Airframe and Powerplant

Charge It!

September 1, 2006

Keeping up with the changes in aircraft batteries

"In this writer's opinion, a battery is the most important component in your electrical system," says Bob Nuckolls, author of The AeroElectric Connection: Information Service and Guide to Theory, Operation, Design and Fabrication of Aircraft Electrical Systems.

A battery does three things — it delivers a strong shot of electrical energy to start the engine; it acts like an electrical shock absorber by dampening system surges; and it is the only source of electrical power when the alternator or generator gives up. Up until the mid-1980s, when recombinant-gas battery technology was introduced, batteries hadn't changed much since the 1940s.

The flooded-cell battery

In the past 20 years the light-airplane-battery landscape has changed, and owners — and technicians — need to keep up with these changes. The old, familiar flooded-cell lead acid battery — which also is called a "dry-charged lead acid battery" — is still around and still is the least expensive battery on the market. But for just a few dollars more, owners can install a more powerful and safer battery. These are RG (recombinant gas) batteries, which are sometimes called "valve-regulated lead-acid" or "absorbed glass mat" (AGM) batteries. It's easy to tell the difference — flooded-cell batteries have one removable filler cap per cell, and AGM batteries are sealed.

A flooded-cell aircraft battery consists of closely stacked negative and positive plates submerged in diluted sulfuric acid within an acid-proof box. To keep the plates, which are suspended from the respective positive and negative terminals in the acid bath, from touching, they are separated by thin slices of plastic. When a flooded-cell battery is charging, explosive gases and droplets of sulfuric acid are given off and pass out of the battery through vented filler caps into the battery box.

This nastiness, termed "off gassing," is completely absent from RG batteries. This toxic cloud produced by flooded-cell batteries is the primary reason that vented battery boxes are required in general aviation airplanes and that the battery box and the airplane structure surrounding these boxes usually are coated with layers of black acid-proof paint.

RG battery advantages

RG battery technology eliminates all the problems of flooded-cell batteries. RG batteries are available in all sizes from both Gill Batteries and Concorde Battery, the two battery vendors in the United States. RG batteries provide more cranking power than a comparable-size flooded-cell unit, do not off-gas, and can be mounted upside down and yet never spill a drop of acid. In fact, this battery completely eliminates the need for a battery box. These continue to be installed in battery boxes because no one has obtained approval from the FAA to do away with the boxes, but the box-free installation is done all the time in kitbuilt aircraft.

RG batteries (the third name for these batteries is "valve-regulated sealed lead-acid" batteries) also retain their charge more than three times better than flooded-cell batteries during periods of inactivity.

Because the electrolyte (dilute sulfuric acid) in RG batteries is absorbed in glass mat separators, and because each cell has a pressure relief valve that's designed to maintain a positive pressure in each cell, the hydrogen and oxygen gases, which are produced during the charge and discharge cycles, are quickly reabsorbed (recombined). In addition, because the glass mats provide support for the individual plates, more plates can be packed into the same-size box, and are more resistant to shock and vibration damage than the plates in flooded-cell batteries. Although cost was once an issue, prices of RG batteries have come down since Concorde first introduced them in 1984. One vendor sells a 12-volt 35-amp-hour RG battery for only $10 more than the same company's top-of-the-line flooded-cell equivalent.

Since the plates in flooded-cell batteries can't be as tightly packed together as the plates in RG-type batteries, they produce less power, have a greater self-discharge rate, and have a greater internal electrical resistance than RG-type batteries.

Another difference is that flooded-cell batteries must be tended regularly to ensure that the electrolyte level doesn't drop below the top of the plates because of evaporation and off gassing. The rate of this electrolyte loss is affected by many variables such as the alternator or generator voltage regulator settings, and even seasonal air temperature variations. Anyone who has ever had to hold a flashlight between her teeth, with a mirror in one hand and a cup of distilled water in the other, while trying to peer into each cell of a battery that's nestled in a battery box, which is in turn buried in the tailcone of an airplane, will breathe a sigh of relief when she hears that RG batteries don't vent electrolytes and are considered to be maintenance free. The question must have now surfaced — is there any reason not to buy an RG battery?

The internal-resistance equation

Since the flooded-cell batteries have a higher internal resistance to electron flow than the RG types, they're more suited for airplanes with low-output generator-based charging systems. The low internal resistance of RG batteries is the second reason (more plate area is the first) they deliver more power and why they accept a charge faster. Accepting more charge can cause small generators to overheat. Skip Koss of Concorde Battery said any airplane with a generator with an output of less than 50 amps (generator output is stamped on the data plate) should use flooded-cell batteries for this reason.

The other soft spot for RG batteries is less resistance to high charging voltages. If an RG-type battery is exposed to extremely high voltages, the gases produced can crack or even blow apart the battery case.

On the other hand, the low internal electrical resistance that's part of the RG battery does mean that these batteries recharge faster than comparable flooded-cell batteries.

Overcharging can be a problem with any charging system, but is most often seen in systems with vibrating-point-type voltage regulators. Hopefully, all of these dinosaur-era regulators have been replaced by now with solid-state regulators (available for both generators and alternators) from companies such as Zeftronics, of Longview, Texas.

