Chargingprocedure for lithium battery have three stages. The first phase is apre-conditioning stage, followed by constant-current stage and thenconstant-voltage stage.
When battery is deeply discharge, pre-conditioningstage let battery to be charged with 0.1C until it is ready to receive fullcurrent. This is to prevent the battery from overheat (1).Most of the modern battery is equipped with switch which disallowed battery tobe discharged below its cut-off discharged voltage. Once the voltage of the batteryrises to a safe level, constant-current charging start where usually batterywill be charge with a constant current of 0.5C to 1C until it reaches itscut-off charging voltage.
Most lithium-ion have a 4.2V charging cut-offvoltage. However, during constant-current stage, battery will only be chargedup to 40%-70% of its maximum capacity. Using higher current for charging causevoltage of the battery to increase faster. However, its capacity will decreaseas it reaches its cut-off charging voltage. Constant-voltage stage help batteryto be charged to its maximum capacity without overcharging the battery whichmay damage the battery and cause explosion. In this stage, charging of thebattery continue until charging current slowly reduce to cut-off chargingcurrent which is usually 50mA. Chargingtemperature for lithium battery should be in between 0°C and 45°C 1.
Lithium battery perform better at elevatedtemperature but at the expense of reducing life cycle. However, highertemperature increases the possibility of cell’s cylinder to vent due to gas generationwithin the battery, leading to thermal runaway. Some lithium batteries are speciallydesigns to have the capability to operate at extreme temperature. Lithiumbattery used in oil drilling and high temperature environment can operate at maximumtemperature of 120C. Battery subjectedto high temperature environment may loss its capacity. Another precaution that shouldbe made is lithium battery should not be charge below freezing point.
Althoughcharging lithium battery below freezing point appear normal, plating will occurat the anode. This effect is not reversible through cycling, making batterymore vulnerable towards stressful condition. Lastyear project implement constant voltage charging method where current above50mA are supplied to the battery while its voltage maintained at 4.2V.
Withcharging current of 100mA, this method required 56 hours to fully charge acompletely discharge battery. To minimized time required for battery to befully charge, combination of pre-conditioning charging, constant-current chargingand constant-voltage charging will be test in this design. Tofacilitate change of phases during charging process, PMIC is used. PMIC candetect battery voltage and change the charging stages accordingly.
LinearTechnology LT3652 is chosen as this IC is a step-down battery charger whichcapable to receive input from 4.95V up to 32V and having an output up to 2A.Therefore, plane power supply can be connected directly to this IC, eliminatingthe need of buck converter module in PCB. Float voltage and charging can beprogram externally by using resistor. Thermistor can be connected to NTC pin tomonitor battery temperature during charging. When LT3652 detect battery voltageto be very low, it will automatically execute pre-conditioning charging wherecharging current will be reduced to 15% of the charging current programmed.This chip also equipped with auto-restart function where new cycle of chargingwill be initiate when its detect battery voltage at feedback pin fall 2.5% below float voltageDesiredoperation requirement can be achieved by connecting batteries in series orparallel connection.
Connecting batteries in series increase its total terminalvoltage with the same capacity as a single battery while connecting batteriesin parallel give same terminal voltage as a single battery but with a highercapacity. For some operation, batteries pack can be connected by combining bothseries and parallel connection of the battery to have a higher nominal voltageand boost in a capacity. Batteries with same parameter need to be used toprevent imbalance in the pack.
This imbalance will lead to weakest battery toexhaust quicker compare to other batteries when connected to a load. Thisparticularly important when connecting batteries in series since its willaffect total terminal voltage of the pack. Besides than that, failure of asingle battery will cause total failure of the packs. During charge, weakestbattery will fill up and reach maximum voltage quicker compare to otherbatteries. Prolong overcharge will lead to battery damage and increase the riskof an explosion.
