|Light Plane Maintenance Editor-in-Chief|
Aircraft starting batteries have a tough life; much more so than their automotive counterparts. They are left idle for days or even weeks at a time, then asked to start a 500 cubic-inch engine in cold weather with three gallons of oil the consistency of molasses in the sump. To make matters worse, aviation batteries have significantly lower capacity than an auto battery, and multi-weight oil can help only to a degree.
Twenty-four volt batteries provide some help in high-current-demand situations such as speeding up electric retractable landing gear, but in terms of battery cost compared to 12 volts, a 24-volt system leaves a lot to be desired on cost vs. benefits.
When left idle, batteries self-discharge about one percent or so a day due to the side effects of components (antimony) used to keep battery-grid structures from shedding and damage during use. This self-discharge process has a long-term, deleterious effect on the battery if not corrected by frequent recharging. The term is sulfation, and when left unchecked it means the gradual self-destruction of the battery. Recharging and the replacement of lost electrolyte, a natural process in wet-cell batteries, staves off permanent sulfation.
When a battery is allowed to remain in a state of sulfation, either from lack of use or from lack of recharge, it tends to lose both capacity and life. Initially reversible sulfation hardens with time and it becomes less and less reversible. As sulfation flakes off and falls to the base of the case, it can short out the cells.
First, let's look at volts and amps, two often-confused terms. We will use the common (but not perfect) water analogy.
The rate of flow through a pipe (amps) is governed by two things: the pressure (voltage) and the size of the pipe (wire diameter). A third factor also has influence -- pipe length. The longer the pipe the more resistance (ohms) and pressure (voltage) drop.
Increasing pressure (pipe size) increases the potential flow. More flow, more work. Greater pressure or voltage allows delivery of more fluid (amps) in a given diameter pipe (wire).
Excessive pressure and flow can cause a pipe too small for the application to burst (a wire would overheat and melt). Thus, there must be a balance of pipe (wire) size for a given flow (amps) and length combination. That's why starting batteries need such large wires leading to the starting motor. There are hundreds of amps required at 12 or even 24 volts (not much pressure) to deliver the large quantity of amps to spin the starter. (Volts times amps equals watts, or power, in a properly sized circuit for DC electricity. AC can get a little more complicated.)
You can have more volts and fewer amps or fewer amps and more volts to equal the same power, given the wire size is appropriate for the application. We are over-simplifying here, but our goal is to give the basic concepts for understanding a battery, not a course in DC electronics.
The battery is a simple device, allowing the chemical storage of electricity. In some batteries, the chemical fuel -- once exhausted -- means a throwaway.
In batteries such as a wet cell (liquid electrolyte), the process is reversible, and you can restore an exhausted battery by adding electrical energy and restoring the chemical potential.
Batteries like this have been around for well over a hundred years, and the classic model is still the basis of operation of the majority of today's starting batteries, but the times are slowly changing.
If you wanted to make a battery, all it would take is suspending a few strips of lead in a solution of sulfuric acid (see drawing at right). To charge the battery, a piece of lead is connected to become the positive terminal and the other the negative terminal. Connect an outside source of voltage, say 3 volts. Bubbles will form and rise in the acid, coating the positive terminal with a brown coating called lead dioxide, yielding a positive potential. Your cell has a voltage of about 2 volts DC. Connect a small bulb to the terminals and -- voila! -- light.
The discharge process converts the lead dioxide of the positive terminal and the lead strip of the negative terminal to lead sulfate. Water is also released, diluting the acid. Connect the outside charging source and charge again. Connect six 2-volt cells in series and you have a 12-volt battery.
Capacity is roughly a function of the amount of lead. Typically, the lead plates are alloyed with antimony for plate ruggedness and resistance to vibration. The down side is antimony promotes shedding of the active plate material. As a battery begins to discharge at peak rates for more than a few seconds, significant amounts of lead sulfate immediately begin to form, slowing the reaction as you crank.
Starting batteries have thin plates with fiberglass separators to provide the maximum active surface area for the greatest burst of current. It's also why a short rest (20 minutes is better) will allow an apparently dead battery to spontaneously recover a bit, sometimes for one last shot to start. The remaining unsulfated lead is being contacted by the electrolyte through diffusion. Wet-cell batteries are least efficient when you demand maximum discharge.
When a battery is charged, and especially when excess voltage is applied, some of the water in the electrolyte is converted to gaseous hydrogen and oxygen and then vents. As this condition continues, electrolyte is lost to the atmosphere, thus the normal requirement to replenish the water in the cells ... preferably with distilled water, please. Aircraft battery caps also have internal stoppers to help prevent loss of electrolyte in unusual attitudes.
