Aircraft Battery Care & Maintenance

by Richard Morey
© Copyright 2023. All rights reserved!
Published in Midwest Flyer Magazine February/March 2023 Digital Issue

Winter is upon us. Typically, this involves less flying, and certainly colder weather, both of which are detrimental to an aircraft battery. Rental pilots just want the airplane to start with no issues. So long as the last pilot did not leave the master switch on and drain the battery and the aircraft is warm, all should and generally does go as planned.

But what if the battery is low, and will not crank over the engine?

What if the engine is cold and the pilot tries to start it and runs the battery down?

What if the master was left on overnight and the battery is completely dead?

Aircraft owner/operators have the same problems of course, as well as the responsibility of deciding how to maintain the battery. These cases require different actions, and depending on what is done, have different effects on the longevity and capacity of the battery. Knowing and following the manufacturer’s recommendations will maximize the battery life and make for safer flying.

Aircraft batteries are a compromise between weight and capacity. As such they are more fragile than car batteries. For the purposes of this article, lead acid batteries will be discussed. Nickel-cadmium batteries are used in some general aviation applications but are rare and will not be discussed in this article.

Batteries store electricity chemically. A lead acid battery has lead plates suspended in a sulphuric acid water solution, called electrolyte. Electrical energy is created during the chemical reaction between the acid and the lead plates. Sulphuric acid reacting with lead creates lead sulphate bond, and releases electrical energy as part of the reaction. This reaction is reversible by forcing electricity back into the battery which breaks the lead sulphate bond, returning the reactants to lead, water and sulphuric acid.

As with any chemical reaction, cold temperatures slow down the process. A cold battery has less power than a warm one. Heat is generated, both during discharging, and especially during charging. Hydrogen and oxygen gas are released as part of this reaction. To avoid the accumulation of a very combustible mixture of hydrogen and oxygen, charging batteries should be done only in a well-ventilated area.

The slight charging and discharging of the battery is part of the flight. The battery discharges during start up and charges during engine operation if the charging system alternator or generator are working properly. The aircraft charging system is designed for small charging operations, more in the lines of maintaining the battery rather than charging a low battery.

Every time a battery goes through a charge discharge cycle, it loses a bit of its capacity to hold a charge. Over time the lead plates “sulphate up,” which means part of the lead sulphate bond does not reverse during charging. Sulphated plates leave less lead available to react, thus reducing the battery’s capacity.

What does this mean to the aircraft owner/operator? Batteries must be thought of as “life limited parts.” In our flight school, we are happy to get three (3) years of life out of a battery, and often only get two (2).

To maximize battery useful life, I would strongly recommend that every aircraft owner spend some time on their battery manufacturer’s website reading the service requirements and recommendations. I found Gill’s website to be particularly useful.

Some useful gems from Gill: “4.4 LIFE EXPECTANCY 4.4.1 Typical battery failure mode should be complete non-reversible utilization of active material. A battery that is operated and maintained per the manufacturer’s guidelines should deliver several years of useful service. However, actual service life varies due to several factors: 4.4.2 Charging variations.” In layman terms, this means if the battery is maintained as recommended, expect “several years of service” before the battery will not hold a charge. This section goes on to mention that undercharging or overcharging will shorten the battery’s life.

Gill on charging a battery: “WARNING. The battery must be removed from the installation and serviced in a well-ventilated designated area. During servicing, the battery will generate oxygen and hydrogen gases, which can be explosive under the right conditions.” In other words, do not charge the battery in the aircraft, but rather remove it. My read on the Gill website is that even for trickle charging, which Gill recommends during periods of reduced flight, that the battery should be removed from the aircraft.

So, what does Gill suggest if we have a low battery and cannot start the aircraft?

“7.11 JUMP STARTING AN ENGINE 7.11.1 Teledyne Gill batteries that have been discharged to the point where their cranking power has been diminished must NOT be jumped with another power source. The discharged battery may not be airworthy because it does not have the necessary capacity required to operate the aircraft avionics and electrical system in the event of generator failure.”

I know that most flight schools do jump start aircraft so long as the aircraft’s battery has enough charge to close the battery and alternator solenoids. The click you hear when you turn on the master switch are solenoids closing. Solenoids are simply electrical switches which require a minimum threshold voltage to close. If the battery is drained to the point of not being able to close a solenoid, then jumping the aircraft may start the engine, but will not result in the electrical system operating. This is because an alternator needs some electricity to energize the electromagnetic coils to turn mechanical energy to electricity. But I digress.

Jump starting – or more precisely using a battery boost to start an aircraft – is not recommended for a number of reasons. The most important reason, according to Gill, is a potentially unsafe situation if the aircraft’s charging system fails.

If your mission for the day is to fly around the patch during daylight hours, then an alternator failure is less of a factor than it would be for a night instrument cross-country flight. If the charging system fails, ether due to a malfunctioning alternator or alternator control unit, the pilot must complete the flight with what electricity the battery has stored. Radios, navigation equipment, lights, fuel gauges, pitot heat, and flaps (if electric) all go away when the battery runs out of electricity.

For safety reasons, Gill recommends removing the low battery and charging it fully, then testing the battery for capacity prior to flight. This procedure will maximize battery life as well. Charging a low battery by flying may work but will also charge initially at a higher rate than recommended, shortening the life of the battery. Pilots who routinely fly in IMC, at night or cross-country, should keep battery capacity in mind. An older battery may well retain enough charge to reliably start your aircraft, but not have much capacity beyond that. If you have a charging system failure, the only source of electricity is that older battery. Being in the clouds with a dead electrical system is not an appealing proposition.

If the battery is so discharged as to not be able to close solenoids, then the battery needs to be removed and charged.

In summary, aircraft batteries are built as lightly as possible and as such have limited capacity. A battery only has so much useful life. Periods of idleness, overcharging, undercharging, and age all reduce a battery’s capacity to hold a charge. Common procedures, such as boost starts, are not recommended, and will shorten the life of a battery. Finally, consider the capacity of the battery when planning both the flights and the aircraft’s maintenance. Deferring replacement of a questionable battery until the next annual, may be penny wise but pound foolish.

The battery should be removed at each annual inspection, the battery and terminals cleaned, the battery box inspected and cleaned, and the battery capacity checked. I would encourage aircraft owners to talk with their aircraft technician prior to their annual inspection and make sure battery maintenance is done.

EDITOR’S NOTE: Richard Morey was born into an aviation family. He is the third generation to operate the family FBO and flight school, Morey Airplane Company at Middleton Municipal Airport – Morey Field (C29). Among Richard’s diverse roles include charter pilot, flight instructor, and airport manager. He holds an ATP, CFII, MEII, and is an Airframe and Powerplant Mechanic (A&P) with Inspection Authorization (IA). Richard has been an active flight instructor since 1991 with over 15,000 hours instructing, and almost 19,000 hours total time. Of his many roles, flight instruction is by far his favorite! Comments are welcomed via email at Rich@moreyairport.com or by telephone at 608-836-1711. (www.MoreyAirport.com).

DISCLAIMER: The information contained in this column is the expressed opinion of the author only. Readers are advised to seek the advice of their personal flight instructor, aircraft technician, and others, and refer to the Federal Aviation Regulations, FAA Aeronautical Information Manual, and instructional materials concerning any procedures discussed herein.

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