b1 - "What do you mean the batteries are no good? I just removed them from the airplane!"
by Joseph F. Mibelli (JFM Engineering) - 24 June 1997

b2 - "Why do you condemn so many of my batteries?"
by Joseph F Mibelli (JFM Engineering, Inc.) - 14 July 1997

b3 - "Taking Care of Batteries - How to avoid costly mistakes"
by Joseph F. Mibelli (JFM Engineering) - 1995

b4 - "Notes on Trickle Charging of Batteries"
by Joseph F. Mibelli (JFM Engineering) - 5 July 1999

b5 - "The Importance of Battery Testing"
by Joseph F. Mibelli (JFM Engineering) - 2 April 2011

b1 - "What do you mean the batteries are no good? I just removed them from the airplane!"
by Joseph F. Mibelli (JFM Engineering) - 24 June 1997

    Emergency batteries that are connected to the bus are constantly in charge and thus continuously evaporate water from the electrolyte. As the electrolyte level drops and the plate separator begins to be exposed (dried out in extreme cases), the separator material begins to deteriorate which results in cell heating and shorts in extreme cases.
    Batteries that are subject to continuous charging and have little or no opportunity to deliver power, need to be removed periodically, first to check the water level and second to check for capacity.
    Water level checking cannot be performed on the aircraft. It can only be performed under bench test conditions with a constant current charger and only when the battery has reached full charge. Excessive water consumption can be indicative of overcharging (bus voltage too high) or infrequent servicing, or both. The time required for this test will range from one day for a "good" battery to several days for a "problem battery".
    Since emergency batteries are basically in stand-by condition and are subject to continuous charging, their capacity to deliver current when needed slowly diminishes (capacity fading), so it is also necessary to periodically perform a capacity test. If this test is passed marginally, or not at all, the cells have to be deep cycled (total discharge) to restore the rated capacity. Depending on the severity of the fading, the total discharge and subsequent recharge must be performed several times before proper capacity restoration will occur. The time required for this type of testing will require from two days for a "good" battery to a full week for a "problem" battery.
    Batteries that do not pass the required tests can be repaired by replacing the individual cells that fail the specific tests, but not more than 20% of the total number of cells in the battery (4 to 5 cells) should be replaced. If more than 20% of the cells need to be replaced, the entire battery needs to be replaced (this is done to minimize the mismatching between new cells and old cells).
    Under normal conditions, most batteries are expected to last five to six years, provided that they are serviced properly (Including occasional cell replacement). This is true even for the larger batteries that are used to start engines or APU’s. But, with improper maintenance (basically infrequent maintenance) the life of the batteries will be significantly shorter. If servicing is infrequent, by the time that the batteries are finally removed for testing, it may be too late.
    Proper servicing is costly. Time to do it, proper personnel, availability of a replacement battery, service charges by the battery shop, etc. But, if as a result of inadequate servicing the battery must be replaced, its cost far exceeds the cost of proper servicing. This is also true if a battery failure results in a grounded airplane. Finally, the cost of an in-flight battery failure (Overheating, little or no capacity to provide power, etc.) could have more severe consequences.

b2 - "Why do you condemn so many of my batteries?"
by Joseph F Mibelli (JFM Engineering, Inc.) - 14 July 1997

    Batteries are condemned when they no longer meet the performance requirements set by the manufacturer of the battery and/or the manufacturer of the aircraft and when it is not economical to repair it by the replacement of all cells. Basically, not meeting performance requirements means not passing the capacity test, but batteries can also be condemned for failing charge tests.

Discharge:

    The most comprehensive of battery tests and also the one requested/performed more often is the deep cycle/capacity test. In this test, the battery must be discharged at a specific rate (usually 1C) for one hour and must not fall below 20.0V (for a 20 cell battery). Also, individually, none of the cells must fall below 1.00V. If any of the cells fail this test, even if the total battery voltage is above 20V, the battery does not pass the capacity requirements and must be discharged totally (each cell discharged fully) and subsequently recharged, as the first attempt to restore the capacity of the cells that failed.
    If a second capacity test shows that the battery still fails but there is an improvement in the cells that failed, the cycle is repeated and there is a great probability that the battery will pass the third capacity test. But, if the second capacity test shows little or no improvement, then, there is a great probability that not only the battery will fail the third test, but that additional cells will also fail. (When a battery is in a bad condition, additional testing often results in more cell failures).
    If the total number of cells that need replacement is greater than 20% (more than four or five depending on how good the rest of the cells are), then, any further cell replacement is not advisable due to the mismatch between the old and new cells, hence total cell replacement is recommended. In most cases, however, the price of a new battery is favorably close to or better than the price of new cells, so it is much better to condemn the old battery and to replace it with a new one. (Sometimes, cost of temperature sensors or special connectors dictates that cell replacement, rather than total battery replacement is the preferred method).