For best performance, airplane charging-system voltage regulators should be adjusted to deliver 14.1 volts (or 28.2 because aircraft have either 14- or 28-volt systems) to the positive terminal on the battery when the battery is fully charged. This number is a good target number at 50 degrees Fahrenheit with less voltage required at higher temperatures and more required at lower temperatures. At minus 4 degrees F, 15.0 volts is appropriate for a battery within a 14-volt system.

Instructions for continued airworthiness

Within the past decade the FAA has implemented regulations requiring that the holders of design approvals include these instructions with each product. Often these instructions are so simple, logical, and straightforward that they are contained in a single written paragraph. Instructions for continued airworthiness (ICAs) for batteries by Concorde and Gill run from 10 to 30 pages, are free on the respective company Web sites, and spell out detailed procedures that owners and maintenance shops must use to determine the health of batteries.

Before delving into this, a few terms need explanation. Batteries are rated in amp-hours. GA airplanes typically use 25- or 35-amp-hour batteries. Simply put, a healthy 25-amp-hour battery is warranted to supply 25 amps for one hour, or 50 amps for one-half hour. It's important that pilots understand the amp-hour equation because the aircraft storage battery becomes the only source of electrical power for avionics, lights, and landing-gear extension if the alternator or generator fails in a single-engine airplane.

FAR 23.1353(h) requires that aircraft storage batteries "must be capable of providing at least 30 minutes of electrical power to those loads that are essential to continued safe flight and landing." This is termed the "essential power requirement." Grasping the intent of this regulation also provides insight for pilots who may not understand why it's a bad idea to use an external power source to jump-start an engine when the aircraft battery is not capable of providing enough power for an engine start. Jump-starting will probably get the engine started, but should the alternator or generator fail early in the flight, the battery will not be charged enough to provide backup electrical power for more than a few short minutes.

Charging and capacity testing

Koss of Concorde Battery explained that "just because a battery can crank an engine does not mean it meets the essential power requirement." Koss says that an engine start uses approximately only 1 percent of the power available in a healthy battery.

Both Gill and Concorde ICAs describe capacity testing procedures that are used to determine battery health. Since RG batteries are sealed, the old standby method of determining individual cell health in flooded-cell batteries — measuring electrolyte specific gravity with a hydrometer — is not an option.

Capacity testing requires that a battery be discharged at what manufacturers call the "C1 rate" until the battery voltage drops to what's called the "end point voltage" (EPV) or "cut-off voltage," which is 10 volts (for 12-volt batteries) or 20 volts (for 24-volt batteries).

The C1 rate is equal to the rated capacity of the battery in amp-hours. This simply means that a 25-amp-hour battery should be discharged at 25 amps and a 35-amp-hour battery at 35 amps. After fully charging the battery, apply the appropriate C1 load and note the time it takes to discharge the battery to EPV. The residual capacity of the battery is determined by multiplying the hours (or portion of hours) from the start of the discharge to EPV by the ampere rate of discharge. If the residual capacity falls below 85 percent of new-battery capacity after this test, both manufacturers require battery replacement.

Gill and Concorde manufacture and sell battery capacity testers that permit the technician to safely discharge both flooded-cell and RG batteries at the proper C1 rates down to the EPV. Both the Gill TCT-1000 and the Concorde BC-3000 battery capacity testers are portable, can test both 12- and 24-volt batteries, and display an end-of-test percent of capacity readout, making testing a simple matter. One commercially available battery capacity tester retails for around $850.

This is a little rich for many small maintenance facilities (and many owners) but there are options. Nuckolls' book has a parts list and schematic for a locally built capacity tester, or a tester can be built by creating a load bank consisting of a series of sealed-beam automobile headlights. These lamps are rated in watts. The formula is: Amps equals watts divided by volts. Substitution of known values reveals that four 100-watt bulbs and one 55-watt bulb (455 watts) divided by 13 volts equals 35 amps. The correct C1 load for a 25-amp-hour battery can be constructed of three 100-watt bulbs and one 25-watt bulb. It's not critical to load exactly to the C1 rate but it makes the math easier. A fully charged 35-amp-hour battery supplies 35 amps for one hour. It's deemed to pass the essential power regulation if it can supply the 35 amps for 51 minutes (85 percent of 60 minutes) or more.

Each capacity test begins by fully charging the battery. For flooded-cell batteries a full charge is indicated when the electrolyte specific gravity readings are stabilized in three successive readings taken an hour apart.

Both Gill and Concorde require that RG batteries be charged using what are called "constant potential" chargers. These chargers maintain a constant voltage (14.1 and 28.2 volts). Current flow is displayed on the charger and is controlled by the battery internal resistance. These chargers are markedly different from the typical off-the-shelf 12- or 24-volt charger, which most often permits the technician to select a rate of current flow (2 amps/6 amps on one popular model).

RG batteries are considered to be charged when the current flow stabilizes for one hour.

Keeping batteries alive

Tips for keeping batteries healthy are simple. Fly often, adjust the aircraft alternator- or generator-charging rate to ensure the battery is being fully charged, keep the battery and connections clean, and don't add anything to a flooded-cell battery except distilled water. For more detailed maintenance instructions, go online to either the Gill ( or Concorde ( Web sites and download the information.

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