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Batteries :
Charges can be separated by several means to produce
a voltage. A battery uses a chemical reaction to produce energy and
separate opposite sign charges onto its two terminals. As the charge is
drawn off by an external circuit, doing work and finally returning to
the opposite terminal, more chemicals in the battery react to restore
the charge difference and the voltage. The particular type of chemical
reaction used determines the voltage of the battery, but for most
commercial batteries the voltage is about 1.5 V per chemical section or
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Batteries with higher voltages really contain
multiple cells inside connected together in series. Now you know
why there are 3 V, 6 V, 9 V, and 12 V batteries, but no 4 or 7 V
batteries. The current a battery can supply depends on the speed
of the chemical reaction supplying charge, which in turn often
depends on the physical size of the cell and the surface area of
the electrodes.
The size of a battery also limits the amount of chemical reactants
stored. During use, the chemical reactants are depleted and
eventually the voltage drops and the current stops.
Even with no current flow, the chemical reaction proceeds at a
very slow rate (and there is some internal current flow), so a
battery has a finite storage or shelf life, about a year or two in
most cases.
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In
some types of batteries, like the ones we use for the robot, the
chemical reaction is reversible: applying an external voltage and
forcing a current through the battery, which requires work,
reverses the chemical reaction and restores most, but not all, the
chemical reactants. This cycle can be repeated many times.
Batteries are specified in terms of their terminal voltage, the
maximum current they can deliver, and the total current capacity
in ampere-hours.
You should handle batteries carefully, especially the ones we use in
this course. Chemicals are a very efficient and compact way of
storing energy. Just consider the power of gasoline or explosives,
or the fact that you can play soccer for several hours powered only
by a slice of cold pizza for breakfast. Never connect the terminals
of a battery together with a wire or other good conductor. The
battery we use for the RoboBoard is similar to the battery in cars,
which uses lead and sulphuric acid as reactants. Such batteries can
deliver very large currents through a short circuit, hundreds of
amperes. The large current will heat the wire
and possibly burn you; the resulting rapid internal chemical
reactions also produce heat and the battery can explode, spreading
nasty, reactive chemicals about. Charging these batteries with too
large a current can have the same effect. Double check the circuit
and instructions before connecting a battery to any circuit.
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