Aircraft Battery Maintenance

Aircraft battery maintenance

A dead battery can be a major inconvenience but there are ways to prevent premature failure

OF ALL THE things that can go wrong with your plane, a dead battery has to be one of the worst – usually due to lack of battery maintenance. Although you might be able to jump-start the engine so you can take off, what are you going to do if your alternator is fried or has thrown a belt?

Now you’re in the air with a run-down battery, without enough voltage to power your radios, avionics or, worse yet, flaps and landing gear. What can you do to prevent this from happening?

Just be aware of the basics of a battery’s workings and how to keep it in good shape. With proper maintenance, it’s relatively easy to make sure your battery stays in top condition for many years of dependable service.

A battery is a device that converts chemical energy into electricity. Normally, two or more cells are connected in series or parallel to achieve the desired voltage and power, but there are also single-cell batteries.

In their simplest form, batteries consist of an electrolyte (which can be a liquid, paste or solid) and two electrodes, one positive and one negative. In operation, one of the electrodes will react to the acidic electrolyte, producing electrons, and the other will accept electrons.

When a load is connected, an electrical current will flow. There are essentially two types of batteries: primary or voltic cells, which cannot be recharged; and those that can, called secondary cells, rechargeable cells, storage cells or accumulators.

From Flashlights to Aircraft Georges Leclanche invented the most common form of primary cell battery, usually found in flashlights and other small devices or toys, around 1860. In almost the same manner now as then, the electrolyte consists of a paste of ammonium chloride and zinc chloride.

The negative electrode and outside case are made of zinc, while the positive electrode is a carbon rod, surrounded by a mixture of carbon and manganese dioxide. The Leclanche cell produces about 1.5 volts. As for secondary cells, the lead-acid and nickel-cadmium batteries are the two types most commonly found in aircraft.

The French physicist Gaston Plante invented the lead-acid battery in 1859. Its three principal parts are a diluted sulfuric acid electrolyte, a lead negative electrode and a lead-dioxide positive electrode. It operates, in very simplistic terms, by the negative electrode giving off free electrons and positive lead ions.

The electrons travel through the external electric circuit, while the ions combine with the sulfate ions in the electrolyte to form lead sulfate. As the electrons re-enter the battery through the positive electrode, they combine with the lead-dioxide and the hydrogen in the electrolyte to form water.

lead acid battery construction

Different types of batteries need special attention. Read the owner’s manual and heed all instructions.

As more and more of the acid is converted into water, the charge is reduced. Recharging the battery is accomplished by reversing the process. Most mechanics recommend replacing the battery every three to five years.

The nickel-cadmium (or Nicad®) battery is a close relative of a nickel-iron battery developed by Thomas Edison around the turn of the century. It operates in a manner similar to the lead-acid battery, except the potassium hydroxide electrolyte is a base (rather than acid), the negative electrode is cadmium and the positive electrode is nickel oxide.

Battery Failure Modes

To hurt a battery, you can either cook it or freeze it or drop it. There’s not much to say about damage from dropping, so we’ll discuss cooking and freezing. To overheat a battery, it can either be operated in an ambient temperature above its design limit or recharged with excessive voltage.

In either case, this can lead to boiling away of the electrolyte and subsequent battery failure. For lead-acid batteries, AC 43.13 recommends a maximum recharge rate of 2.35 volts per cell.

For nickel-cadmium batteries, the rate should be between 1.4 and 1.5 volts per cell. As with everything else in aviation, it’s best to find out what the manufacturer recommends on battery maintenance are and go along with that.

Freezing is a serious concern for lead-acid batteries in particular. The temperature at which the electrolyte freezes depends on its specific gravity (the weight of the electrolyte as compared to pure water).

Fully charged, with a specific gravity of about 1.275 or higher, the electrolyte is able to withstand temperature s of -50 degrees F or less. Bear in mind, though, that there’s a constant discharge in lead-acid batteries, whether they are used or not.

