Aircraft Battery Maintenance, Battery and Charger Characteristics, Inspection, Ventilation Systems, Installation Practices and Troubleshooting | Aircraft Systems

Aircraft Battery Maintenance, Battery and Charger Characteristics, Inspection, Ventilation Systems, Installation Practices and Troubleshooting

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 depends upon the battery’s age, state of health, state of charge, and mechanical integrity, which you can determine according to the following:
  • 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.
  • 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 battery charging system, or an overcharge condition.
  • Use a hydrometer to determine the specific gravity of the lead-acid 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. Do not add electrolyte.
  • Battery state of charge is determined by the cumulative effect of charging and discharging the battery. In a normal electrical charging system, the aircraft generator or alternator restores a battery to full charge during a flight of 1 hour to 90 minutes. 
  • 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 oriented in accordance 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.


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. [Figure 1] Products may vary from these descriptions due to different applications of available technology. Consult the manufacturer for specific performance data.

aircraft batteries
Figure 1. Battery charger

NOTE: Never connect a lead-acid battery to a charger, unless properly serviced.

Lead-Acid Batteries

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.

Lead-acid batteries are usually charged by regulated DC voltage sources. This allows maximum accumulation of charge in the early part of recharging.


NiCd Batteries

NiCd 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 5 or more years. Full discharge is not harmful to this type of battery.

NiCd sealed batteries are similar in most respects to NiCd 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 NiCd 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 NiCd battery is by a measured discharge with a NiCd battery charger and following the manufacturer’s instructions. After the battery has been fully charged and allowed to stand for at least 2 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 results in electrolyte spewage during charging. This causes corrosive effects on the cell links, self-discharge of the battery, dilution of the electrolyte density, possible blockage of the cell vents, and eventual cell rupture.

Constant current battery chargers are usually provided for NiCd batteries because the NiCd cell voltage has a negative temperature coefficient. With a constant voltage charging source, a NiCd battery having a shorted cell might overheat due to excessive overcharge and undergo a thermal runaway, destroying the battery and creating a possible safety hazard to the aircraft. Pulsed-current battery chargers are sometimes provided for NiCd 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.

Aircraft Battery Inspection

Aircraft battery inspection consists of the following items:
  1. Inspect battery sump jar and lines for condition and security.
  2. Inspect battery terminals and quickly disconnect plugs and pins for evidence of corrosion, pitting, arcing, and burns. Clean as required.
  3. Inspect battery drain and vent lines for restriction, deterioration, and security.
  4. Routine preflight and postflight inspection procedures should include observation for evidence of physical damage, loose connections, and electrolyte loss.


Ventilation Systems

Modern airplanes are equipped with battery ventilating systems. The ventilating system removes gasses and acid fumes from the battery in order to reduce fire hazards and to eliminate damage to airframe parts. Air is carried from a scoop outside the airplane through a vent tube to the interior of the battery case. After passing over the top of the battery, air, battery gasses, and acid fumes are carried through another tube to the battery sump. This sump is a glass or plastic jar of at least one pint capacity. In the jar is a felt pad about 1 inch thick saturated with a 5-percent solution of bicarbonate of soda and water. The tube carrying fumes to the sump extends into the jar to within about 1⁄4 inch of the felt pad. An overboard discharge tube leads from the top of the sump jar to a point outside the airplane. The outlet for this tube is designed so there is negative pressure on the tube whenever the airplane is in flight. This helps to ensure a continuous flow of air across the top of the battery through the sump and outside the airplane. The acid fumes going into the sump are neutralized by the action of the soda solution, thus preventing corrosion of the aircraft’s metal skin or damage to a fabric surface.

Installation Practices

  1. External surface—Clean the external surface of the battery prior to installation in the aircraft.
  2. Replacing lead-acid batteries—When replacing lead-acid batteries with NiCd 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 NiCd battery installation. Acid residue can be detrimental to the proper functioning of a NiCd battery, as alkaline is to a lead-acid battery.
  3. Battery venting—Battery fumes and gases may cause an explosive mixture or contaminated 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 should 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.
  4. 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.
  5. Installing batteries—When installing batteries in an aircraft, exercise care to prevent inadvertent shorting of the battery terminals. Serious damage to the aircraft structure (frame, skin and other subsystems, avionics, wire, fuel, etc.) can be sustained by the resultant high discharge of electrical energy. This condition 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. Connect the grounding lead of the battery last to minimize the risk of shorting the hot terminal of the battery during installation.
  6. 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.
  7. Quick-disconnect type battery—If a quick-disconnect type of battery connector that prohibits crossing the battery lead is not employed, ensure that the aircraft 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.


Battery Troubleshooting

See Figure 2 for a troubleshooting chart.

Trouble
Probable Cause
Corrective Action
Apparent loss of capacity
Very common when recharging on a constant potential bus, as in aircraft

Usually indicates imbalance between cells because of difference in temperature, charge efficiency, self-discharge rate, etc., in the cells

Electrolyte level too low Battery not fully charged

Reconditioning will alleviate this condition.








Charge. Adjust electrolyte level. Check aircraft voltage regulator. If OK, reduce maintenance interval.

Complete failure to operate
Defective connection in equipment circuitry in which battery is installed, such as broken lead, inoperative relay, or improper receptacle installation

End terminal connector loose or diengaged Poor intercell connections

Open circuit or dry cell
Check and correct external circuitry.




Clean and retighten hardware using proper torque values.


Replace defective cell.

Excessive spewage of electrolyte
High charge voltage
High temperature during charge
Electrolyte level too high

Loose or damaged vent cap



Damaged cell and seal

Clean battery, charge, and adjust electrolyte level.


Clean battery, tighten or replace cap, charge and adjust electrolyte level.

Short out all cells to 0 volts, clean battery, replace defective cell, charge, and adjust electrolyte level.

Failure of one or more cells to rise to the required 1.55 volts at the end of charge
Negative electrode not fully charged
Cellophane separator damage

Discharge battery and recharge. If the cell still fails to rise to 1.55 volts or if the cell’s voltage rises to 1.55 volts or above and then drops, remove cell and replace.

Distortion of cell case to cover
Overcharged, overdischarged, or overheated cell with internal short


Plugged vent cap

Overheated battery

Discharge battery and disassemble. Replace defective cell. Recondition battery.


Replace vent cap.

Check voltage regulator: treat battery as above, replacing battery case and cover and all other defective parts.

Foreign material within the cell case
Introduced into cell through addition of impure water or water contaminated with acid

Discharge battery and disassemble, remove cell and replace, recondition battery.

Frequent addition of water
Cell out of balance

Damaged “O” ring, vent cap

Leaking cell




Charge voltage too high
Recondition battery.

Replace damaged parts.

Discharge battery and disassemble.
Replace defective cell, recondition battery.

Adjust voltage regulator.

Corrosion of top hardware
Acid flumes or spray or other corrosive atmosphere

Replace parts. Battery should be kept clean and kept away from such environments.

Discolored or burned end connectors or intercell connectors
Dirty connections Loose connection Improper mating of parts

Clean parts: replace if necessary.
Retighten hardware using proper torque values. Check to see that parts are properly mated.

Distortion of battery case and/or cover
Explosion caused by:
Dry cells
Charger failure
High charge voltage
Plugged vent caps
Loose intercell connectors

Discharge battery and disassemble.
Replace damaged parts and recondition.