U.S. Department

of Transportation

Federal Aviation

Administration

 Advisory

 Circular

AC 00-33A

DATE: 02/14/1973

 AC No: 00-33A

 Date: 14 Feb 73

 Initiated

 by: AFS-350/130

 Subject: NICKEL-CADMIUM BATTERY OPERATIONAL, MAINTENANCE, AND

 OVERHAUL PRACTICES

 1. PURPOSE. This circular provides guidelines for more reliable

 nickel-cadmium battery operation through proper operational

 and maintenance practices, and has been reissued to include

 reconditioning information.

 2. CANCELLATION. Advisory Circular 00-33, dated August

 1971, is cancelled.

 3. BACKGROUND. An increasing number of potentially hazardous

 incidents involving nickel-cadmium batteries, during flight

 and ground operations, have been reported. The failures are

 more prevalent where the batteries are charged directly from

 the DC bus rather than by a separate battery charger.

 Although the nickel-cadmium battery is capable of delivering

 large amounts of current, the battery is inherently

 temperature sensitive and a majority of the reported

 incidents can be attributed to overheating. The overheat

 conditions can be minimized or averted by following proper

 operational, maintenance, and overhaul practices.

 4. THERMAL EFFECTS ON NICKEL-CADMIUM BATTERIES. The

 nickel-cadmium battery is capable of performing to its rated

 capacity when the ambient temperature of the battery is in

 the range of approximately 70 degrees to 90 degrees F. An

 increase or decrease in temperature, from this range, results

 in reduced capacity. A combination of high battery

 temperature (in excess of 100 degrees F) and overcharging can

 lead to a condition called "thermal runaway." Basically,

 "thermal runaway" is an uncontrollable rise in battery

 temperature that will ultimately destroy the battery. This

 condition can occur when a nickel-cadmium battery is operated

 at above normal temperatures and is subjected to high
 charging currents associated with constant voltage charging.

 As the temperature of the battery increases, the effective

 internal resistance decreases and higher current is drawn

 from the constant voltage charging source. The higher

 current increases the battery temperature which in turn

 results in even higher charging currents and temperatures.

 5. BATTERY OVERHEAT FACTORS. Battery overheating can be caused

 or accelerated by the following factors:

 a. Frequent engine starts and excessive engine cranking.

 b. Aircraft generator bus voltage too high.

 c. Improper charging and infrequent battery

 reconditioning.

 d. Unnecessary use of the aircraft batteries to run

 auxiliary equipment such as lights, avionics equipment,

 ventilation systems, etc. during ground operations.

 e. Poor or no ventilation of the battery compartment during

 high ambient temperatures particularly during ground

 operations.

 f. Loose cell-to-cell connections (commonly called links).

 g. Current leakage between cell and battery container and

 airframe ground.

 h. Cells low on electrolyte.

 i. Ground operations using power units with voltage settings

 higher than the recommended aircraft bus voltage, or

 power units with poor regulation.

 j. Cell imbalance.

 6. OPERATIONAL PRACTICES TO PREVENT BATTERY OVERHEATING.

 a. Reduce the number of consecutive engine starts by

 programming the use of a well regulated external power

 supply when a series of short duration flights or

 consecutive engine starts are planned. This procedure

 will allow the battery to dissipate some of its

 accumulated heat. Avoid prolonged engine cranking and

 follow the manufacturer's recommended rest periods

 between starts to minimize battery over-heating.

 b. Frequent inflight monitoring of the aircraft bus voltage

 and load current will provide an indication of any

 increase, decrease or fluctuations of the aircraft bus

 voltage or load current indicating an abnormal condition.

 c. An increase in load or charge current as indicated on the

 aircraft load meter, especially during normal cruise,

 with no additional circuits being energized may be an
 indication of battery overheat or failure. Initiate

 corrective action as soon as possible.

 7. MAINTENANCE PRACTICES TO PREVENT BATTERY OVERHEATING.

 a. Service batteries at the interval recommended by the

 aircraft and battery manufacturer; however, more frequent

 servicing may be necessary depending upon the type of

 operation you are conducting.

 b. The aircraft voltage regulator setting should be checked

 periodically to correct for out-of-calibration units and

 replacement of defective units thereby reducing the

 possibility of an inadvertent increase in charging

 voltage/current and a resultant rise in battery

 temperature.

 c. During extended ground operation, under high outside

 ambient temperatures, keep the battery loads to a minimum

 and ensure there is adequate battery compartment

 ventilation. Additional ventilation may be provided by

 opening the battery compartment access door or using

 forced air ventilation.

 d. Check and maintain the manufacturer's recommended torque

 values on intercell connections during routine

 maintenance inspections. This will reduce the

 possibility oflocalized heating that can be caused by

 high currents passing through poor connections and

 feeding back into a cell or cells.

 e. Periodic measurement of battery leakage current and

 removal of any electrolyte that may have accumulated

 around and between the cells will prevent high leakage

 currents and short circuits from developing and heating

 the battery.

 f. Cell electrolyte level should be monitored frequently and

 if below the minimum requirement the battery should be

 removed from service for reconditioning. This will

 reduce the possibility of localized cell overheating.

 g. When charging a battery in the aircraft assure that:

 (1) The battery compartment is well ventilated.

