Par 11-15 Page 11-486
SECTION 2. STORAGE BATTERIES
GENERAL. Aircraft batteries may be used for many functions, e.g., ground power, emergency power, improving DC bus stability, and fault-clearing. Most small private aircraft use lead-acid batteries. Most commercial and military aircraft use NiCad batteries. However, other types are becoming available such as gel cell and sealed lead-acid batteries. The battery best suited for a particular application will depend on the relative importance of several characteristics, such as weight, cost, volume, service or shelf life, discharge rate, maintenance, and charging rate. Any change of battery type may be considered a major alteration.
Storage batteries are usually identified by the material used for the plates. All battery types possess different characteristics and, therefore, must be maintained in accordance with the manufacturer’s recommendations..
WARNING: It is extremely dangerous to store or service lead-acid and NiCad batteries in the same area.
Introduction of acid electrolytes into alkaline electrolyte will destroy the NiCad and vice-versa.
BATTERY CHARGING. Operation of storage batteries beyond their ambient temperature or charging voltage limits can result in excessive cell temperatures leading to electrolyte boiling, rapid deterioration of the cells, and battery failure. The relationship between maximum charging voltage and the number of cells in the battery is also significant. This will determine (for a given ambient temperature and state of charge) the rate at which energy is absorbed as heat within the battery. For lead-acid batteries, the voltage per cell must not exceed 2.35 volts.
In the case of NiCad batteries, the charging voltage limit varies with design and construction. Values of
1.4 and 1.5 volts per cell are generally used. In all cases, follow the recommendations of the battery manufacturer.
BATTERY FREEZING. Discharged lead-acid batteries exposed to cold temperatures are subject to plate damage due to freezing of the electrolyte. To prevent freezing damage, maintain each cell’s specific gravity at 1.275, or for sealed lead-acid batteries check “open” circuit voltage. (See table 11-1.) NiCad battery electrolyte is not as susceptible to freezing because no appreciable chemical change takes place between the charged and discharged states. However, the electrolyte will freeze at approximately minus 75 °F.
NOTE: Only a load check will determine overall battery condition.
TABLE 11-1. Lead-acid battery electrolyte freezing points.
Specific
Freeze point State of Charge (SOC) for sealed lead-acid batteries at 70° Gravity C. F. SOC 12 volt 24 volt
1.300 -70 -95 100% 12.9 25.8
1.275 -62 -80 75% 12.7 25.4
1.250 -52 -62 50% 12.4 24.8
1.225 -37 -35 25% 12.0 24.0
1.200 -26 -16
1.175 -20 -4
1.150 -15 +5
1.125 -10 +13
1.100 -8 +19
TEMPERATURE CORRECTION. U.S.
manufactured lead-acid batteries are considered fully charged when the specific gravity reading is between 1.275 and 1.300. A 1/3 discharged battery reads about 1.240 and a 2/3 discharged battery will show a specific gravity reading of about 1.200, when tested by a hydrometer and the electrolyte temperature is 80 ºF. However, to determine precise specific gravity readings, a temperature correction (see table 11-2) should be applied to the
Par 11-15 Page 11-487 hydrometer indication. As an example, a hydrometer reading of 1.260 and the temperature of the electrolyte at 40 °F, the corrected specific gravity reading of the electrolyte is 1.244.
TABLE 11-2. Sulfuric acid temperature correction.
Electrolyte Temperature
Points to be subtracted or added to °C °F specific gravity readings 60 55 49 43 38 33 27 23 15 10 5 -2 -7 -13 -18 -23 -28 -35 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 -10 -20 -30 +24 +20 +16 +12 +8 +4 0 -4 -8 -12 -16 -20 -24 -28 -32 -36 -40 -44
BATTERY MAINTENANCE.
Battery inspection and maintenance procedures vary with the type of chemical technology and the type of physical construction. Always follow the battery manufacturer’s approved procedures. Battery performance at any time in a given application will depend upon the battery’s age, state of health, state of charge, and mechanical integrity.
Age. To determine the life and age of the battery, record the install date of the battery on the battery.
During normal battery maintenance, battery age must be documented either in the aircraft maintenance log or in the shop maintenance log.
