Aircraft battery
Introduction(or Brief Introduction)
The basic function of any battery is to convert chemical energy into electrical energy.
A battery can be regarded as a chemical means of storing electrical energy.
Electrons are removed from the cathode(positive electrode)and deposited onto the anode(negative electrode).The electrolyte serves as the physical medium for electron transfer between the cathode and the anode.The mutual attraction of electrons between the cathode and the anode creates a potential difference across the battery terminals;the cathode and the anode are connected to external terminals for interfacing with equipment or systems.The types of materials used for the cathode,anode,and electrolyte will determine the battery voltage.
The battery is divided into
Primary battery(can only be used once)
Secondary battery(rechargeable)
Open-circuit voltage(OCV)and closed-circuit voltage(CCV)
The voltage measured when no load is applied to the battery is called open-circuit voltage(OCV),while the voltage measured when a load is applied to the battery is called closed-circuit voltage(CCV).Since no current flows out of the battery,the open-circuit voltage is always greater than the closed-circuit voltage.
CCV is a function of the applied load and the battery's state of charge.Unless the battery is fully discharged,OCV is almost independent of the state of charge.
Internal resistance
The internal resistance(IR)of a battery when connected to a load is determined by the applied load and the state of charge of the battery.The internal resistance of a battery is equal to the difference between the open-circuit voltage(OCV)and the closed-circuit voltage(CCV),divided by the applied load current.
Internal resistance(IR)=(Open circuit voltage(OCV)-Closed circuit voltage(CCV))/Reading current.
The internal resistance(IR)of a battery increases as it discharges,because unless the battery is fully discharged,the decrease in closed-circuit voltage(CCV)and open-circuit voltage(OCV)remain almost unchanged.The difference between them increases,hence the internal resistance also increases.
Aircraft battery type
Aircraft batteries are typically classified based on the materials used for the plates.The two most commonly used battery types are
Lead-acid battery
Nickel-cadmium battery
Most small private aircraft use lead-acid batteries.Most commercial and corporate aircraft use nickel-cadmium batteries
(Ni-Cd)battery.However,other types of lead-acid batteries have also been introduced,such as valve-regulated lead-acid(VRLA)batteries.
The most suitable battery for a specific application depends on the relative importance of characteristics such as weight,cost,volume,service life or shelf life,discharge rate,maintenance,and charging rate.
Aircraft batteries are used for many functions,such as
Ground power supply
Emergency power supply
Improve the stability of DC busbars
Fault clearance
Lead-acid battery
Lead-acid batteries consist of lead compound plates immersed in an electrolyte(a solution of sulfuric acid and water).The positive plate is filled with lead dioxide,while the negative plate is filled with pure sponge-like lead.The electrolyte is composed of a mixture of 30%sulfuric acid and 70%water by volume,achieving a specific gravity of 1.2.
The open-circuit voltage(OCV)of each battery cell when fully charged is approximately 2.1V,and the closed-circuit voltage(CCV)is around 2V.
Lead-acid batteries used in aircraft are typically rated at 12V or 24V;hence,they consist of six or twelve battery cells connected in series.
The structure of lead-acid batteries
Each unit of a battery is composed of alternating positive and negative plates,which are insulated by separators.Each plate consists of a frame called a grid and an active material filled within the grid.The grid is made of 90%lead and 10%antimony.The purpose of adding antimony is to harden lead and make it less prone to chemical reactions.Other metals such as silver are also used to enhance durability.
The plate is made by coating lead compounds on the grid frame.After mixing the paste with dilute sulfuric acid,magnesium sulfate,or ammonium sulfate to achieve the appropriate consistency,it is applied onto the grid frame.
