AS
THE WRENCH TURNS
Aircraft
Batteries
Hello WVFC
members! The maintenance subject this month will be about aircraft batteries.
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 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.
Most of us are flying around with lead-acid
batteries. While 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.
Even in the
battery world, the trick is balancing the benefits of the technology with the
cost to aircraft operators. New technologies aren’t as inexpensive as what we
have today, and it remains to be seen where batteries will go in aviation. So,
think about your humble battery next time you look at that ammeter during your
run-up. Consider the work it has to do and what you as a pilot can do to keep
it—and your entire electrical system—happy and functioning.