Charge:

    During charging, a Nickel-Cadmium battery in good condition must show a continuous increase in terminal voltage (no significant drop of voltage in any of the cells), and must not show any increase in temperature.
    If a battery has been in service (just pulled from the aircraft) and shows a noticeable warming during charge, it is a sure sign that one or more of the cells have separator damage (usually caused by low water level due to evaporation from overcharge in the aircraft and infrequent service). There are no alternatives here, once the separator is damaged, the cell(s) must be replaced.
    Failure of the cell separator also prevents cells from reaching the minimum end of charge voltage of 1.50 to 1.55V/cell. Or it may cause cells to show a drop of terminal voltage even after achieving a good initial charge voltage. This is also remedied only by cell replacement.

Note: Unlike in the aircraft, where batteries are charged under constant voltage conditions by being connected to a 28V bus (on a 20 cell battery this means 1.4V/cell), bench charging is done with constant current to allow the cells to reach the minimum charge voltage of 1.5 to 1.55V/cell. Also, it is only at the end of this type of charge that the water level can be tested and adjusted.

Other:

    Another reason to condemn a battery is the finding of unauthorized cells. Battery manufacturers are very specific in determining that only genuine OEM cells can be used to overhaul/repair a battery. This means that cells from another manufacturer, however good or better that they may be (form, fit and function), cannot be approved. Also, cells that have been remanufactured are not approved, even if they are the correct type. The original manufacturers abandoned this practice long ago because it proved to be an unreliable process.          The performance of remanufactured cells cannot be guaranteed. (A recent advisory from the FAA provides additional information on this subject).

One last note:

    Batteries that have been in storage for a long time (six months or more) without proper maintenance (charge and water) may show warming/overheating in charge and/or faded capacity in discharge. Depending on the original condition of the battery, complete revival may or may not be possible. This is particularly true of batteries that are part of excess lots that are disposed off at very enticing low prices after being on the shelf for a long time (years).
    Proper battery testing takes time. Batteries in good condition can be tested in one day, but batteries that have problems require multiple cycles that can stretch for many days. There are techniques that can shorten some of the testing, particularly the charge, but the time savings is only a few hours. In AOG situations there may not be time to do all the cycling needed to revive cells, so, outright replacement with new cells (or a new battery) may be the only way to guarantee that the battery will be ready when needed.

Recommendations:

    Nickel-Cadmium batteries have a life expectancy of 5 to 6 years provided that they are serviced properly, even when used in heavy duty situations such as engine start-up applications. But, batteries can also fail in applications where they are seldom required to provide power, such as in emergency stand-by applications. The key to long battery life is maintenance.
    First, to check the water level, since batteries are always losing water due to evaporation, particularly when they are under continuous charge conditions. And second, to check for capacity and to deep cycle it when cells show signs of severe imbalance and/or lost capacity.
    Please contact JFM Engineering (305-599-6893) for additional details on battery servicing and maintenance plan recommendations aimed at reducing the overall cost of battery maintenance.

b3 - "Taking Care of Batteries - How to avoid costly mistakes"
by Joseph F. Mibelli (JFM Engineering) - 1995

