 800 times) and peak cutting and valley filling (more than 1200 times).
Theremanufactured battery is used in the battery pack of base station backup powerafter remanufacturing. Its cost structure includes the remanufacturing processof cell purchase, transportation, testing, screening, and reorganization.According to the indicators of the 13th Five-Year Plan, it is expected that thenumber of retired batteries will increase significantly in the future, and therecycling and remanufacturing system will form a scale effect, and the cost isexpected to further reduce.
Interms of disposal of scrapped power batteries, since the base station mainlyuses LiFePO4 batteries decommissioned for commercial vehicles, its mainmaterial value is not high, so the residual value of scrapped LiFePO4 batteriesis very low. However, there are already some manufacturers of waste batterytreatment starting this business, and it is expected to recycle the wastebatteries free of charge.
Inshort, the application of ladder batteries should follow the principles ofsmall modules, low voltage, high redundancy, small current, and non-mobile, socommunication base stations are more suitable for ladder battery applicationsthan other scenarios. Ladder batteries have certain advantages over lead-acidbatteries in terms of cycle life, energy density, and high-temperatureperformance, and various performance indicators are superior to lead-acidbatteries. Ladder batteries technically fully meet the backup powerrequirements of various operating conditions on the existing network. Differentcycle life ladder batteries are suitable for different application scenariosand also have certain economic advantages. Ladder battery application is amajor innovation in the development of national emerging industries such asenergy conservation, environmental protection, and new energy. It has veryimportant practical significance for promoting the development of a low-carboneconomy, a green economy, and a circular economy, which benefits both thecountry and the people.