Lead-Acid Storage Batteries
BATTERY COMPONENTS AND OPERATION
Cells vs. Batteries
A battery is a device that converts the chemical energy contained in its active materials into
electrical energy by means of an electrochemical reaction. While the term "battery" is often
used, the basic electrochemical element being referred to is the cell. A battery consists of
two or more cells electrically connected in series to form a unit. In common usage, the terms
"battery" and "cell" are used interchangeably.
Primary and Secondary Cells and Batteries
Batteries are either primary or secondary. Primary batteries can be used only once because
the chemical reactions that supply the electrical current are irreversible. Secondary (or
storage) batteries can be used, charged, and reused. In these batteries, the chemical
reactions that supply electrical current are readily reversed so that the battery is charged.
Primary batteries are common since they are cheap and easy to use. Familiar primary
battery uses are in flashlights, watches, toys, and radios. The most common use for
secondary (storage) batteries is for starting, lighting, and ignition (SLI) in automobiles and
engine-generator sets. Other applications include uninterruptible power supplies (UPSs) for
emergency and backup power, electric vehicles (traction), telecommunications, and portable
tools. The remainder of this Primer will be concerned only with storage batteries except
where general operating characteristics of batteries are discussed.
A cell has five major components as shown in Figure 1.
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 (PbO ). 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 thus conduct electrical current. The
electrolyte completes the internal circuit in the battery by supplying ions to the positive and
negative electrodes. Dilute sulfuric acid (H SO ) is the electrolyte in lead-acid batteries. In
a fully charged lead-acid battery, the electrolyte is approximately 25% sulfuric acid and 75%
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