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How Lead Acid
Batteries Work
The negative electrode
supplies electrons to the external circuit (or load) during discharge. In a
fully charged lead-acid storage battery the negative electrode is composed
of sponge lead (Pb). The positive electrode accepts electrons
from the load during discharge. In a fully charged lead-acid battery the
positive electrode is composed of lead dioxide (PbO2). It should
be noted that the electrodes in a battery must be of dissimilar materials
or the cell will not be able to develop an electrical potential and conduct
electrical current. The electrolyte completes the internal
circuit in the battery by supplying ions to the positive and negative
electrodes. Diluted sulfuric
acid (H2SO4) is the electrolyte in lead-acid
batteries. In a fully charged lead-acid battery, the electrolyte is
approximately 37.52% sulfuric acid and 62.48% water, or 1.285 specific
gravity.
The separator
is used to electrically isolate the positive and negative electrodes. If
the electrodes are allowed to come in contact, the cell will short-circuit
and become useless because both electrodes would be at the same
potential. The type of separator
used varies by cell type. Materials used as separators
must allow ion transfer between the electrolyte and electrodes. Many
separators are made of a porous plastic or glass fiber material. The above
components are housed in a container commonly called a jar to
form a cell.
Cells and batteries may be connected
in series, parallel, or combinations of both. Cells or batteries connected in
series have the positive terminal of one cell or battery connected to the
negative terminal of another cell or battery. This has the effect of
increasing the overall voltage but the overall capacity remains the same.
For example, a 12V lead-acid automobile battery contains 6 cells connected
in series with each cell having a potential difference of about 2V.
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