    Let's face it, the battery, even on an aircraft, does not rank high in anybody's list of important items to take care of. Until it fails that is, then, all of a sudden it generates an instant awareness and a dire need to be educated, sort of, on the subject of battery care. This, after first recuperating from the minor shock caused by the cost of battery overhaul or replacement.
    The batteries in an aircraft are part of an overall system that includes emergency backup for various items, some of them very critical. The main battery in a small aircraft is used for engine starting and for main emergency backup. In larger aircraft, the main battery is used to start the APU and also for main emergency backup. Smaller battery packs are also used to provide emergency illumination and emergency power backup for avionics.
    Battery maintenance, then, has to be considered as important (more in some cases) as the maintenance for engines, structure and other vital parts. Improper battery care can result in problems that range from nuisance to deadly. From a simple "the aircraft cannot leave" to "the engines are out and the APU cannot be started", and everything else in between.
    Even if battery problems do not generate life threatening situations, the cost of overhauls or outright replacements can be very extensive. Unlike the typical car battery, the aircraft battery is a precision device and must be serviced accordingly.
    The main battery in the majority of larger aircraft is a Nickel-Cadmium type, with sealed Lead-Acid gradually gaining acceptance, particularly in the smaller aircraft. But, regardless of the technology, all batteries are required to perform the same task: supply current when required. Then, how do we know that the battery will deliver the required amount of power?
    Bench testing, under specific conditions, as set forth by the battery manufacturer, is the only reliable way to determine the condition of a battery.
    There are no simple direct measurements, such as placing a voltmeter across the terminals, to determine the condition of the battery. The voltmeter reading may tell us something about the state of charge (with an enormous margin of error), but it cannot tell us how well the battery will deliver current when demanded. This is particularly true for Nickel-Cadmium batteries that have a very shallow discharge curve, but it is also true for Lead-Acid batteries, even though they have a more pronounced discharge profile.
    The most basic and crucial bench test is the capacity test. This test determines if the battery will deliver the rated current in the minimum time interval, while maintaining a terminal voltage above some minimum. A typical 40A-Hr battery is required to deliver 34A (85%), for one hour with a terminal voltage of no less than 20V.
    The currents under real conditions such as engine starting are many times higher, or they can be many times lower, such as in backup situations. The bench test, even though it is not a realistic condition, it gives us a reliable and uniform method to determine the condition of the battery.
    As with any kind of a test, where the resulting numbers will lead to a pass or fail decision, the equipment used to test the batteries has to be of instrumentation quality. The equipment must allow the operator to program and monitor the test parameters as set by the manufacturer.
    Unlike the car battery, where a simple and inexpensive charger is all that it needed (plus a voltmeter for those that know what they are doing), the equipment required for the testing of aircraft batteries, whether Nickel-Cadmium or Lead-Acid, is many times more complex and demanding.
    Why do batteries have to be tested in the first place? Just as with any other part of the aircraft, normal use wears out the various elements of the battery system (plates, separators and electrolyte), which slowly reduces the capacity of the battery.
    In addition, normal use under extreme conditions, abuse and improper maintenance will significantly shorten the life of the battery. Of these, inadequate maintenance can become the major cause of short battery life.
    The proper approach for long battery life begins with periodic maintenance, most of the times at intervals much shorter than those established by the aircraft manufacturer.
    Test intervals, as given by aircraft manufacturers are for reference only. The actual time between tests has to be established by usage (number of starts) and by the results of bench tests (water consumption).

