Airframe and Powerplant

Versed in Volts

September 1, 1995

A little respect for the beleaguered battery

As with so many under-respected parts of the airplane, most pilots notice the battery only when it's dead. Never given a second thought on the sunny days when it puffs up and offers a surge of energy to crank over a large piston powerplant, it is nonetheless loudly criticized when it fails to so much as turn a blade on a sub-freezing morning, 11 months out of the airplane's annual. And yet without a faithful storehouse of electrical energy, we'd be back to hand- propping engines and honing our dead-reckoning skills. Few pilots want that.

History has been kind to our most common of electricity storers, the lead-acid wet-cell battery. Invented in the late nineteenth century, it has since evolved to a durable, inexpensive means of getting us aloft without risking fingers flung into the grass. In fact, beyond getting the engine running, the battery offers a type of electrical reservoir for the charging system — it smooths out variances in voltage and helps remove residual alternating-current noise — and it offers backup power for aircraft systems.

By name alone the lead-acid battery describes its main components. Individual plates made of lead and lead alloys (specifically lead dioxide and so-called pure spongy lead) are immersed in a mixture of sulfuric acid and water called electrolyte. A chemical reaction occurs in a charged lead-acid battery that gives the negative plates an excess of electrons (negatively charged ions), while the positive plates have too few. When connected to an external load, the excess electrons on the negative plate flow to the positive plate, creating an electrical charge. A side effect of this action is a gradual formation of lead sulfate on both sets of plates; eventually the sulfate will interfere with the chemical reaction and the battery will cease to produce voltage. This is the discharged state — or, the left-the-dang-master-on-again condition. By applying current to the battery in what is essentially reverse flow — via the airplane's charging circuits, for example — this electrochemical reaction can be reversed, dissipating the lead sulfate.

A typical lead-acid aircraft battery consists of six or 12 sets of plates arrayed in cells. Each cell produces about 2 volts, hence the multiplicity; the cells are wired in series to obtain a desired voltage. In the most common of batteries, the plates in the cells are separated by plastic dividers and immersed completely in the liquid electrolyte. Vented caps on the battery case allow the cells to breathe. During the charging sequence, the lead-acid battery expels hydrogen and oxygen; these gases must be vented to the atmosphere. Typically, this will occur through special caps that prevent spillage during bouts of turbulence, or through a manifold system to external vents. (That's why aircraft with a non-manifold battery use a sealed battery box. In the event of spillage, you don't want the acid to be eating through the aircraft structure or your favorite windshield- cleaning diapers at the back of the baggage bay.)

Though there have been several manufacturers of lead-acid batteries for general aviation, today we're down to two. Teledyne- Gill (800/456-0070) is indisputably the volume leader, with a wide variety of batteries for everything from Cessna 150s to business jets. Its most common product is the straightforward lead-acid wet cell with liquid electrolyte. For the most part, this battery is little different from the one in your Cub Cadet tractor, save for detail differences in plate design and enclosure construction. Generally, aviation batteries use lighter cases and have thinner plates than comparable general-purpose batteries, all in the name of weight savings.

But Gill has recently jumped on the bandwagon of new lead- acid batteries called recombinant gas (RG) — you could also say sealed maintenance-free if that label reminds you of aftereffects of Cajun food. This is a trend, incidentally, that Concorde Battery Corporation (818/813-1234), the other big charger in the battery field, has been following for more than nine years now. B&C Specialty Products (316/283-8000) sells RG batteries, as well.

Rather than using liquid electrolyte, the RG battery employs something called absorbed glass mat to maintain the acid in suspension. Looking like scratchy toilet paper, this mat is 90- percent saturated in acid and is held tightly against the lead plates. Since the mat contains the liquid, there's no chance of spillage. This construction also allows the plates to be closer together, a circumstance that the battery makers have used to cram more plates into each cell. Therefore, an RG battery of a conventional size will be heavier but have greater capacity than a comparable flooded-cell model.

Concorde has been perfecting the manufacturing techniques used in RG batteries for nearly a decade, and its latest line benefits from that experience. Besides the additional plates for extra power, Concorde's RG battery uses proprietary tricks to ensure that the plates remain affixed tightly in place. The company has also come up with a venting system that maintains slight positive pressure within the battery, which is supposedly good for efficiency.

So could the RG battery be the equivalent of a complimentary midday repast? Very likely. The RG battery can't leak, doesn't need a battery box, and generally has greater electrical capacity for its size. It also has lower internal resistance than a conventional lead- acid cell, leading to improved cranking power. The RG also has a slower self-discharge rate. Finally, the RG battery costs about the same as a regular lead-acid type when you take into account that conventional flooded-cell batteries require you to purchase electrolyte separately and often pay more for shipping. (The RG batteries aren't considered hazardous by your UPS driver.)

One major drawback is that you can't easily ascertain the RG battery's condition. In a regular lead-acid model, you can test the specific gravity of the electrolyte to determine cell health. In an RG that's not possible. Concorde has come up with a test that will help identify battery strength. It involves loading the battery to 80 percent of its rated output for one hour. Then a measurement of its no-load voltage is used to determine its condition.

Unfortunately, few maintenance facilities currently have the equipment necessary to test the RG battery, so Concorde has taken an extremely conservative view with respect to replacement times. If no maintenance or testing of the battery is performed at all, Concorde recommends pulling it from service at 600 hours or one year, whichever comes first. To keep the battery in service past this limitation, Concorde recommends the aforementioned test. The company's rationale is that a battery is an emergency backup, with roles more important than merely starting the engine. And while a severely depleted battery might indeed be able to get you to the end of the runway, there's no guarantee it'll last for a reasonable amount of time in the event of an in-flight charging system shutdown. Concorde is probably playing this aspect too conservatively. Right now relatively few owners replace a battery until it shows signs of severe depletion, whether it's an RG or lead-acid model.

In specialized applications, the so-called gel-cell battery has been popular. Its construction is similar to that of a regular lead- acid cell, except that the electrolyte is in gel form. This allows the battery to be sealed, like the RG, and is essentially maintenance- free. So far, these gel cells have found most favor in aerobatic and other special-case installations in which freedom from acid spillage is paramount. But since gel cells are somewhat less efficient and appear to have a shorter lifespan than conventional batteries, it seems that the applications for them will remain specialized.

How long, then, should a battery last? That depends on many factors, but two or three years seems to be the average; a few have been known to go five or six years in light-duty applications. And yet a battery that's ignored and over-charged (more on that later) may not see its third birthday. Airframes placing the battery on the firewall may not get exceptional life, either. (Although, as recompense, this setup also means that the battery doesn't have to work quite as hard at engine start.)

Proper charging conditions are vital to its long-term health. The greatest cause of premature battery demise is over-charging. It's worthwhile to connect a precision voltmeter to your airplane's battery bus once in a while to make sure the voltage regulator isn't working too aggressively. Ideal charging rates vary by battery type and temperature — the lower the temperature, the more charging a battery can handle — but an overage of as little as half a volt can lead to long-term degradation.

Cold electrolyte has much higher internal resistance than when hot — that's because of viscosity increase, which doubles between 77 and 32 degrees Fahrenheit. Now you know why it's so much harder to start a cold-soaked engine in the winter.

The engine itself is more resistant to coming to life at exactly the same time that the battery's performance is at a low ebb. And these are the conditions in which a battery that has been ignored all during the summer (and maybe even through several summers past) allows us to rediscover the joys of hand-propping — or at least the warmth and comfort of the FBO's power cart.