Battery Buying Advice

AGM technology is the best choice these days. Heres a primer on making the right choice.

by Kim Santerre

Considering that every flight depends on it, the aircraft battery doesnt get any respect. It lives in the tailcone or engine room, out of sight and out of mind getting little care or attention. In our experience, most batteries-particularly the flooded or vented electrolyte wet cells-don’t die-they are killed by neglect or improper maintenance. This damage can start as early as improperly activating a dry-charged wet-cell battery.

Just as most of us don’t pay much attention to maintenance of batteries, most of us havent followed the advancements in battery technology, either. When a battery dies, we simply replace it with something, often taking the shops recommendation at face value. Yet the savvy owner can save a few bucks and enjoy better performance and longevity from an aircraft battery by buying the right type and taking care of it.

In this article, the first of two on the subject, we’ll examine basic battery technology, with an eye toward illuminating the things you need to know to make a battery buying decision. In the second article in the January 2004 issue, we’ll publish the data on specific brands of batteries we have tested, along with buying recommendations.

At the outset, we’ll note that the new sealed absorbed glass mat batteries have come down in price since they first appeared. Theyre usually less than $20 more than a conventional flooded-cell model so we see no reason not to buy the newer technology.

Battery Physiology
While most owners are familiar with the typical lead-acid wet-cell or flooded-cell battery, sealed batteries are far less we’ll understood and that may be the reason they havent really caught on in light aircraft. Aviation wet cells are nearly the same as the typical modern auto battery, with the exception of a higher specific gravity electrolyte (1.285 versus 1.265 for the car battery), positive gas venting, lower capacity to save weight and non-spill vent caps to prevent the loss of electrolyte with aircraft attitude changes. These things havent changed much in the past four decades.

The new technology is called absorbed glass mat (AGM) and its not exactly cutting-edge new, either. This technology was introduced in 1985 by Concorde Battery Company to support a military contract for a high-reliability, rugged, maintenance-free and powerful (for its size) leak-proof battery. The AGM was to have none of the shortcomings of the messy wet-cell battery or the labor-intensive nature of nickel-cadmium aircraft batteries used in heavier aircraft where high-capacity is critical. The AGM battery is also called a valve-regulated, sealed lead-acid battery (VRSLAB), a starved-electrolyte battery or a recombinant-gas (RG) battery. All of these things mean the same thing so we’ll just call them AGM batteries.

While the AGM has been we’ll received in the marine world, there has been little market penetration in the single-engine aircraft arena, although operators of heavier aircraft have embraced the AGM battery to a larger degree. Why is this so? Were not so sure, since the prices are competitive but the performance should be much better, thanks to higher capacity and better temperature range.

One of the purposes of our head-to-head testing is to see just how much better AGMs perform. These days, both Concorde and Gill are in the AGM battery business but Gill came to the party much later. AGM technology has continued to advance to the point that gel cells-one of the few improvements in aircraft lead-acid batteries-are now pass. Concorde stopped making gel cells in 1987.

The AGM battery is so powerful that it can serve as a replacement for high-current demand-and potentially dangerous-NiCad batteries used in heavy twins and jets. The AGM also has superior cold-weather starting performance. This is so because of the internal design, which gives this battery low internal resistance to current flow and no liquid electrolyte to thicken up at cold temperatures. Its capable of safe, repeated high discharge and charge currents-far greater than an equivalent size wet cell.

AGM batteries are also sometimes called starved electrolyte batteries because there is no electrolyte liquid to spill. There’s just enough electrolyte to saturate the glass mats. The aviation VRSLAB uses the same lead-acid chemistry as the wet cell, but its internally packaged in a much more usable way. When a wet-cell battery is charging, explosive hydrogen gas is vented to the atmosphere. Conversely, the sealed AGM battery uses one-way pressure relief vent valves. It operates with a positive internal pressure, which forces the gasses produced during charging to recombine and remain within the battery, hence the term recombinant gas.

Only under excessive charging voltages will an AGM sealed battery vent to the atmosphere, which does result in water loss and permanent capacity reduction. Continued excessive charging voltages will destroy it, but it wont become a fire hazard, as will a NiCad, even if the AGM is forced into a thermal runaway.

Charging it Right
Charging is critical to a batterys output and longevity, both in storage and in service. Wet-cell batteries self-discharge about 50 percent in 30 days at 80 degrees F, whereas AGM batteries self-discharge much more slowly-about 50 percent in 90 days at 80 degrees F. The warmer the temperature, the faster the self-discharge. This is not always intuitive; batteries work better when its warm.

Conversely, a battery stored in a cold environment will loose its charge more slowly, but the batterys ability to provide rated starting current drops. There’s a particularly steep loss of starting capacity when a wet-cells temperature drops below 32 degrees F. The thickening electrolyte moves less efficiently.

There are two basic charging methods. One is called constant voltage, also known as constant potential, and is by far the most common method. The other is called constant current and is generally found in expensive shop chargers or some smart chargers. Some high-end chargers are capable of multiple modes-constant current or constant voltage-and other smart-sensing functions. Constant current can be time-efficient but needs careful monitoring to avoid damaging the battery.

Constant current charging is also used to try to revive a sulfated battery to gain back lost capacity. Sulfation occurs when a wet-cell battery sits for prolonged periods in the airplane or in your hangar in a discharged state, something that occurs as a natural consequence of battery chemistry. It can also occur in a charging system that never completely charges a battery due to insufficient voltage or inadequate charging intervals. Sulfation is the formation of hard, recharge- and discharge-resistant material on the plates of the battery during the discharge cycle. It can happen to an AGM battery as well.