In automotive use, "maintenance free" generally means a wet-cell battery with simply more electrolyte and different chemical additive. An exception to this is the true maintenance-free battery from Optima, and a few other specialty companies, which will run from $150-200 -- and worth it.
Aviation-type maintenance-free batteries for certified airplanes -- available from Gill and Concorde -- are also called sealed, valve-regulated batteries and are truly maintenance free from adding liquids. Terminals still require periodic cleaning. They can be damaged by overcharging with excess voltage (over 14.7 volts for a long time).
They may be installed at any angle, have superior resistance to self-discharge, and can discharge more completely under high loads, such as with an alternator failure.
Being chemical beasts, the ability to supply current is not really linear. The greater the draw in amps, the less efficient the typical wet cell battery. For example, a battery that has a 25 amp-hour rating of 60 minutes will not sustain a 50 amp load for 30 minutes, but substantially less time due to the inefficiencies of the plate design.
A battery has an internal resistance of fractions of an ohm, but it's meaningful. The greater the amp drain per unit of time, the more energy is lost as internal battery heat due to that small resistance. As batteries age and/or sulfate, this resistance goes up.
Bottom line: The discharge curve is very sharply sloped. Every battery has an optimal maximum rate for current draw. This concept is important to understand because in an "alternator-out" situation, you will get much more than double the battery time if you cut the current drain on the battery in half when running electronics.
Capacity Testing is a greatly ignored annual requirement, because most shops don't have the proper equipment. You must have an annual capacity test performed on your starting battery. It's part of the Instructions for Continued Airworthiness that should come with any new battery.
Our conversations with the FAA indicated they interpret the rules that the capacity test is mandatory for Part 23 certified airplanes and recommended for CAR 3 certified airplanes. That's a rule interpretation we don't understand and don't agree with. Do yourself a favor and do the test -- it's your neck.
The test must show that your battery is capable of sustaining 80 or 85 percent of the amp-hour rating of your battery (depending on the type of test protocol performed). See each maker's ICA for the specifics -- they do differ.
If you want the best chance of passing this test (and longest-lived battery), then the best bet is to use a multi-stage battery charger that reaches up to 14.5 volts, but not more, at the conclusion of the charging cycle.
We recommend the BatteryMINDer from VDC Electronics. They have a line of 12 and 24-volt chargers optimized for aircraft batteries. With battery costs rising, it makes economic sense to use a proper charger -- it will make the battery last significantly longer.
Once you've discharged a storage battery, promptly recharge it. In the simplest terms, if you apply a voltage to the battery that's higher than the voltage the battery produces, you reverse the chemical action that produced the electricity. This converts the lead sulfate and water back to lead, lead dioxide and sulfuric acid.
If the process were 100-percent efficient -- which it isn't -- you'd have to put back exactly as much electricity as you drew from the battery when you were discharging it to return it to a fully charged condition. In real life, lead sulfate, lead dioxide and lead are materials with different densities and different rates of expansion and contraction with changing temperature. When one replaces another, small pieces can flake off the plates and fall to the bottom of the cell. Then too, the chemical reactions that occur during charging take time to occur.
If you try to charge a battery faster than it can accept a charge, some of the electrical energy goes to producing additional lead dioxide that will flake off and fall uselessly to the bottom of the cell. Some will also break down the water portion of the sulfuric acid electrolyte into gaseous hydrogen and oxygen and vent out the caps. An undesirable but unavoidable consequence of recharging causes some of the electrical energy you supply to a battery to be lost as heat.
As a rule of thumb, you have to supply about 120 percent of the electrical energy you've taken from the battery to recharge it. It should be supplied in a controlled manner specific to the battery if you are to achieve maximum battery life.
One of the best ways to charge a battery is at a constant current rate in amps equal to 20 to 40 percent of the battery's capacity in amp-hours (Ah) until the battery reaches an optimal voltage for its type. The level of charge at this point is equivalent to about 75 percent of the battery's capacity. This first phase of charging is called bulk charging.
Once the bulk charging is complete, the charging device should maintain the charging voltage at a constant value and allow the charging rate in amps to drop steadily. When the battery accepts current at only about 1 percent of its capacity (e.g., a 25-Ah aircraft battery accepting 0.25 amps), it can be considered fully charged. A current of 5 percent of capacity represents about an 85 percent charge. This phase, which can take several hours, is called the acceptance phase of the charge cycle.
