Batteries
Various batteries
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Schematic symbol
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So far, we have treated batteries as if they were perfect voltage
sources. That is, we have assumed that a battery will produce the same
voltage under all circumstances and has no other effects on a circuit.
In reality, batteries have their own characteristics and batteries with
different chemistry have different characteristics.
Terminology
What we usually call a battery should be called a voltaic cell or
galvanic cell. A "battery" is technically a group of voltaic cells.
However, it has become common to refer to a single voltaic cell as a
battery and a group of voltaic cells "batteries".
How batteries are constructed
A battery is constructed by combining two dissimilar metals with an
electrolyte (an acid or base) that chemically reacts with the metals.
The unequal chemical action causes an imbalance of free electrons and
therefore an electric potential between the electrodes of the cell.
Different metal / electrolyte combinations will result in different
voltages as well as differences in other characteristics. For example,
a Carbon-Zinc cell produces 1.6 volts. The voltage appears to
drop off steadily as the battery is used. A mercury-oxide cell produces
1.35 volts and the voltage remains constant through most of the life of
the battery.
Internal resistance
Internal resistance is a theoretical limitation of how much current a
battery can deliver. As stated above, a carbon-zinc cell appears to
lose voltage over time. In theory this is because the internal
resistance increases as the battery is used. Internal resistance is
calculated by dividing the open circuit voltage by the closed circuit
current. This is the same procedure for determining the Thevenin
equivalent impedance of a circuit (see
Thevenin's Theorem).
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Equivalent circuit for a battery |
The internal resistance of a battery is the same concept as output
impedance with other circuits (Thevenin equivalent impedance, output
impedance and internal resistance are different names for the same
thing). If you try the above method to determine the internal
resistance of a battery (measuring the open circuit voltage and the
short circuit current), be sure to have the current meter across the
battery for as short a time possible. Shorting batteries for too long
can cause fire or cause the battery to explode.
Batteries and EMF
Earlier (see What is Voltage), we discussed the difference between EMF
and voltage. Recall that EMF is a force that tends to move electricity, and
voltage is an electrical pressure differential that develops where the flow
of electrical current is blocked. The standard battery model (shown above as
the equivalent circuit for a battery) shows a source of EMF (represented by
a battery symbol) and a resistance in series with that EMF source. The
voltage across the terminals of a battery is usually assumed to equal the
total EMF. This assumes no current flow since a voltmeter has a very high
impedance. If there is current flow, the voltage at the terminals will be
the total EMF minus the voltage differential across the internal
resistance. This concept has been discussed previously under Ohm's Law and
Thevenin's Theorem, Output Impedance and Input Impedance.
Applications and Safety
The terminals of batteries should never be shorted together except for
short periods in order to test the battery. Some shorted batteries,
particularly alkaline batteries, will explode.
Fresh batteries should never be mixed with old batteries. Fresh
batteries will force a reverse charge on old batteries near exhaustion.
This is likely to cause the old batteries to leak.
Some batteries contain powerful acid (lead-acid batteries for example).
Others contain other hazardous materials such as mercury.
Batteries that are not designed to be recharged should never be
recharged. For example, standard alkaline batteries are prone to leak
if recharged. However, rechargeable alkaline batteries are available
and labeled as such.