As a rule, trying to fix a sulfated battery with a conditioning constant current charge at 1/10C (.1 of the one-hour capacity rating) for 18 to 24 hours is worth a try. A capacity check is required to prove that the conditioned battery capacity has been restored. Otherwise, you could think you have a restored battery that will not hold a charge and one that would almost certainly let you down upon starting or, worse, after an alternator failure.

Old style, low-cost taper chargers simply pump out a slightly higher voltage than a charged battery and let the internal resistance of the battery control the charge voltage and thus current. Its simple and cheap but can be hard on the battery when excess voltage or excess current is used to speed up the process intentionally or by out-of-specification chargers that do the same inadvertently. Even trickle charging a battery can be harmful unless there are feedback electronics to control and periodically shut down the voltage as the battery peaks. Or, you can use a timer to shut off the power to the charger for a period every day.

Unmonitored and particularly high-powered dumb chargers can ruin or shorten the life of wet-cell batteries because they can cause excessive water loss and/or excessive charge rates for a given battery capacity, leading to overheating, plate damage and shedding of active plate material. Weve all seen a battery in a shop with the caps off and under a fast charge to get in service quickly. When this happens, the electrolyte bubbles vigorously and the case is hot to the touch. This is a good way to ruin a battery.

The more frequently you charge a battery used for starting from a deeply discharged state, the shorter its life. Fortunately, a typical engine start consumes only 3 percent of the capacity, so thats not what kills a battery. A deeply discharged battery, whether from sitting or excessive cranking, is prone to generator or alternator charging damage unless properly bench-charged first.

Think about that when jump-starting a dead battery in your airplane. Moreover, fly charging a battery means it will not be fully charged for up to five hours, rendering it unsafe-and unairworthy-should you need it following an alternator or generator failure. To get the longest life and highest performance out of any battery type, a smart charger is the way to go. Low output types are available for as little as $50. The Battery Tender is a good choice, in our view, and is available from Aircraft Spruce for $59. (

Just as an improper ground charge can do in a battery, so can an out-of-spec aircraft charging system. Aircraft charging systems do best when they keep up with or stay ahead of the electrical demands of a sufficiently charged battery in use. A deeply discharged battery can suffer overheating or excess gassing and electrolyte loss from high charging current and the sometimes marginal voltage regulation of aircraft charging systems.

A high-current, 100-amp alternator can damage a discharged wet-cell battery by overheating it with more current than it can properly handle-particularly if the battery is in a deeply discharged state. (Not so an AGM.) Aircraft charging systems are even more primitive than automobile systems in that there is no temperature compensation, which leads to frequent over or undercharging, depending on ambient temperature.

Battery Capacity
There are many batteries in service that would fail the FAA recommended capacity test-see FAR 23.1353(h) and the sidebar on page 21. This test is designed so that if the aircraft alternator or generator fails, the battery should provide an emergency reserve of at least 30 minutes to keep critical avionics alive. The FAA standard is 30 minutes reserve, while the newer, tougher JAA and international standard is 1 hour, with different voltage test parameters.

Capacity testing should be conducted annually for any aircraft battery. (For some sealed batteries, the first check is at two years or 1200 hours, whichever occurs first.) This is according to the Instructions for Continued Airworthiness (ICA) that now come with any new Concorde battery. Similar ICA tests are prescribed for Gill batteries.

These capacity tests have been prescribed for the last few years in battery ICAs in response to the FAAs continued emphasis on ICAs and promoting safety. In the past, battery capacity testing has largely been ignored and not we’ll understood. A review of the amended FAR 43.16 and 91.403 with respect to continued airworthiness indicates that all ICAs must be followed and since annual capacity checks are called for, conducting one during the annual would seem to be required.

That said, there doesnt appear to be widespread compliance with capacity checking-at least according to local shops we checked with. Many followe the old and as-yet-unchanged FAR standard, the annual/100 hour mandatory checklist calling for checking battery security and for insufficient charge. The FARs don’t seem to be at all in agreement with required ICAs.

Notwithstanding the FAA-required or optional status of the battery capacity test, if you fly at night or IFR or land where FBO assistance is not to be found, you should request one. (A capacity check has nothing to do with a hydrometer or voltage reading alone; its more complex.)

Does it Fit?
Both Gill and Concorde warn in writing that some battery box mods or, in extreme cases, airframe mods may be required even on batteries that have approval for your airplane under an STC from them. Things as simple as plastic handles have caused interference problems with battery case tops. Fit issues lead the list of complaints we have heard about AGM batteries so if you decide to replace a flooded-cell with an AGM, check out the dimensional sizes carefully.

Concorde has recently obtained new STCs for Cessna and Piper, which cover a few minor mods required such as trimming some Cessna plastic battery box tops, or trimming the battery handle in other Cessna applications. This eliminates any field approval requirements for covered aircraft, which is nearly all. Chamfering the battery edge has cured the old Piper problem of battery top interference with the case.

In an effort to correct the battery terminal problems which have shown up in service difficulty reports, Concorde has also installed improved terminal designs and materials over the antiquated wing nut designs. These older wing nut/stud terminals can be easily over or under torqued and that can lead to failure of the terminals or even fire in rare instances.

Bottom line: both battery companies have done much to address the fit problems that bedeviled AGM batteries at the outset. In our view, if you want one, you’ll be able to find one that fits, for most aircraft. In the next installment of this series, we’ll look at how the individual batteries measured up in terms of construction and capacity.


Also With This Article
Click here to view “Checklist.”
Click here to view “Is Your Battery About to Croak?”
Click here to view “AGM Technology.”

-Kim Santerre is editor of Light Plane Maintenance.