State of Health. Lead-acid battery state of health may be determined by duration of service interval (in the case of vented batteries), by environmental factors (such as excessive heat or cold), and by observed electrolyte leakage (as evidenced by corrosion of wiring and connectors or accumulation of powdered salts). If the battery needs to be refilled often, with no evidence of external leakage, this may indicate a poor state of the battery, the batte ry charging system, or an over charge condition.
Use a hydrometer to determine the specific gravity of the battery electrolyte, which is the weight of the electrolyte compared to the weight of pure water.
Take care to ensure the electrolyte is returned to the cell from which it was extracted. When a specific gravity difference of 0.050 or more exists between cells of a battery, the battery is approaching the end of its useful life and replacement should be considered.
Electrolyte level may be adjusted by the addition of distilled water.
State of Charge. Battery state of charge will be determined by the cumulative effect of charging and discharging the battery. In a normal electrical charging system the battery’s generator or alternator restores a battery to full charge during a flight of one hour to ninety minutes.
Mechanical Integrity. Proper mechanical integrity involves the absence of any physical damage as well as assurance that hardware is correctly installed and the battery is properly connected. Battery and battery compartment venting system tubes, nipples and attachments, when required, provide a means of avoiding the potential buildup of explosive gases, and should be checked periodically to ensure that they are securely connected and orie nted in accord ance with the maintenance manual’s installation procedures. Always follow procedures approved for the specific aircraft and battery system to ensure that the battery system is capable of delivering specified performance.
Par 11-19 Page 11-488 Battery and Charger Characteristics. The following information is provided to acquaint the user with characteristics of the more common aircraft battery and battery charger types. Products may vary from these descriptions due to different applications of available technology. Consult the manufacturer for specific performance data.
NOTE: Under no circumstances connect a lead-acid battery to a charger, unless properly serviced.
Lead-acid vented batteries have a two volt nominal cell voltage. Batteries are constructed so that individual cells cannot be removed. Occasional addition of water is required to replace water loss due to overcharging in normal service. Batteries that become fully discharged may not accept recharge.
Lead-acid sealed batteries are similar in most respects to lead-acid vented batteries, but do not require the addition of water.
The lead-acid battery is economical and has extensive application, but is heavier than an equivalent performance battery of another type. The battery is capable of a high rate of discharge and low temperature performance.
However, maintaining a high rate of discharge for a period of time usually warps the cell plates, shorting out the battery. Its electrolyte has a moderate specific gravity, and state of charge can be checked with a hydrometer.
Do not use high amperage automotive battery chargers to charge aircraft batteries.
NiCad vented batteries have a 1.2 volt nominal cell voltage. Occasional addition of distilled water is required to replace water loss due to overcharging in normal service. Cause of failure is usually shorting or weakening of a cell. After replacing the bad cell with a good cell, the battery’s life can be extended for five or more years. Full discharge is not harmful to this type of battery.
NiCad sealed batteries are similar in most respects to NiCad vented batteries, but do not normally require the addition of water. Fully discharging the battery (to zero volts) may cause irreversible damage to one or more cells, leading to eventual battery failure due to low capacity.
The state of charge of a NiCad battery cannot be determined by measuring the specific gravity of the potassium hydroxide electrolyte.
The electrolyte specific gravity does not change with the state of charge. The only accurate way to determine the state of charge of a NiCad battery is by a measured discharge with a NiCad battery charger and following the manufacturer’s instructions. After the battery has been fully charged and allowed to stand for at least two hours, the fluid level may be adjusted, if necessary, using distilled or demineralized water. Because the fluid level varies with the state of charge, water should never be added while the battery is installed in the aircraft. Overfilling the battery will result in electrolyte spewage during charging. This will cause corrosive effects on the cell links, selfdischarge of the battery, dilution of the electrolyte density, possible blockage of the cell vents, and eventual cell rupture.
Lead-acid batteries are usually charged by regulated DC voltage sources. This allows maximum accumulation of charge in the early part of recharging.
Constant-current battery chargers are usually provided for NiCad batteries because the NiCad cell voltage has a negative temperature coefficient. With a constant-voltage charging source, a NiCad battery
Par 11-19 Page 11-489 having a shorted cell might overheat due to excessive overcharge and undergo a thermal runaway, destroying the battery and creating a po ssible safety hazard to the aircraft.
