Plane & Pilot
Tuesday, September 6, 2011

Caring For Your Aircraft


Part V: Understanding your aircraft’s battery is simple, useful and important


The aircraft battery is a seldom-noticed and somewhat-ignored component of the machine’s life-giving systems. In past articles, we’ve discussed the aircraft’s ignition system, and noted that it doesn’t require a battery; the engine runs just fine with its magnetos creating the electrical pulses necessary to make the cylinders fire. But because few of us hand-prop airplanes any more, and because of our increased reliance on electronics and technology in the cockpit, the battery has become the star of the show in modern aircraft.

Our batteries—and in fact all batteries—essentially are energy-storage modules. In our aircraft, we rely on the battery to store enough energy, so when we turn the ignition key, there’s enough power to turn the starter, which turns the propeller and creates that initial spark that ignites the fuel-air mixture in the cylinders. Once the engine is firing on its own, the battery provides power to all the aircraft’s electronics, like radios, glass-panel displays, GPS units and the zillion other things in the aircraft that run on electricity.

Most think the battery’s job is done once the engine has started, since the alternator is now engaged by the turning engine and constantly feeds power to the battery and other electrical components. While that’s true, the battery still exists to provide emergency power in the event of an alternator failure. Anyone who has experienced an alternator failure at night knows the value of a well-charged battery.

Lead-Acid Batteries
Most of us are flying around with lead-acid batteries. While it’s true that most electronics have moved to nickel-cadmium, and more recently, Li-Ion technology, general aviation has stayed with lead acid for a number of reasons, including ease of maintenance and low cost. Lead-acid technology is robust, proven, and—like the lowly magneto—reliable. The technology is simple.

Lead-acid aircraft batteries contain six or 12 “flooded” lead-acid cells connected in a series to make a 12-volt or 24-volt battery (each cell is nominally rated at 2V). The cells are encased in a plastic container equipped with electrical terminals (or sometimes a receptacle for mating to the aircraft). Each cell consists of positive plates made of lead dioxide and negative plates made of spongy lead (a form of metallic lead). The “flooded” part refers to the electrolyte that fills each cell, made of sulfuric acid and water.

The positive and negative plates are separated by layers of polyethylene to prevent the plates from shorting together. Electrons flow from the negative plate, out the battery terminal to whatever electrical component is being powered, then back through the positive plate. The electrons leaving the negative plate cause an oxidation reaction that converts the spongy lead into lead sulfate. The gathering of electrons at the positive plate causes a chemical reaction that converts the lead dioxide into lead sulfate. The whole process continues until most of each plate is converted to lead sulfate and the battery is fully discharged.

Meanwhile, the engine is turning the alternator, which sends an electrical charge back to the battery, keeping it from discharging completely. During the charging process, current is passed through the cells in the reverse direction. The reverse current causes a reverse of the chemical reaction, returning the positive plates to lead dioxide and the negative plates to spongy lead. When this process is complete, the battery is fully charged.



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