Once a battery is charged, a float cycle helps to hold it in that condition. A float cycle is simply a voltage maintained slightly above the battery's rested, open-circuit voltage (13.1 volts nominal for a 12-volt battery). This float voltage doesn't really charge the battery but helps maintain a charge to compensate for internal losses in the battery. If your charger is trickle charging at more than 13.2 volts, it is slowly cooking the battery -- don't leave it on all the time.
This phase charging is what a "smart" charger does. A typical alternator/regulator charging system is not as sophisticated in recharging a battery, nor is the typical automotive battery charger.
A quick start takes very little charge out of a battery, so sophisticated charging is not critical. Batteries that sit idle, though, need more capable battery chargers for long life to reverse any sulfation that always take place when a battery sits.
Never recharge a battery that's low on electrolyte (plates showing to air). Add distilled water before starting the charging cycle, not after. Only fill it to the bottom (or a bit below) of the split rings, not to the top, or else the electrolyte will overflow during the charging cycle. Tap water is not good since it contains minerals that tend to accelerate adverse reactions. Adding more battery acid is even worse, and usually kills the battery in a matter of days.
A hydrometer is a simple instrument used to measure the state of charge of a battery. It does this by measuring the specific gravity -- the weight as compared with water -- of the electrolyte. You can get a suitable hydrometer at a local auto parts store, but be sure that it only requires an ounce or two of electrolyte for a reading. That's about all you'll get from an aircraft battery. Also, get a numerically calibrated one; do not buy a pith-ball type.
Checking the charge is simply a matter of taking a sample of electrolyte from a cell and reading the value where the fluid line meets the hydrometer's floating scale. Write the number down. A specific gravity reading between 1.300 and 1.275 indicates a high state of charge; between 1.275 and 1.240 is a medium state of charge, and between 1.240 and 1.200 is a low state of charge.
A battery in a low state of charge has 50 percent or less of its capacity left and needs recharging. Check all the cells; then compare the readings. Usually, if there's a spread of more than 0.050 between the worst cell and the average of the others, the battery is on its way out.
When testing a battery with a hydrometer, observe temperature factors; the temperature of the battery can make a big difference in the readings. When its temperature is between 70 and 90 degrees F, the readings can be used as-is. Outside of this range, a correction chart is needed. Some hydrometers have this chart printed on floating scales; others have the chart on or in their packaging. Also, hydrometer readings are taken after the battery has been off the charger for a couple of hours minimum; overnight is even better for accuracy.
Batteries, as we've noted, are relatively simple beasts, although some complex things happen to them during their operation. Battery failures, for the most part, are also relatively simple to understand.
The flaking and separation of lead sulfate -- and, to a lesser extent, lead dioxide -- from the plates that occurs even in normal operation can eventually cause a sludge in the bottom of the battery that actually short-circuits the plates. That can't be helped, but this process is exacerbated by overcharging and by excessively high charging rates. Even worse is when a battery is left badly discharged (more about this in a minute).
Gassing and subsequent loss of electrolyte is another possible failure mode. It's caused largely by overcharging the battery and by neglecting to check electrolyte levels from time to time. Check the vent caps once a month for electrolyte level. If you find that every couple of months you need to add water to the battery, have the airplane's charging system checked. It shouldn't be gassing that much, and the charging voltage may be set too high -- i.e., over 14.3 or 28.6 volts -- or the regulator is malfunctioning. If the charge voltage is too low, it can sulfate the battery.
The most common cause of battery failure is sulfation. It occurs when a battery is left in a discharged state, in which much of the plate area is covered with a fine deposit of lead sulfate. This deposit grows into larger, harder crystals of lead sulfate that clog the spongy surface of the lead plates, acting as insulators. The crystals don't readily break up, and so the battery loses effective plate area, and therefore capacity.
It's a very poor idea, in terms of longevity, to discharge a battery down to less than 50 percent of its capacity. In fact, a battery that remains in a low or discharged condition for a long period of time will be permanently damaged.
Simply put, it's not so much what needs to be done as what should not be done, to get the most out of your investment.
A special note: If you buy an aviation battery in a dry-charged state via mail order and plan to buy automotive acid locally, don't. Auto acid is not the same specific gravity and will reduce battery life.
More aircraft repair and prevention articles are available in AVweb's Maintenance Index. And for monthly articles about aircraft maintenance, subscribe to AVweb's sister publication, Light Plane Maintenance.