The paste for the positive plate is primarily composed of a large amount of red lead and a small amount of lead oxide.The paste for the negative plate,on the other hand,is mainly composed of a large amount of lead oxide and a small amount of red lead.When preparing the paste for the negative plate,a material known as an expander is added.This material is chemically inert and accounts for 1%of the mixture.Its function is to prevent the negative plate from losing its porous nature.Common expander materials include charcoal black,barium sulfate,graphite,fine sawdust,and ground carbon.
After applying the active material,it needs to be dried until the paste hardens.Then,the formation process is carried out,where the positive plate is connected to the positive terminal of the charger and the negative plate is connected to the negative terminal of the charger.Next,they are placed in a solution of sulfuric acid and water(electrolyte)and subjected to a long period of slow charging.
After several rounds of charge and discharge cycles,lead compounds are transformed into active materials.The resulting positive plate is lead dioxide,which appears chocolate brown;the negative plate is sponge-like lead,which appears pearl gray.After formation is complete,the plates need to be cleaned and dried,and then assembled into a plate group by connecting the same-type plates to a common busbar.
Each plate is equipped with a lug on its top for welding with the plate strap.All positive lugs are interconnected,and all negative lugs are also interconnected.Compared to the positive plates,we have one additional negative plate;the negative plates are positioned at the beginning and end,so there is a negative plate on both sides of each positive plate.This helps distribute the chemical reaction evenly on both sides of the positive plates.
The separator used in lead-acid batteries is made of glass fiber,rubber,or other insulating materials.Its function is to separate the plates and prevent internal short circuits.The separator material must be highly porous to provide minimal resistance to current flow.
Once the battery cells are assembled,they are placed into battery containers made of hard rubber or plastic.
Service life
The service life of lead-acid aviation batteries depends on their usage patterns(such as discharge rate,frequency,and depth),environmental conditions(such as temperature and vibration),charging methods,and maintenance.Depending on the specific application,the service life is typically between 1 and 5 years.
Nickel-cadmium battery
A nickel-cadmium battery in a charging state consists of nickel hydroxide on the positive plate and metal cadmium on the negative plate;during discharge,the positive plate converts to nickel hydroxide,and the negative plate converts to cadmium hydroxide.The electrolyte consists of 70%distilled water and 30%potassium hydroxide to achieve a specific gravity of 1.3.Typically,the specific gravity of a nickel-cadmium battery ranges from 1.24 to 1.32.
The open-circuit voltage of each battery cell is 1.28V,and the closed-circuit voltage ranges from 1.2 to 1.25V,depending on the battery temperature,the last charging time,and the applied discharge current.
A typical 24V nickel-cadmium battery consists of 19 or 20 battery cells connected in series.
Aircraft equipped with nickel-cadmium batteries typically feature a fault protection system that monitors the battery status.The battery charger serves as the unit responsible for monitoring the battery status,and the following conditions will be monitored.
Overheated state
2.Low temperature conditions(below–40°F)
3.Battery cell imbalance
4.Open circuit
5.Short circuit
If the battery charger detects a fault,it will shut down and send a fault signal to the Electrical Load Management System(ELMS).
The structure of a nickel-cadmium battery
Each unit of the battery consists of a negative plate,a positive plate,a separator,an electrolyte,a battery case,a battery cover,and an exhaust valve.
The electrode plate is made of a metal billet,which is impregnated with active materials for both the positive and negative electrodes.Sintering is a process of heating fine metal particles,causing them to weld together where they come into contact with other particles,thus forming a porous material.
The sintered material of the positive plate is nickel or carbonyl nickel,while the sintered material of the negative plate is cadmium.After sintering,the blank plate forms a porous material with a porosity of 80%to 85%and a solid material content of 15%to 20%.The porous blank plate is impregnated with nickel salt to make the positive plate,and with cadmium salt to make the negative plate.
After the plates have absorbed enough active material to provide the required capacity,they are placed in the electrolyte and a current is passed through,converting the nickel and cadmium salts into their final forms.Subsequently,the plates are cleaned,dried,and cut into electrode plates.A nickel tab is welded to the corner of each electrode plate,which serves as a means of connecting the electrode plates into a plate group.