DEFINITION: Thermal runaway can result in a chemical fire and/or explosion of the NiCad battery under recharge by a constant-voltage source, and is due to cyclical, ever-increasing temperature and charging current.
One or more shorted cells or an existing high temperature and low charge can produce the cyclical sequence of events: (1) excessive current, (2) increased temperature,
(3) decreased cell(s) resistance,
(4) further increased current, and
(5) further increased temperature.
This will not become a self-sustaining thermal-chemical action if the constant-voltage charging source is removed before the battery temperature is in excess of 160 °F.
Pulsed-current battery chargers are sometimes provided for NiCad batteries.
CAUTION: It is important to use the proper charging procedures for batteries under test and maintenance. These charging regimes for reconditioning and charging cycles are defined by the aircraft manufacturer and should be closely followed.
Shop-Level Maintenance Procedures. Shop procedures must follow the manufacturer’s recommendations. Careful examination of sealed batteries and proper reconditioning of vented batteries will ensure the longest possible service life.
Aircraft Battery Inspection.
Inspect battery sump jar and lines for condition and security.
Inspect battery terminals and quick-disconnect plugs and pins for evidence of corrosion, pitting, arcing, and burns. Clean as required.
Inspect battery drain and vent lines for restriction, deterioration, and security.
Routine pre-flight and post-flight inspection procedures should include observation for evidence of physical damage, loose connections, and electrolyte loss.
ELECTROLYTE SPILLAGE. Spillage or leakage of electrolyte may result in serious corrosion of the nearby structure or control elements as both sulfuric acid and potassium hydroxide are actively corrosive. Electrolyte may be spilled during ground servicing, leaked when cell case rupture occurs, or sprayed from cell vents due to excessive charging rates. If the battery is not case enclosed, properly treat structural parts near the battery that may be affected by acid fumes. Treat all case and drain surfaces, that have been affected by electrolyte, with a solution of sodium bicarbonate (for acid electrolyte) or boric acid, vinegar, or a 3 percent solution of acetic acid (for potassium hydroxide electrolyte).
CAUTION: Serious burns will result if the electrolyte comes in contact with any part of the body. Use rubber gloves, rubber apron, and protective goggles when handling electrolyte. If sulfuric acid is splashed on the body,
Par 11-20 Page 11-490 neutralize with a solution of baking soda and water, and shower or flush the affected area with water. For the eyes, use an eye fountain and flush with an abundance of water. If potassium hydroxide contacts the skin, neutralize with 9 percent acetic acid, vinegar, or lemon juice and wash with water. For the eyes, wash with a weak solution of boric acid or a weak solution of vinegar and flush with water.
NOXIOUS FUMES. When charging rates are excessive, the electrolyte may boil to the extent that fumes containing droplets of the electrolyte are emitted through the cell vents.
These fumes from lead-acid batteries may become noxious to the crew members and passengers; therefore, thoroughly check the venting system. NiCad batteries will emit gas near the end of the charging process and during overcharge. The battery vent system in the aircraft should have sufficient air flow to prevent this explosive mixture from accumulating. It is often advantageous to install a jar in the battery vent discharge system serviced with an agent to neutralize the corrosive effect of battery vapors.
INSTALLATION PRACTICES.
External Surface. Clean the extern al surface of the battery prior to installation in the aircraft.
Replacing Lead-Acid Batteries. When replacing leadacid batteries with NiCad batteries, a battery temperature or current monitoring system must be installed. Neutralize the battery box or compartment and thoroughly flush with water and dry. A flight manual supplement must also be provided for the NiCad battery installation. Acid residue can be detrimental to the proper functioning of a NiCad battery, as alkaline will be to a lead-acid battery.
Battery Venting. Battery fumes and gases may cause an explosive mixture or c ontaminated compartments and should be dispersed by adequate ventilation.
Venting systems often use ram pressure to flush fresh air through the battery case or enclosure to a safe overboard discharge point. The venting system pressure differential shoul d always be positive, and remain between recommended minimum and maximum values. Line runs should not permit battery overflow fluids or condensation to be trapped and prevent free airflow.
Battery Sump Jars. A battery sump jar installation may be incorporated in the venting system to dispose of battery electrolyte overflow. The sump jar should be of adequate design and the proper neutralizing agent used.
The sump jar must be located only on the discharge side of the battery venting system. (See figure 11-1.)