• Taking care of the aircraft's batteries, the Owner/Pilot perspective

    As the owner/pilot of the aircraft, you have enough problems to worry about. The last thing you want to hear from your mechanic is that the aircraft is grounded because of problems with a battery. Or if inflight, you do not want to find yourself having to make an unscheduled landing because of a "battery hot" indication.
    Worse yet, if a generator malfunction forces you to depend entirely on the batteries to make an emergency landing, do you have the confidence that the batteries will give you the necessary power?
    Therefore, you need to make sure that not only are the batteries tested as often as required, but that you also maximize the life of the batteries by reducing usage under extreme or abusive conditions.
    The first part is relatively easy, just test the batteries as often as required.
   The second part, however, is not so simple. First of all, what constitutes usage under extreme or abusive conditions? The starting of engines is the mechanism that rapidly wears out the main batteries.
    The peak discharge current, often exceeds 1,000A and lasts for several seconds. It is followed by a drain of several hundred amps, lasting for several tens of seconds. Finally, as the engine fires up, the battery is hit with a high recharge current as the starter motors become generators.
    Usage of the batteries under extreme temperature conditions further contributes to the wear-out mechanism. Multiple short flights (less than 30 minutes) where there is not enough time to replenish the charge used to start the engines also contribute to the wear of the battery.
    Remember also that the batteries are used to power other electrical devices, such as air conditioning, while the aircraft is on the ground. When the engines are at idle, the RPM's are not high enough to allow the generators to replenish the current consumed by the equipment.
    If the batteries are not properly recharged, they will have less and less charge reserve every time that they are asked to deliver starting current. If this practice is continued to the point where one or more of the cells reach full discharge, subsequent starts will cause those depleted cells to reverse, severely cutting the overall available capacity and also reducing their useful life.
    The engines will also suffer with batteries in poor conditions. With a battery that cannot deliver proper power, the result is a hot start, which results in costly engine overhauls (well in excess of the cost of maintenance or replacement of the batteries).
    The simplest and best solution to this problem is to use ground power equipment for engine starting. The next option is to shorten significantly the period for testing of the batteries. If the manufacturer of the aircraft calls for battery testing every 100 hours, reduce the interval to 75 hours.
    Better yet, alternate electrolyte check and deep cycle every 50 hours for best performance. This, at least, will insure that the batteries are in proper conditions to meet the heavy demand.
    Another condition that can shorten the life of the battery is overcharging while in flight. Batteries can be literally cooked when subjected to overcharge by an improperly set or mal- functioning voltage regulator. This condition can also lead to catastrophic results. The battery can go into thermal runaway with heavy release of toxic fumes and it can also explode if the charge is not terminated.
    Emergency batteries, such as used to power the artificial horizon, lights and other types of essential equipment, are normally not in use. They remain in stand-by under continuous charge while the aircraft is active and they do not deliver current unless called for in emergencies or routine equipment tests.
    Batteries that are subject to such a state of inactivity will gradually degrade in capacity, and will fail miserably when called upon to deliver power (such as in case of a main power failure).
    This type of failure is known as capacity fading, and can be prevented by testing all emergency batteries (capacity test, deep cycle) as often as the main batteries are tested.
    Failure to do so, will result in little or no backup power under emergency situations (the consequences of which need no additional reminders).
    When such batteries are taken for service after prolonged periods of inactivity, the cells may be so degraded that no amount of deep cycling will restore the required capacity. Thus resulting in costly overhauls.

• Taking care of the aircraft's batteries, the Repair Station perspective

When an aircraft is brought in for routine service, the main battery and the many additional batteries may also be serviced if called for at that time. No problems here. What constitutes a problem for batteries is when the aircraft is going to be inactive for a prolonged period of time. Depending on climatic conditions (usually high temperature), the batteries will experience from a minor self-discharge to severe loss (evaporation) of electrolyte. The time periods could be several months to as short as a few weeks. If the time period is short, all that is needed is to top charge the battery (which includes verification of electrolyte level).
    Batteries that have been "abandoned", however, will require a deep cycle (several) to bring them back to proper levels. If the evaporation of the electrolyte has exposed the cell separators, the affected cells may be ruined and thus require replacement.
    It is important, then, to immediately remove and to service the batteries of any aircraft that are expected to be inactive for a prolonged period of time.
    If this is done, the batteries will be ready for service when the work on the aircraft is completed. Batteries that are serviced and are not immediately placed on the aircraft to be used, will require a top charge prior to installation if they have remained on the shelf for more than two weeks.
    One way to maintain the batteries always ready is to connect them to a trickle charger. Batteries can then be kept in standby for several months without any detrimental effects (the low charge current is controlled to simply offset the loss due to self-discharge).

• Types of tests performed on batteries

(Description of tests refer primarily to Nickel-Cadmium batteries. Additional comments are provided for Lead-Acid batteries).

Top charge:

    Top charge is the simplest type of service for all types of batteries. Batteries are "topped off" before being put on the aircraft to compensate for self-discharge while in storage. Top charge is also used to determine the proper electrolyte level. Water is normally lost during usage and it is also lost due to evaporation. When the battery reaches full charge, the electrolyte is at its maximum level; distilled water is added as required.
    The top charge process is also used to measure cell voltages, to determine that each cell reaches the proper charge voltage and to check if any cells exhibit a temperature rise and/or drop in their voltage under constant charge current (topping current).
    Lead-Acid batteries are also top charged, and if of the flooded type, the specific gravity and level of electrolyte is also tested and adjusted as necessary. On sealed batteries, the specific gravity of the electrolyte cannot be measured.