The separator of a nickel-cadmium battery is a thin,porous,multilayer composite material consisting of woven nylon and a layer of cellophane.
The battery container consists of a plastic battery can and a lid,which are permanently connected during assembly.
Working principle
When the battery discharges,hydroxide ions in the electrolyte combine with cadmium in the negative plate,releasing electrons into the negative plate.During this process,cadmium is converted into cadmium hydroxide.At the same time,hydroxide ions in the positive plate(nickel hydroxide)enter the electrolyte,carrying excess electrons.At this point,the positive plate lacks electrons,while the negative plate has an excess of electrons.Once a conductor is connected,current will flow through the conductor.
When a nickel-cadmium battery is being charged,hydroxide ions are forced to leave the negative plate and enter the electrolyte.Consequently,the cadmium hydroxide on the negative plate is converted back into metallic cadmium.The hydroxide ions in the electrolyte recombine with the nickel hydroxide on the positive plate,and the active material is transformed into a higher oxidation state known as nickel hydroxide.This process continues until all active materials have been converted.
If the battery is overcharged,the water in the electrolyte will decompose through electrolysis.Hydrogen gas is released at the negative plate,and oxygen gas is released at the positive plate.
During the reaction process,the rate at which hydroxide ions are added to the electrolyte is equal to the rate at which they are consumed.Therefore,the specific gravity of the electrolyte remains essentially constant at any stage of the discharge process.
Nickel-cadmium battery chemical reaction
Types of nickel-cadmium batteries
Exhaust type
Vent-type batteries utilize liquid electrolytes,and during the charging process,hydrogen and oxygen gases are expelled from the battery container.
Recombinant
The phenomenon of electrolyte depletion exists in recombinant cells,and oxygen will be generated at the positive electrode during charging.
The recombination reaction inhibits hydrogen evolution at the negative electrode,thereby enabling the battery to be sealed.Unlike valve-regulated lead-acid batteries,recombinant nickel-cadmium batteries are sealed with high-pressure vent valves that release gas only under abnormal conditions.Therefore,these batteries remain sealed under normal charging conditions.However,it is still necessary to design the battery case with a fuel gas escape channel,as abnormal conditions may occur periodically(such as when an overcurrent condition arises due to a charger failure).
Battery charging
Operating aircraft batteries beyond the ambient temperature or charging voltage limits may lead to excessive temperature of the battery cells,causing electrolyte boiling,rapid cell degradation,and battery failure.The relationship between the maximum charging voltage and the number of battery cells is also crucial.It determines the rate at which energy is absorbed internally in the battery(under specific ambient temperature and charging conditions)in the form of heat.
For lead-acid batteries,the voltage of each individual cell must not exceed 2.35 volts.
For nickel-cadmium batteries,the charging voltage limit varies depending on the design and construction.The commonly used values are 1.4 and 1.5 volts per cell.
In all cases,please follow the recommendations of the battery manufacturer.
Constant voltage charging(CP)
The aircraft's battery charging system employs a constant voltage type.An engine-driven generator is capable of providing the required voltage and is directly connected to the battery through the aircraft's electrical system.A battery switch is installed in the system to disconnect the battery when the aircraft is not in operation.The voltage of the generator is precisely controlled by a voltage regulator connected in the generator's excitation circuit.
For a 12-volt system,the voltage of the generator is adjusted to approximately 14.25 volts.In a 24-volt system,the adjustment range should be between 28 and 28.5 volts.When these conditions are met,the initial charging current flowing through the battery is relatively high.As the state of charge increases,the battery voltage also rises,resulting in a gradual decrease in current.When the battery is fully charged,its voltage is almost equal to the generator voltage,and the current flowing into the battery is very small.
