BGA AIRWORTHINESS AND MAINTENANCE PROCEDURES

PART 4, LEAFLET 4-9

MAINTENANCE AND TESTING OF GLIDER BATTERIES

GENERAL
1. Most gliders have batteries installed to provide power for radios, GPS navigation systems and some flight instrumentation such as electric variometers. The batteries used are normally of the Gel type. These offer advantages over liquid (wet) types in that they are less prone to spillage. As more reliance is placed on electric instrumentation and radio in recent times it is desirable to establish the capacity of the battery so that it may be of use for the intended flight. On CAA registered aircraft it is a requirement to have a battery of at least 80% capacity. In gliders since there is no starter motor demand on the battery a lower figure can be acceptable. The capacity may be as low as 50%, below this figure then the battery is unlikely to last for the majority of flights depending on the load on it. It is also worth bearing in mind that temperature affects the capacity of the battery, the colder it is the lower the capacity. See section on ‘Capacity testing’.
This leaflet only deals with Lead Acid Batteries. (For Ni-cad batteries please refer to the aircraft or battery manufacturer)

MAINTENANCE

2. (a) Case: The battery should be inspected for condition, cracks or excessive wear in the outer casing, if fill plugs are fitted these should be secure and free from signs of leaks. If any battery gel or liquid acid has leaked out please refer to AMP 4-3, 8 (I) for corrective action for the spillage of battery acid. (b) Terminals: The terminals should be checked for security or looseness, cracks or corrosion. Check that the connectors or disconnect plug (s) are in good condition and a good fit. If the plating is worn and corrosion evident either; replace the terminals, or remove the deposits using hot water and reprotect with petroleum jelly. Do not use grease. (c) Fuse: All glider wiring should be protected by a fuse situated as close to the battery terminal as possible. By convention the fuse is usually in the Positive + line. Motor gliders are not normally fused at the battery due to the high starter motor load, but rely on individual system fuses or circuit breakers. The majority of glider applications use a 5A fuse, refer to manufacturers information if available.
(d) Mounting: Check the security of the battery mounting and the means of retaining the battery. As most glider batteries are located in the centre section behind the pilot’s head it is imperative that the battery is retained in the event of any accident. Some mechanical means of securing the battery is required. The practice of using an elastic band is not acceptable.
(e) Fit of Battery: The battery must fit the battery stowage correctly. If a smaller battery is used than initially designed for, then suitable packing must be used to prevent the battery moving. The packing must be secured in place and also not be capable of absorbing any spilt battery fluid. (f) Battery Stowage: Glider batteries are normally housed in either wooden or GRP battery stowage and gel batteries pose no significant corrosion problem. If a ‘wet’ battery is used then precautions against corrosion should be taken especially if a metal battery stowage is used. A metal stowage other than Stainless Steel, should be protected with acid proof paint. Acid proof paints are available

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through normal consumable suppliers. Some installations are fitted with vent and drain tubes. Ensure that these are clear and correctly fitted. Some have clamps in the drain line – these should be drained each annual inspection. (g) Servicing: Gel batteries require no servicing and are filled for life. ‘Wet’ batteries require servicing (unless sealed & maintenance free) due to evaporation of battery fluid. Only use clean distilled water to top up the battery and do not overfill. Do not add acid. If a battery requires constant topping up the charging system may be at fault and overcharging or the battery may be leaking.
CHARGING
3. Two level charging is recommended, the initial charge must not exceed the manufacturers recommended current and when the battery is charged the charge rate should automatically fall to a float charge rate. This prevents damage to the battery plates and minimises the loss of electrolyte due to evaporation. Do not use fast chargers under any circumstances, as permanent damage to the battery will most likely be caused.
CAPACITY TESTING
4. The capacity of a battery needs to be established from time to time, this is usually once a year, to ascertain if the battery will reliably last for the intended flight. Most electric varios and GPS units will work satisfactorily at a voltage down to 11 volts (on 12-volt nominal systems) but radios often do not transmit satisfactorily at this voltage. All electronic equipment on stand-by will use some current, so there is a constant drain on the battery. A battery with a low capacity may work satisfactorily at the launch, but quickly becomes exhausted and may not work when needed during the flight or landing.
Batteries are rated in Voltage and Amp hours. Some may have maximum amperage stated.
The most common glider battery is a 12 volt 7 amp hour (Ahr) battery. This means for example; a 12v/7Ahr battery will at 100% capacity, nominally supply 1 amp for 7 hours or 7 amps for 1 hour. The Max. Amperage really only comes into play if you have a starter motor to operate as the battery will have larger terminals and internal components to supply the heavier load. For Motor Gliders and Self Sustainer Sailplanes the Parts Catalogue should be consulted for the correct battery to ensure that the battery is capable of delivering sufficient amperage for starter loads.
5. Quick Check If a battery is suspect, a quick check of the open circuit terminal voltage will show if the battery
needs to be recharged or if a cell has failed. Assuming the battery is not just off charge, a voltage of about 12½v or more should indicate a good battery. If this check can be done under load e.g. by momentarily pressing the transmit button on the radio the voltage should not fall below 12v.
6. Capacity test By convention the rated capacity of a lead acid type battery is stated for the current which will
fully discharge the battery in 20 hours. For practical purposes this means when the terminal voltage, under load, falls to 10½v for a nominal 12v battery. One can arrange to discharge at a faster rate but the apparent capacity (amp hours) will be slightly less. For a practical approach it is convenient to discharge at a rate such that the battery is fully discharged in 5 hours (i.e. down to 10.5v), in which case the nominal 20 hour capacity will be about 10% more than measured.
Note: Two 12v/21w car stop light bulbs wired in series provides a convenient 1.1 to 1.2A load which draws almost a constant current at 12v as the battery discharges. Two 12v/21w bulbs wired in parallel will draw a load of 3.5A at 12v.