FIGURE 11-1. Battery ventilating systems.
Installing Batteries. When installing batteries in an aircraft, exercise care to pr event inadvertent shorting of the battery terminals. Serious damage to the aircraft structure (frame, skin and ot her subsystems, avionics, wire, fuel etc.) can be sustained by the resultant high discharge of electrical energy. This condition
Par 11-30 Page 11-491 may normally be avoided by insulating the terminal posts during the installation process.
Remove the grounding lead first for battery removal, then the positive lead. C onnect the grounding lead of the battery last to minimize the risk of shorting the “hot terminal” of the battery during installation.
Battery Hold Down Devices. Ensure that the battery hold down devices are secure, but not so tight as to exert excessive pressure that may cause the battery to buckle causing internal shorting of the battery.
Par 11-30 Page 11-492 Quick-Disconnect Type Battery. If a quick-disconnect type of battery connector, that prohibits crossing the battery lead is not employed, ensure that the ai rcraft wiring is connected to the proper battery terminal. Reverse polarity in an electrical system can seriously damage a battery and other electrical components. Ensure that the battery cable connections are tight to prevent arcing or a high resistance connection.
11-23.11-29. [RESERVED.]
Par 11-30 Page 11-493 SECTION 3. INSPECTION OF EQUIPMENT INSTALLATION
GENERAL. When installing equipment which consumes electrical power in an aircraft, it should be determined that the total electrical load can be safely controlled or managed within the rated limits of the affected components of the aircraft’s electrical power supply system. Addition of most electrical utilization equipment is a major alteration and requires appropriate FAA approval. The electrical load analysis must be prepared in general accordance with good engineering practices. Additionally, an addendum to the flight manual is generally required.
INSTALLATION CLEARANCE PROVISIONS. All electrical equipment should be installed so that inspection and maintenance may be performed and that the installation does not interfere with other systems, such as engine or flight controls.
WIRES, WIRE BUNDLES, AND CIRCUIT PROTECTIVE DEVICES. Before any aircraft electrical load is increased, the new total electrical load (previous maximum load plus added load) must be checked to determine if the design levels are being exceeded. Where necessary, wires, wire bundles, and circuit protective devices having the correct ratings should be added or replaced.
OUTPUT RATING. The generator or alternator output ratings and limits prescribed by the manufacturer must be checked against the electrical loads that can be imposed on the affected generator or alternator by installed equipment. When electrical load calculations show that the total continuous electrical load can exceed 80 percent output load limits of the generator or alternator, steps must be taken to reduce the electrical load or increase the generating capacity of the charging system. When a storage battery is part of the electrical power system, the battery will be continuously charged in flight.
ALTERNATOR DIODES. Alternators employ diodes for the purpose of converting the alternating current to direct current. These diodes are solid-state electronic devices and are easily damaged by rough handling, abuse, over heating, or reversing the battery connections. A voltage surge in the line, if it exceeds the design value, may destroy the diode. The best protection against diode destruction by voltage surges is to make certain that the battery is never disconnected from the aircraft’s electrical system when the alternator is in operation. The battery acts as a large capacitor and tends to damp out voltage surges. The battery must never be connected with reversed polarity as this may subject the diodes to a forward bias condition allowing very high current conduction and will generally destroy them instantly.
STATIC ELECTRICAL POWER CONVERTERS.
Static power converters employ solid-state devices to convert the aircraft’s primary electrical source voltage to a different voltage or frequency for the operation of radio and electronic equipment. They contain no moving parts (with the exception of a cooling fan on some models) and are relatively maintenance free. Various types are available for AC to DC or AC to AC conversion.
Location of static converters should be carefully chosen to ensure adequate ventilation for cooling purposes. Heat-radiating fins should be kept clean of dirt and other foreign matter that may impair their cooling properties.
Par 11-37 Page 11-494 (and 11-14) Static power converters often emit unacceptable levels of EMI that may disrupt communication equipment and navigation instruments. Properly shielded connectors, terminal blocks, and wires may be required, with all shields well grounded to the air frame.
CAUTION: Do not load converters beyond their rated capacity.
ACCEPTABLE MEANS OF CONTROLLING OR MONITORING THE ELECTRICAL LOAD.
The use of placards is recommended to inform the pilot and/or crew members of the combination(s) of loads that may be connected to each power source.