When the charging current is low,the battery can remain connected to the generator without being damaged.When using a constant voltage system in a battery store,a voltage regulator that can automatically maintain a constant voltage is integrated into the system.High-capacity batteries(such as 42 ampere-hours)have lower resistance than low-capacity batteries(such as 33 ampere-hours).Therefore,when both are at the same state of charge and the charging voltage is equal,the charging current of the high-capacity battery will be higher than that of the low-capacity battery.The constant voltage method is the preferred charging method for lead-acid batteries.
Touch voltage charging
Constant current charging
Constant current charging is the most convenient method for external charging of aircraft,as it allows for the simultaneous charging of multiple batteries with different voltages on the same system.The constant current charging system typically consists of a rectifier,which converts normal AC power into DC power.Before the power passes through the rectifier,a transformer is used to reduce the available 110V or 220V AC power to the required level.If a constant current charging system is used,multiple batteries can be connected in series,provided that the charging current is maintained at a level that prevents the batteries from overheating or generating excessive gas.
The constant current charging method is the preferred method for charging nickel-cadmium batteries,as it ensures better cell balance and total battery charging capacity,while also preventing the possibility of thermal runaway.
Typically,nickel-cadmium batteries are charged at a constant current of 1CA until the voltage of all battery cells reaches at least 1.55V.Subsequently,another round of charging is performed at a rate of 0.1CA until all battery cells reach 1.55V again.Overcharging or supplementary charging is performed at the end of the charging process,usually at a rate of 0.1CA for at least 4 hours.
The purpose of overcharging is to vent as much as possible(if not all)of the gas accumulated on the electrodes,namely hydrogen on the anode and oxygen on the cathode;some of this gas will recombine to form water,thereby raising the electrolyte level to its maximum level,after which the electrolyte level can be safely adjusted.
During overcharging or recharging,the battery voltage will exceed 1.6V and then slowly start to decrease.The voltage of any battery should not exceed 1.71V(for dry batteries)or fall below 1.55V(when the fuel barrier is ruptured).When charging,it is necessary to loosen or open the exhaust cap.Blocking the exhaust port may lead to an increase in internal pressure within the battery.Additionally,this allows water to be replenished to the correct level before the end of recharging,while the charging current remains connected.
However,once the cleaning and inspection of the ventilation vents are completed,the battery cover should be closed immediately,as carbon dioxide from the outside air will dissolve and cause carbonation inside the battery,thereby accelerating battery aging.constant current charging
Thermal runaway
When the ambient temperature of a nickel-cadmium battery is within the range of approximately 60–90°F,it can operate at its rated capacity.Temperatures outside this range can lead to a reduction in capacity.Nickel-cadmium batteries have a ventilation system to control battery temperature.An excessively high battery temperature(over 160°F)combined with overcharging can lead to a state known as thermal runaway.Battery temperature must be continuously monitored to ensure safe operation.Thermal runaway can cause chemical fires and/or explosions in nickel-cadmium batteries when charged by a constant voltage source,due to the periodic and continuously rising trend of temperature and charging current.
One or multiple short-circuited batteries,or existing high temperatures and low battery levels,may trigger a cyclic sequence of events:
1.Excessive current,
2.Temperature increases,
3.The battery resistance decreases,
4.The current further increases,and
5.Further increase the temperature
If the constant voltage charging source is removed before the battery temperature exceeds 160°F,it will not evolve into a self-sustaining thermochemical reaction.
Aircraft battery maintenance
Battery inspection and maintenance procedures vary depending on the chemical technology and physical structure type.It is essential to follow the procedures approved by the battery manufacturer.In specific applications,the performance of the battery depends on its age,health status,charging status,and mechanical integrity,which can be determined through the following methods:
To determine the battery's service life and usage duration,please record the installation date on the battery.During regular battery maintenance,the battery usage duration must be recorded in the aircraft maintenance log or shop maintenance log.