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If the glider is only equipped with an electric vario or vario plus GPS then a capacity of about 3½ amp hours will be sufficient for a typical days flying, however if a radio is fitted in addition then there should be a minimum of 4½ amp hours available. (Approximately 50% and 65% of a 7Ahr battery respectively)
8. Temperature effect The available capacity of the battery will fall with a drop in temperature. At 0°c only 80% is
available and at -15°c only 65% is available compared with room temperature.
9. Method Note: this is a simplified method of capacity testing applicable to smaller batteries used in gliders and motor gliders and using equipment readily available to most BGA inspectors. For Tugs and larger batteries, 20Ahr and above, it is recommended that specialist equipment (CAA approved if necessary) be used.
Equipment required: Battery charger, Known load, Accurate Voltmeter & Ammeter, Clock, Calculator and label or marking paint. Note: Two meters are required, digital if possible. If only one multimeter is available then use the voltmeter function, but you will need to periodically check the current during the discharge procedure.
a) Fully charge the battery and leave to stand off charge or allow to float charge overnight. b) Measure the terminal no load voltage. You should have approximately 13.6v. c) Connect known load (2 x 12v/21w bulbs in series = 1.2A approx.) (2 x 12v/21w bulbs in
parallel = 3.5A approx.) and ammeter across the battery as per diagram below. d) Record time it takes the battery to discharge to 10½v. Monitor battery voltage & record
current against time at 15 minute intervals for first hour, then half hour intervals until the voltage drops to about 11 volts, then every 10 minutes until the voltage falls to 10½ volts. Record the time taken in minutes. e) To Calculate capacity – Take the mean current between two readings and multiply by the time interval and record amp minutes. Summate all the amp minutes and divide by 60 to convert to amp hours. The capacity at the 20 hour rate is then (measured Amp hours ÷ nominal Amp hours) x 110%. f) Recharge battery g) Record capacity and date on battery. (Use label or paint). h) Establish that battery capacity is suitable for the particular application taking into account the low temperature effect if applicable.
Capacity test set up:

Ammeter

Load

+ Battery

-

Example of a capacity test, over page;

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Voltmeter

Example: 12v / 7Ahr battery

Elapsed time
0 min 15 min 30 min 45 min 60 min 90 min 120 min 150 min 180 min 210 min 240 min 270 min 280 min 290 min 300 min 310 min 320 min

Battery Voltage
13.6v 13v 13v 12.8v 12.6v 12.2v 12.0v 11.8v 11.6v 11.4v 11.2v 11.0v 11.0v 10.8v 10.7v 10.6v 10.5v

Current draw
0A 1.2 A 1.2 A 1.2 A 1.1 A 1.1 A 1.1 A 1.0 A 1.0 A 1.0 A 1.0 A 1.0 A 1.0 A 1.0 A 0.9 A 0.8 A 0.7 A

Amp/minutes (to nearest .5)
18 A min 18 A min 18 A min 17.5 A min 33 A min 33 A min 31.5 A min 30 A min 30 A min 30 A min 30 A min 10 A min 10 A min 9.5 A min 8.5 A min 7.5 A min 334.5 A min

334.5 ÷ 60 = 5.6 Ahr ÷ 7 Ahr = 0.8 x 110 = 88% @ 20 hr rate

≈ 7 Ahr x 88% = 6.1 Amp hours available @ 20 hr rate
If in the above example an assumed current of 1.1A was used without the use of an ammeter then a result of 352 A min would have been recorded. This would have given an artificially high value of 91.6% battery capacity. This may be of no concern in the majority of applications but may give problems if battery capacity is tight for longer flights or in cold conditions.

Thanks to F J (Jim) Tucker, Southdown GC for assistance in compiling this leaflet.

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