Installation of warning lights can be installed that will be triggered if the battery bus voltage drops below 13 volts on a 14 volt system or 26 volts on a 28 volt system.
For installations where the ammeter is in the battery lead, and the regulator system limits the maximum current that the generator or alternator can deliver, a voltmeter can be installed on the system bus. As long as the ammeter never read s “discharge” (except for short intermittent loads such as operating the gear and flaps) and the voltmeter remains at “system voltage,” the generator or alternator will not be overloaded.
In installations where the ammeter is in the generator or alternator lead, and the regulator system does not limit the maximum current that the generator or alternator can deliver, the a mmeter can be redlined at 100 percent of the generator or alternator rating. If the ammeter reading is never allowed to exceed the red line, except for short intermittent loads, the generator or alternator will not be overloaded.
Where the use of placards or monitoring devices is not practical or desired, and wher e assurance is needed that the battery will be charged in flight, the total continuous connected electrical load should be held to approximately 80 percent of the total generator output capacity. When more than one generator is used in parallel, the total rated output is the combined output of the installed generators.
When two or more generators and alternators are operated in parallel and the total connected system load can exceed the rated output of a single generator, a method should be provided for quickly coping with a sudden overload that can be caused by generator or engine failure. A quick load reduction system or procedure should be identified, whereby the total load can be reduced by the pilot to a quantity which is within the rated capacity of the remaining operable generator or generators.
DETERMINATION OF ELECTRICAL LOAD. The connected load of an aircraft’s electrical system may be determined by any one or a combination of several acceptable methods, techniques, or practices. However, those with a need to know the status of a particular aircraft’s electrical system should have available accurate and up-to-date data concerning the capacity of the installed electrical power source(s) and the load(s) imposed by installed electrical powerconsuming devices. Such data should provide a true picture of the status of the electrical system. New or additional electrical devices should not be installed in an aircraft, nor the capacity changed of any power source, until the status of the electrical system in the aircraft has been determined accurately and found not to adversely affect the integrity of the electrical system.
Par 11-38 Page 11-495 (and 11-14) JUNCTION BOX CONSTRUCTION.
Replacement junction boxes should be fabricated using the same material as the original or from a fire-resistant, nonabsorbent material, such as aluminum, or an acceptable plastic material. Where fire-proofing is necessary, a stainless steel junction box is recommended. Rigid construction will prevent “oil-canning” of the box sides that could result in internal short circuits. In all cases, drain holes should be provided in the lowest portion of the box. Cases of electrical power equipment must be insulated from metallic structure to avoid ground fault related fires.
(See paragraph 11-7.)
Internal Arrangement. The junction box arrangement should permit easy access to any installed items of equipment, terminals, and wires. Where marginal clearances are unavoidable, an insulating material should be inserted between current carrying parts and any grounded surface. It is not good practice to mount equipment on the covers or doors of junction boxes, since inspection for internal clearance is impossible when the door or cover is in the closed position.
Installation. Junction boxes should be securely mounted to the aircraft structure in such a manner that the contents are readily accessible for inspection. When possible, the open side shoul d face downward or at an angle so that loose metallic objects, such as washers or nuts, will tend to fall out of the junction box rather than wedge between terminals.
Wiring. Junction box layouts should take into consideration the necessity for adequate wiring space and possible future additions. Electrical wire bundles should be laced or clamped inside the box so that cables do not touch other components, prevent ready access, or obscure markings or labels. Cables at entrance openings should be protected against chafing by using grommets or other suitable means.
11-39.11-46. [RESERVED.]
Par 11-47 Page 11-496
SECTION 4. INSPECTION OF CIRCUIT-PROTECTION DEVICES
GENERAL. All electrical wires must be provided with some means of circuit protection. Electrical wire should be protected with circuit breakers or fuses located as close as possible to the electrical power source bus.
Normally, the manufacturer of electrical equipment will specify the fuse or breaker to be used when installing the respective equipment, or SAE publication, ARP 1199, may be referred to for recommended practices.
DETERMINATION OF CIRCUIT BREAKER RATINGS. Circuit protection devices must be sized to supply open circuit capability. A circuit breaker must be rated so that it will open before the current rating of the wire attached to it is exceeded, or before the cumulative rating of all loads connected to it are exceeded, whichever is lowest. A circuit breaker must always open before any component downstream can overheat and generate smoke or fire. Wires must be sized to carry continuous current in excess of the circuit protective device rating, including its time-current characteristics, and to avoid excessive voltage drop. Refer to section 5 for wire rating methods.