Capacity Test:

    This is the most important test, for it determines if the battery is fit to return to service. After receiving a full charge, the battery is subjected for one hour to a typical discharge current of 85% of its rating, or it can be discharged at the 2 hour rate at 100% of its rated capacity. If none of the cells drop below 1V, the battery passes the capacity test. It is then recharged and returned to service. If one or more of the cells drop below 1V (even if the battery as a unit does not drop below 20V) the battery fails the capacity test.
    What happens next depends on the individual condition of the cells. If the cell voltages are reasonably similar (balanced), the battery is recharged and retested for capacity. If the cells are heavily unbalanced, then, the battery is subjected to a full discharge (deep cycle).
    If after three tries, one or more cells fail to meet the capacity test, they are replaced. If more than 20% of the cells need to be replaced, it is recommended that either all the cells be replaced or that the entire battery be replaced .
   Lead-Acid batteries are similarly tested for capacity, but with no individual cell readings (terminals for individual cells are not available).

Deep Cycle:

    A battery where the cells are heavily unbalanced, either as received for testing or after failing a capacity test, must be fully discharged. This allows all cells to start from zero in the subsequent recharge, thus restoring the balance in the cell voltages.
   Lead-Acid batteries are never discharged to zero during testing

Overhaul:

    Batteries are not repairable, at least not with the same meaning that we apply to other devices. The basic component of the battery is the cell (or cell block in a Lead-Acid battery). If cells fail to perform, they are replaced. The same is true for other parts such as temperature sensors. Interconnecting hardware, (connectors, links, screws, nuts and washers), are normally cleaned. If burned or corroded, they are replaced.

Conclusion:

• Test batteries at the intervals given by the aircraft's manufacturer. (The Main batteries and the Emergency Batteries).
• Reduce the time periods if there is a heavy engine starting demand on the battery, or if prior test records deem it necessary (ie: high water consumption and loss of capacity).
• Reduce the engine starting load on the battery by the use of ground support equipment (battery carts).
• Service and properly store batteries that are not immediately needed.

b4 - "Notes on Trickle Charging of Batteries"
by Joseph F. Mibelli (JFM Engineering) - 5 July 1999

The purpose of trickle charging is to offset the self discharge that is inherent in all battery mechanisms. For Nickel-Cadmium batteries, as are commonly used on aircraft, the self discharge is estimated to be about 3mA per A-hour. Thus, a 40A-hr battery is expected to leak (and hence require) about 120mA. The exact amount of trickle charging, however, is dependent on several factors: the age and condition of the battery and more importantly the ambient temperature. The higher the ambient temperature the higher the leakage, and vice-versa.

A standard method of trickle charging is to connect the battery to a power supply with an output equivalent to 1.4V/cell. This makes it equivalent (for a 20 cell battery) to the 28V bus charging that occurs in the aircraft. A trickle charger, however, does not need to have a high current capability or an extremely well regulated output voltage.

Batteries , however, cannot be maintained on trickle charge indefinitely. First, any amount of charge current results in water evaporation. If the current is low (about 2 mA per A-hr) it is not a big problem to maintain a battery in trickle charge for one or two months, and expect it to be ready to be put in the aircraft without any further checking. After three months, however, it is necessary to subject the battery to a water level test (top charge) and to add water as required. Note: the condition of the battery and ambient temperature will have a big factor in determining how long a battery can be kept in trickle charging without any testing.

Another side effect of continuous trickle charging (under constant voltage mode) is the progressive imbalance of the cells. The same condition occurs in the aircraft, because batteries are continuously connected to the 28V bus. This condition is particularly detrimental to emergency batteries that are not normally used (as opposed to the main batteries that are required to start engines). In this case, the same top charging that is used to check for the water level will get all of the cells to "peak" at around 1.55V/cell.

Therefore, if batteries will be maintained in trickle charging (stand-by) for prolonged period of time, it is imperative to establish a program to routinely test the batteries (very short test) to insure that they are always in condition to be put in service. In conjunction with this program, the trickle chargers could be adjusted to voltages lower than the typical 1.4V/cell, such as 1.375V/cell or 1.35V/cell. The objective here is to minimize the required trickle current. In addition, if the batteries are kept at low ambient temperatures, the trickle current requirement is further reduced.

The standard Trickle Chargers manufactured by JFM Engineering provide the means for effective and safe trickle charging of any size (up to about 70 A-Hr) and type (Nickel-Cadmium or Lead-Acid) of aviation battery. Custom Trickle Chargers can be provided for specialized applications.

Simplificando la Prueba de Baterías

Testing Intervals for Aircraft Nickel-Cadmium Batteries

Battery Testing Guidelines