By contrast, the nickel-cadmium batteries don’t have an appreciable chemical mix going on when not in use, so they’re not as susceptible to cold weather. Even so, their electrolyte will freeze at about -75 degrees F.

Time for a Checkup

Almost everyone uses a hydrometer to check the specific gravity of lead-acid batteries. This battery maintenance tool looks like a turkey baster with a float that has a bulbous lower end and a rod rising from it at the top.

The rod is usually marked with the specific gravity reading and, for those of us who are constantly searching for our glasses, color codes for “good,” “fair” or “recharge.” To operate the tool, you simply suck up enough battery acid to hold the float away form the sides and bottom, and read the specific gravity at the fluid level.

If you’re in a cold climate, however, you can’t just read the indicator on the bulb; you must make a correction for the temperature of the electrolyte. Tables of temperature corrections are in all the battery shops, and most good hydrometers have a thermometer and correction scale built in.

As for checking nickel-cadmium batteries, the process is not so simple. If you have the right equipment, you can use a test called a measured discharge. But be advised that nickel-cadmium batteries are a breed in themselves, so you should know what you’re doing when you work with them.

One of the biggest problems with nickel-cadmium battery maintenance is maintaining the fluid level. The recommended way to add fluid is to wait two hours after charging the battery before you fill it to the correct capacity.

The fluid level varies with the charge of the battery so nothing should be added while the battery is in the airplane. Overfilling will result in the fluid being spewed out during operation, which can lead to corrosion on the cell links, a self discharge of the battery, dilution of the electrolyte and failure.

All in all, the safest way to handle nickel-cadmium batteries is to remove them from the aircraft and perform the manufacturer’s recommended service and maintenance schedules. Nicad® batteries require periodic deep-cycling in a maintenance shop to stay healthy.

Individual cells can become weak, to the point of becoming so discharged that they will reverse polarity and take current from the adjoining cells. The result can be a hydrogen/oxygen explosion, if allowed to progress too far.

A deep-cycle battery maintenance service takes the battery down to complete discharge and recharges it to full capacity, at which point the water can be topped off and the battery returned to service. One of the simpler tests of your battery’s condition is to measure its voltage while starting the engine.

This method is somewhat akin to the load test that a shop will do to your car’s battery. Actually, if your car mechanic will let you borrow his load tester, you can simply test your aircraft battery with it. If not, then find a place to hook up a voltmeter and crank the engine.

Eight to 9 volts is acceptable, allowing for less voltage in colder ambient conditions. Many aircraft have a voltmeter installed, but few pilots remember to monitor it during engine start. If at no other time, try to check the battery’s condition prior to commencing a long trip.

Battery Maintenance – Cleaning Acid Spills

While we’re on the subject of spewing electrolyte (read that as acid), let’s mention how to clean it up. The only way to clean up an electrolyte spill is by neutralizing it with a base material and washing down the area thoroughly.

Battery acid will eat through metal and create more problems than can be recounted here. This is especially true for those of us with the battery in the aft compartment, behind the baggage area. The old saying, “out of sight, out of mind,” is very true and very bad.

For lead-acid battery spills, sodium bicarbonate (baking soda) is the best substance to use. With Nicad batteries, the potassium hydroxide electrolyte can be neutralized with boric acid, vinegar or a three percent solution of acetic acid. There’s nothing terribly technical about cleaning up either type of spill.

different lead acid batter terminals

Batteries have different types of terminals. All connections must be secure and the battery firmly anchored in the battery box.

Simply take a bottle of water and soak the area, add the neutralizer, let it work for a while and then rinse off. Do this as often as needed until there’s no more reaction between these opposing materials (indicated by a hissing and boiling).

Don’t forget that this means treating your skin as well. The acid will burn you faster than it will the fuselage. Rubber gloves and an apron, as well as safety goggles, are really a good idea if the spill is anything more than a few drops.

Battery Maintenance – Out-of-Control Heating

Thermal runaway is a phenomenon peculiar to the Nicad battery. These batteries are quite often used in applications where a current flow, initially up to 2000 amps, is needed for starting, as is usually the case with turbine engines.