 (2) The ground power unit voltage setting does not

 exceed the aircraft bus voltage specified by the

 aircraft manufacturer; is well regulated; and its

 volt/ammeters are accurate.

 (3) The battery cover is off during charging to allow

 visual monitoring and to increase ventilation.

 (4) The battery is not charged when the battery

 temperature or battery compartment temperature is

 above approximately 100 degrees F.
 8. BATTERY INSPECTION. Visually inspect the battery and

 associated hardware on a regular basis. Depending on the

 type of aircraft operation, it is considered good practice to

 establish an electrolyte level inspection interval based on

 the battery and aircraft manufacturer's recommendations.

 Conduct a detailed investigation when any of the following

 conditions are noted:

 a. Cell case distortion indicates the battery may have been

 overheating. The battery should be removed and sent to a

 maintenance facility or factory for cell replacement.

 b. Cell link corrosion.

 c. Burn marks or signs of overheating on battery terminals

 or cell links. This indicates that the connectors

 involved have not been properly tightened.

 d. Electrolyte has spewed or leaked from cells.

 e. Battery and cell vents are obstructed.

 The use of a service log provides an accurate service record

 of battery inspections and malfunctions. It can also be a

 useful tool in determining the optimum period between

 reconditionings.

 9. RECONDITIONING SERVICE. It is characteristic of a

 nickel-cadmium battery to undergo a temporary loss of

 capacity during its normal duty cycle. This temporary loss

 of capacity is normally an indication of imbalance between

 cells. If not regularly maintained, this imbalance can lead

 to cell reversal and premature battery failure. The purpose

 of periodic reconditioning is to restore a battery to its

 full capability and to prevent premature damage and failure.

 The following factors should be considered when establishing

 reconditioning cycles for various types of aircraft.

 a. Battery manufacturer recommendations; for example one

 battery manufacturer recommends the following approximate

 battery reconditioning cycle periods:

 Type Approximate

 Reconditioning Period

 (1) Lear 23, Jet Commander, MU-2 100 hours

 and Turbo Commander

 (2) King Air, Beech 99, Fan Jet Falcon 100 hours

 (3) Hansa Jet, Twin Otter, Merlin I 100 hours

 and IIB

 b. Frequency of engine or auxiliary power unit starting

 service.
 c. Battery duty cycle.

 d. Ambient operating temperatures.

 e. Operator service experience will dictate the need for an

 increase or decrease of time between reconditioning

 periods. One method of determining this is by the amount

 of water consumption between reconditioning. (Each

 manufacturer specifies the amount of water that can be

 expected to be needed after a specific period of

 service.) If during servicing, all of the cells require

 more water than is normally specified by the manufacturer

 it may indicate problems with the aircraft voltage

 regulator (charging voltage too high) or you may have a

 need for more frequent reconditioning.

 Aircraft equipped with battery temperature sensors should

 have the sensor accuracy tested at the time of battery

 reconditioning. It is important that this test be performed

 on a regular basis.

 10. SHOP MAINTENANCE. Follow the battery manufacturer's

 instructions regarding periodic servicing, capacity checks,

 and reconditioning procedures to ensure a reliable and

 properly conditioned nickel-cadmium battery. The following

 area should be given special attention:

 a. Battery facilities. Separate shops, equipment, and

 tools are recommended for servicing nickel-cadmium and

 lead-acid batteries.

 b. Anything associated with lead acid batteries (acid fumes

 included) that comes in contact with a nickel-cadmium

 battery or its electrolyte can cause severe damage.

 11. OVERHAUL PRACTICES. The construction and design of

 nickel-cadmium batteries allows easy overhauling of the

 individual cells. The following guidelines are recommended

 to ensure meeting the original battery manufacturer's

 specifications.

 a. It is recommended cells be overhauled only once. It is

 the repair facility's responsibility that repaired cells

 meet all manufacturer's specifications before approval

 for return to service.

 b. Manufacturers do not recommend mixing cells. Some

 manufacturer's warranties are void if cells are mixed.

 c. New and overhauled cells may be identified as follows:

 (1) New cells by the manufacturer's part number stamped

 on the case.

 (2) Overhauled cells with the manufacturer's or repair

 station's identification stamped on the cell case,

 or color coded on the cell cases.
 (3) The original manufacturer's part number should be

 retained on the overhauled cell to preclude mixing

 of cells. Repair agencies should mark the

 overhauled battery with their identity mark so as to

 not obliterate the original manufacturer's

 identification.

 d. In addition to the standard quality control procedures,

 inspect plates carefully for evidence of burned, crimped,

 or frayed edges, hot spots or other damages.

 e. Damaged cell plates or stacks should be discarded.

 Nickel-cadmium cells are not consistently identical with

 respect to their capacity.

 12. SUMMARY. Optimum and reliable performance can be expected

 from nickel-cadmium batteries only when they are operated,

 maintained, and overhauled in accordance with the battery

 and aircraft manufacturers' instructions. Overheating and

 thermal runaway are the prime causes of battery degradation

 and cell/battery destruction. The degree of reliability is

 directly proportional to the quality of the practices

 followed in their operation, maintenance and overhaul.

 /s/ C. R. MELUGIN, JR.

 Acting Director, Flight Standards Service