DC CIRCUIT PROTECTOR CHART. Table 11-3 may be used as a guide for the selection of circuit breaker and fuse rating to protect copper conductor wire. This chart was prepared for the conditions specified. If actual conditions deviate materially from those stated, ratings above or below the values recommended may be justified. For example, a wire run individually in the open air may possibly be protected by the circuit breaker of the next higher rating to that shown on the chart. In general, the chart is conservative for all ordinary aircraft electrical installations.
TABLE 11-3. DC wire and circuit protector chart.
Wire AN gauge copper
Circuit breaker amp.
Fuse amp.
22 20 18 16 14 12 10 8 6 4 2 1 0 5
7.5 10 15 20 30 40 50 80 100 125 5 5 10 10 15 20 30 50 70 70 100 150 150 Basis of chart:
(1) Wire bundles in 135 °F. ambient and altitudes up to 30,000 feet.
(2) Wire bundles of 15 or more wires, with wires carrying no more than 20 percent of the total current carrying capacity of the bundle as given in Specification MIL-W-5088 (ASG).
(3) Protectors in 75 to 85 °F. ambient.
(4) Copper wire Specification MIL-W-5088.
(5) Circuit breakers to S pecification MIL-C-5809 or equivalent.
(6) Fuses to Specification MIL-F-15160 or equivalent.
RESETTABLE CIRCUIT PROTECTION DEVICES.
All resettable type circuit breakers must open the circuit irrespective of the position of the operating control when an overload or circuit fault exists. Such circuit breakers are referred to as “trip free.”
Automatic reset circuit breakers, that automatically reset themselves periodically, are not recommended as circuit protection devices for aircraft.
CIRCUIT BREAKER USAGE. Circuit breakers are designed as circuit protection for the wire (see paragraph 11-48 and 11-49), not for protection of black boxes
Par 11-47 Page 11-497 or components, and are not recommended for use as switches. Use of a circuit breaker as a switch will decrease the life of the circuit breaker.
CIRCUIT BREAKER MAINTENANCE. Circuit breakers should be periodically cycled with no load to enhance contact performance by cleaning contaminants from the contact surfaces.
SWITCHES. In all circuits where a switch malfunction can be hazardous, a switch specifically designed for aircraft service should be used. These switches are of rugged construction and have sufficient contact capacity to break, make, and continuously carry the connected load current. The position of the switch should be checked with an electrical meter.
Electrical Switch Inspection. Special attention should be given to electrical circuit switches, especially the spring-loaded type, during the course of normal airworthiness inspection. An internal failure of the spring-loaded type may allow the switch to remain closed even though the toggle or button returns to the “off” position. During inspection, attention should also be given to the possibility that improper switch substitution may have been made.
With the power off suspect aircraft electrical switches should be checked in the on position for opens (high resistance) and in the off position for shorts (low resistance), with an ohmmeter.
Any abnormal side to side movement of the switch should be an alert to imminent failure even if the switch tested was shown to be acceptable with an ohmmeter.
Electromechanical Switches. Switches have electrical contacts and various types of switch actuators (i.e., toggle, plunger, push-button, knob, rocker).
Contacts designed for high-level loads must not be subsequently used for low-level applications, unless testing has been performed to establish this capability.
Switches are specifically selected based on the design for the aircraft service current ratings for lamp loads, inductive loads, and motor loads and must be replaced with identical make and model switches.
Proximity Switches. These switches are usually solidstate devices that detect the presence of a predetermined target without physical contact and are usually rated
0.5 amps or less.
Switch Rating. The nominal current rating of the conventional aircraft switch is usually stamped on the switch housing and represents the continuous current rating with the contacts closed. Switches should be derated from their nominal current rating for the following types of circuits:
Circuits containing incandescent lamps can draw an initial current that is 15 times greater than the continuous current. Contact burning or welding may occur when the switch is closed.
Inductive circuits have magnetic energy stored in solenoid or relay coils that is released when the control switch is opened and may appear as an arc.
Direct-current motors will draw several times their rated current during starting, and magnetic energy stored in their