In parallel connection, effect of imbalance is less severe.Failure of a single battery will only cause reduce in total capacity of thepack. However, parallel connected batteries are exposed to electrical shot dueto reverse polarization of the battery where fail battery will consume energyfrom the remaining battery in parallel. Prolong exposure can lead to firehazard.Usingbatteries pack require multiple safety precaution to reduce hazard cause by thebattery.
Usually batteries pack are equipped with charge interrupt device (CID)and positive temperature coefficient switches (PTC) 3to govern current flow into the battery and pressure build up within thebattery. If overcurrent detected or excessive pressure are build up in thebattery, CID and PTC will interrupt charging process, preventing the batteryfrom damage. Packs equipped with serial connection may have a solid-stateswitch equipped to bypasses damage cell to ensure continuity in current flowdespite at a lower total voltage while packs equipped with parallel connectionmay equipped with a fuse at every single battery to disconnect damage cell whenshort occurs.ForLi-ion battery, its voltage should not exceed 4.
2V during charge. This can begoverned by using PMIC which will be explained further in SECTION. TwoLi-ion batteries connected in series will have a cut-off charging voltage of8.4V. To prevent battery from overcharged due to imbalance in seriesconnection, we decide to stop the charging when the batteries voltage reaches8V. Batterieshave been widely used in aviation industry since the beginning of planehistory. During earlier age of aviation, electricity was mostly used to ignitethe airplanes engine. As technology progress, more advanced airplanes and jet aredeveloped and equipped with a sophisticated electrical system.
While enginesable to serve as generator and provide electricity, batteries are required toprovide electricity before the engine started and serve as a part of ignition systemin the engine. This battery also serves as backup power in the event of enginefailure. Battery used for these purposes are called main battery. In most ofthe advanced airplanes, another battery usually installed to serve as auxiliarypower unit (APU). The fundamentalin batteries operation is converting chemical energy to the electrical energy.This process occurs through electrochemical discharge of the reactant.
However,they are limited time in converting this energy since reactant can be depletedand unable to supply electron. Batteriesthat unable to restore their reactant are called primary batteries. Some of thebatteries have the capability to restore their reactant to its initial state bycharging it through direct current. These batteries are called secondary batteries.
In airplanes, secondary batteries are widely used as it able to be usedmultiple time through a cycle process of charge and discharge.Thechemistry of the battery used in airplanes are usually lead acid, nickel cadmiumor lithium. The battery used are big in nature since its consisted of multiplecells connected in a series-parallel connection to give desired operationrequirement. Batteries are rated based on their nominal voltage and capacity.Battery with 1000mAh capacity will have a runtime of 1 hour before completelydischarge when discharge with 1A. This discharge rate is equivalent to 1C.
Subjectingbattery to a discharge rate of 2C means battery are discharge with 2A and willhave a runtime of 30 minutes before it is completely discharge. High rate cell canbe achieved by minimizing the internal resistance of the cell which highlydependent on the internal construction of the cell itself. Theuse of lithium ion battery in aviation is quite popular due to energy densitythat it can offer. Higher energy density means, more power can be supplied witha smaller battery. Space required for battery installation is small as some ofthe battery size can be similar to the size of car battery. Total weight of theaircraft can be reduced leading to reduce in airplanes fuel consumption.
Lithiumion battery also have excellent life cycle, faster recharge time and require minimalmaintenance compared to nickel-cadmium battery. Boeing, an airplanes manufacturingcompany have been knowingly used lithium ion battery in their airplanes, B787.However, due to accident of thermal runaway of the B787 battery in 2013, concernregarding the safety of using lithium ion battery in aviation arises, makingAirbus to drop the use of lithium ion in their A350 fleet. Immature technologyof lithium ion lead to further development in minimizing the risk of thermalrunaway in battery as year progress. Saft, a battery-maker company has succeededin developing more stable lithium-ion battery, making Airbus to deliver their firstA350 equipped with lithium ion battery in 2016.