The state of health of a lead-acid battery can be determined by the duration of maintenance intervals(for vented batteries),environmental factors(such as excessively high or low temperatures),and observed electrolyte leakage(manifesting as corrosion of wiring and connectors or accumulation of powdered salts).If the battery requires frequent topping up and shows no signs of external leakage,this may indicate that the battery itself,the battery charging system,or there is an overcharging issue,indicating poor condition.Use a hydrometer to measure the specific gravity of the lead-acid battery electrolyte,which is the ratio of the weight of the electrolyte to the weight of pure water.Be sure to pour the electrolyte back into the battery cell from which it was removed.When the specific gravity difference between the cells of the battery reaches 0.050 or greater,it indicates that the battery is nearing the end of its service life and should be considered for replacement.The electrolyte level can be adjusted by adding distilled water.Do not add electrolyte.
The battery state of charge is determined by the cumulative effect of battery charging and discharging.In a normal electrical charging system,the aircraft generator or alternator can fully charge the battery within a flight time of 1 to 90 minutes.
Qualified mechanical integrity implies the absence of any physical damage and ensures that the hardware is installed correctly and the battery is properly connected.The pipelines,connectors,and accessories(if applicable)of the battery and battery compartment ventilation system prevent the accumulation of explosive gases and should be regularly inspected to ensure that they are securely connected and comply with the installation procedures outlined in the maintenance manual.It is essential to follow the approved procedures applicable to the specific aircraft and battery system to ensure that the battery system can provide the specified performance.
Battery check
The inspection of aircraft batteries includes the following items:
Check the status and fixation of the battery fluid collection bottle and pipeline.
Inspect the battery terminals and quickly disconnect the plug and pin to check for corrosion,pitting,arc burns,and burn marks.Clean them if necessary.
Check the battery for discharge and the ventilation pipeline for blockage,aging,and fixation.
The standard pre-flight and post-flight inspection procedures should include observation for signs of physical damage,loose connections,and electrolyte loss.
Installation specifications
External surface-Clean the external surface of the battery before installing it on the aircraft.
Replacing lead-acid batteries-When replacing lead-acid batteries with nickel-cadmium batteries,a battery temperature or current monitoring system must be installed.Neutralize the battery box or compartment,thoroughly rinse with clean water,and wipe it dry.Supplementary information for the flight manual must also be provided for the installation of nickel-cadmium batteries.Acidic residues can damage the normal operation of nickel-cadmium batteries,just as alkaline substances affect lead-acid batteries.
Battery exhaust-The smoke and gases generated by the battery may form explosive mixtures or contaminate the cabin,and should be dispersed through adequate ventilation.The exhaust system typically utilizes ram pressure to force fresh air into the battery box or enclosure and expel it to a safe overboard discharge point.The pressure differential of the exhaust system should always be maintained at positive pressure and kept between the recommended minimum and maximum values.The piping layout should not allow the retention of battery overflow liquid or condensate water,which could hinder air circulation.
Battery fluid collection tank-The battery fluid collection tank device can be integrated into the ventilation system to handle battery electrolyte overflow.The design of the collection tank should be reasonable,and appropriate neutralizers should be used.The collection tank must only be installed on the drainage side of the battery ventilation system.
Installing Batteries-When installing batteries on an aircraft,extreme caution must be taken to prevent inadvertent short-circuiting of the battery terminals.The resulting high-current discharge can cause severe damage to aircraft structures(frames,skins,and other subsystems,avionics,wiring,fuel,etc.).This can usually be avoided by insulating the terminal posts during installation.When removing the battery,the grounding wire should be disconnected first,followed by the positive wire.During installation,the battery's grounding wire should be connected last to minimize the risk of short-circuiting the battery's positive terminal during the installation process.
Quick-disconnect batteries-If quick-disconnect battery connectors that prohibit crossing of battery leads are not used,ensure that the aircraft wiring is connected to the correct battery terminals.Reversing the polarity in the electrical system can severely damage the battery and other electrical components.