Now that turbines are becoming more affordable through the resale market (note the features elsewhere in this issue on the subject), we are likely to see an increasing number of this type of battery.

In a condition where an engine is difficult to start due to environmental conditions or a low battery state, the middle cells in the battery will heat up more than the others.

When the cells heat up, they will accept more recharging current than normal when the generator is brought on-line, which will heat them up even more, creating a runaway condition. This cycle can lead to the battery’s failure, as well as the possibility of an on board fire.

Prevention involves monitoring the charge rate for an excessive build-up as the battery is recovering from a start and also taking a generator off-line before the meltdown can begin.

While battery temperature monitoring systems used to be required, by the time the battery gets hot, it may be damaged, so keeping the charge rate under control is the best prevention against a runaway.

Picking the Right One

As far as aircraft batteries are concerned, there are three factors to take into account when buying one. First, the voltage, which will be either 12 or 24 volts. Second, the ampere-hour rating, which is a measure of the strength of the battery, i.e., how many amps of current the battery will discharge in how many hours.

For example a 17 ampere-hour battery will, theoretically, give out 17 amperes for one hour or one ampere for 17 hours. This is really only a guide, and if you have a choice, the higher the rating the better. And lastly, the battery has to physically fit into its container in the aircraft.

Those of you with production planes have it easy. All the manufacturers have cross directories which give the recommended battery for the plane. This information has to be taken with a grain of salt.

Looking at the Gill battery catalog, it simply recommends its G-35 battery for the PA-24 Comanche. This has a rating of 12 volts and 23 amp hours. The catalog makes no distinction between the 180, 250, 260, 400 or even the Twin Comanche, so apparently, the exact battery for the plane isn’t all that critical.

As for those of you who are building your own custom plane, and/or swapping engines around, the cross directories will still be very effective. Just find a listing for a similar plane, engine and installation, and that should be close enough. The Gill directory also has the physical dimensions listed. There is always a tendency to go cheap.

But with respect to batteries, at a minimum, you should buy aircraft grade batteries, even if a lawn mower battery will fit. Aircraft batteries have the proper terminals, are normally lighter and are equipped with self-sealing vent caps that prevent the acid from spilling out if the battery is tipped over.

Also, the cap seals will pull themselves in at any angle over 45 degrees, so for anyone contemplating aerobatics, a sealed battery is a must. Again, all the manufacturers offer these features, and there are cross references for picking the right model.

The Key to Long Battery Life

According to the experts at Concorde Batteries, the main reason batteries fail is that they are not maintained correctly. For instance, the water level is ignored and only checked at the annual inspection or when the battery fails (at which point it may be too late to save it).

If the battery itself is not cleaned, acid spills will cause low voltage leaks. Also, when the charge rate is too high and boils away the electrolyte, battery failure can be expected.

Concorde reports that through good battery maintenance, customers have gotten as much as 10 years of service out of a battery. Although the company makes a number of recommendations, the main one is to add distilled water to the lead-acid batteries when the level is low.

Two Battery Tests

A simple test you can do is to check for acid bridges with a voltmeter. These small paths of conductivity on the surface of the battery act the same as any load and will drain the charge in the same way. An acid bridge is caused from spillage or spewing.

To check the battery, hook one lead of the voltmeter to the appropriate battery terminal, and move the other voltmeter lead around on the battery top, looking for a voltage reading. If there’s any at all, wash the battery with water and baking soda, rinsing thoroughly when done, and recheck until no more leaking voltage is found.

Another excellent test, if you have the knowledge and skill to do so, is to check the charge rate and adjust the voltage regulator for whatever setting the manufacturer recommends. All in all, the important thing is to take care of your battery, and it’ll take care of you. Then you’ll really get a charge out of things.

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Summary
Aircraft Battery Maintenance
Article Name
Aircraft Battery Maintenance
Description
Often overlooked, poor battery maintenance is one of the main contributors to engine failures, especially in aircraft with auto conversions that require 12v
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