Batteries Not Included
It used to baffle me at electronics
trade shows that several big international companies spent a lot of effort putting
together press kits devoted to their battery lines. I couldn't imagine that any of the
journalists attending the show cared much about batteries, and I certainly had never seen
an article about them.
But batteries are definitely a big deal. Just consider how
many things you use every day that employ batteries, even some you may not be aware of.
The computer I am writing this on has a hidden battery, for instance, whose only function
is to keep the internal clock going when the power goes off. Other devices have tiny
batteries to power memory chips that hold the settings you have programmed in.
And then there are the obvious battery-driven things, from
cell phones to portable CD players to TV remote controls. If you do a quick audit of the
things you own, you'll be astounded at how many batteries you use in a day. But chances
are you know very little about them.
Almost all the electricity we use is produced one of two
ways. The first is electromagnetically generated by the power utilities and conducted to
our houses by wires from massive generating plants. It doesn't matter whether coal or
natural gas or nuclear power is the originating energy; each one simply creates motion so
a bank of magnets can induce a current in a coil of wire. It's like an electric motor in
reverse. Mostly such power is alternating current, in which the polarity -- the direction
the power moves through the wires -- switches back and forth some sixty times a second (in
North America, at least; some other parts of the world use fifty).
The other derives electrical energy from a chemical
reaction. Strictly speaking, the basic power-generating unit in this case is called a
"cell"; if a number of cells are strung together they become a battery. In
practice, however, most people use the word battery whether single or multiple cells are
used.
A battery consists of two electrodes made of dissimilar
materials, separated by a medium called an electrolyte. The electrodes react differently
to the electrolyte, one (called the anode) becoming negatively charged, the other (the
cathode) acquiring a positive change. If you were to connect the two directly, using a
piece of wire, these charges would cancel each other out very quickly, as the extra
electrons of the anode would race through the wire to make up a deficiency of electrons in
the cathode.
Instead of such a "short circuit," the movement
of the electrons can be made to do work, by illuminating a light bulb, say, or driving an
electric motor or electronic circuit. This electricity is "direct current," in
which the electrons always flow in the same direction (anode to cathode).
The first cell was developed about 1800 by Alessandro
Volta, who separated disks of silver and zinc by layers of cloth or paper soaked in an
electrolyte such as brine. The unit of electrical potential, the volt, is named after him.
France’s Gaston Planté invented the forerunner of the
modern car battery -- the lead-acid cell -- in 1859. His countryman, Georges Leclanché,
came up with the dry cell about thirty years later. The dry cell was the ancestor of the
familiar flashlight battery, although the name is a bit of a misnomer. While the lead-acid
cell used (and still uses) a liquid electrolyte that can spill, the dry cell used a paste
or jelly, which is much more physically stable.
The dry cell and its kin are what are known as
"primary" batteries. There is a finite amount to the reaction of the electrodes
and the electrolyte, after which electricity ceases to be produced, and the battery must
be discarded.
In a perfect battery, the reaction would only take place
when the electrons were being drawn off to do work; when the circuit is broken, the
reaction should stop. In reality, there are impurities in the materials used to make
batteries that let the reaction continue in a reduced amount even when the battery is not
in use, which means that cells have a shelf life, and will ultimately die even if not
connected to anything.
The lead-acid cell is a "secondary" battery, in
which the chemical reaction that produces electricity can be reversed by applying a
current from an external source to the electrodes in the reverse direction. Such devices
were once known as "accumulators" or storage batteries; now they're generally
called "rechargeable" batteries.
In theory, the number of times you could restore full power
to a secondary battery should be limitless, but again, no device is perfect, and
eventually the battery will no longer take a charge, as any car owner knows.
The most common batteries are primary
zinc-manganese-dioxide cells -- "flashlight batteries." One of the electrodes,
the anode, is the zinc-alloy outer shell of the battery; the cathode is manganese dioxide
formed around a carbon rod in the battery's center. In between is an electrolyte that's
mostly ammonium chloride, with some zinc chloride added.
An improvement on this design was patented in 1899, in
which zinc chloride alone is used in the electrolyte and much purer materials can be used
for the electrodes, extending the life of the battery. Even longer service can be obtained
with a battery which is basically similar to the zinc-manganese sort, but which uses an
alkaline electrolyte.
All of these designs produce a similar voltage -- roughly
1.5V -- which means they are interchangeable. Greater operating voltages can be obtained
by stringing batteries in series, most flashlights using two, for instance, many other
devices using four or more. The size of the battery affects its life, not how much voltage
it produces.
Several other systems - mercuric-oxide-zinc and
silver-oxide-zinc -- typically come in the form of button batteries, useful for watches,
hearing aids, and cameras.
The most recent family of batteries use an anode made of
lithium, which exhibits a high degree of chemical activity. In some applications, lithium
batteries can produce higher voltages than conventional batteries. Other types are able to
operate at extremely low temperatures.
The most common secondary batteries have traditionally been
the lead-oxide variety. The car version of this is perhaps the best known, characterized
by its ability to give hundreds of brief discharges over a number of years. The technology
is also used in things like lift trucks, where they may weigh several tons, the weight
helping to stabilize the vehicle. They are also used for emergency power systems because
of their long shelf life.
More recent are the rechargeable batteries used in a
variety of consumer items, such as camcorders. The most familiar is the Nickel-cadmium
(NiCad), which turns up almost everywhere. The main virtue of the NiCad battery is its
ability to be recharged numerous times. It suffers from a much shorter operating cycle per
charge than most primary batteries, however, and is best recharged only when it has been
completely flattened. Otherwise, it will only take a partial charge, and the
lower-percentage charge is cumulative, so that after a number of charges the battery will
only hold a small amount of power. Some chargers have a "refresh" mode that
intentionally discharges the battery completely before recharging it, but most do not.
A more recent development is the nickel-metal-hydride
(NiMH) battery, which does not exhibit the same sort of charging "memory" as the
NiCad.
Both types of battery take about 12 to 14 hours to charge
with the typical charger. Manufacturers could make the process faster or slower, but the
"overnight" charge is a good compromise because it involves levels of current
that won't damage the battery if it's connected after a full charge is reached. Reducing
the power for the same reason runs the risk that a full charge will never be achieved.
For some, it's tempting to consider replacing all one's
primary batteries with rechargeables, but in reality, both types have their place. In
devices that are used for relatively short periods, but frequently, and require high power
levels, secondary batteries are probably appropriate, and in fact most such units come
with their own charging systems. For things that need little power or are only used
occasionally, old-style throwaway batteries are undoubtedly the more economic choice.
...Ian G. Masters
ian@mastersonaudio.com
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